2022

[1]Nienke van der Marel

[1]Leiden Observatory, Leiden University, P.O. Box 9513, Leiden, 2333 CA, the
Netherlands

# Transition disks: the observational revolution from SEDs to imaging

<EMAIL_ADDRESS>*

###### Abstract

Protoplanetary disks surrounding young stars are the birth place of planets.
Of particular interest are the transition disks with large inner dust cavities
of tens of au, hinting at the presence of massive companions. These cavities
were first recognized by a deficit in their Spectral Energy Distribution
(SED), later confirmed by millimeter interferometry observations. The Atacama
Large Millimeter/submillimeter Array (ALMA) has truly revolutionized the field
of spatially resolved imaging of protoplanetary disks in both dust and gas,
providing important hints for the origin of gaps and cavities. At the same
time, new types of substructures have been revealed. Also infrared
observations show a large range of substructures both in resolved imaging,
interferometry and spectroscopy. Since the last review paper of transition
disks in Protostars and Planets VI, a huge amount of data has been taken,
which led to numerous new insights in the origin of transition disks. In this
review I will summarize the observational efforts from the past decade,
compare their insights with the predictions from SED modeling, analyze the
properties of the transition disk population and discuss their role in general
disk evolution.

##  1 Introduction

The study of protoplanetary disks and their dissipation processes is one of
the most active fields in today’s astronomy (e.g. Williams Cieza, 2011; Testi
., 2014; Ercolano Pascucci, 2017; Andrews, 2020; Manara ., 2022). The
investigation of transition disks, loosely defined as disks with an inner
clearing in their dust distribution (e.g. Espaillat ., 2014), is a fundamental
component of these studies, as these disks were historically the only targets
with direct evidence for clearing with respect to the younger, full
circumstellar disks. Early evidence for clearing of the dust was the deficit
in the mid infrared excess of the Spectral Energy Distribution (SED), whereas
the excess would be high again at longer wavelengths, comparable to that of
full protoplanetary disks (e.g. Calvet ., 2002). The clearing is also referred
to as a ‘hole’ or ‘cavity’. What makes these disks particularly intriguing is
that they still maintain high accretion rates, comparable to those of ‘full’
disks, indicating that some material is still flowing through the gap towards
the star. As transition disks were identified long before the rise of
spatially resolved imaging with e.g. millimeter interferometry, scattered
light imaging and (more recently) optical interferometry, it is important to
keep in mind that early studies and transition disk identifications may
actually represent a large range of actual disk morphologies and physical
processes responsible for the clearing. The literature has become cluttered
with a large range of definitions and classifications within the term
‘transition disk’, both in the observational and the disk modeling
perspectives. The most recent reviews that were specifically focused on
transition disks date from Protostars & Planets VI in 2014 (Espaillat ., 2014)
as well as a PASA review in 2016 (Casassus, 2016) which pre-dates the majority
of the high-resolution millimeter studies with the Atacama Large
Millimeter/submillimeter Array (ALMA) (e.g. Pinilla, Tazzari ., 2018; Francis
van der Marel, 2020) and the efforts on both high contrast imaging of embedded
protoplanets and scattered light imaging with optical/infrared (OIR)
facilities such as VLT/SPHERE, Gemini/GPI and Subaru/SCExAO (Benisty ., 2022).
Therefore, this new review attempts to summarize the different contributions
to the field over the last 10 years, discuss the implications of the different
definitions of transition disks and compare the transition disks as identified
from SEDs and (millimeter) imaging. The focus of the latter will be primarily
on millimeter dust continuum images, as the morphologies and relevant disk
processes of scattered light imaging was recently reviewed in (Benisty .,
2022).

Figure 1: Sketch of a transition disk structure of a large cavity transition
disk where millimeter dust is trapped at the cavity edge. The gap may be
caused by a giant planet but material is still flowing through the gap as
evidenced from the high accretion rates. Figure 2: Gallery of continuum
emission of transition disks imaged with ALMA, showing the large diversity of
substructures.

Historically, the term ‘transition disk’ (sometimes also referred to as
‘transitional disk’) originates from the first discovery of 8 young stars with
infrared excess at $\lambda>10\mu$m and the absence of excess at
$\lambda<10\mu$m in a large sample of 83 pre-main sequence stars in Taurus
with near-infrared (_J, H, K, L_ -band) and _IRAS_ (25, 60, 100 and 160
$\mu$m) photometry (Strom ., 1989). The authors define these disks as being
"in transition", possibly due to "disk clearing" where the "inner disk regions
$r<0.1$ au are relatively devoid of distributed matter". The occurrence of
these disks of approximately 10% of the sample is regarded as a measure of the
"transition time" from massive, optically thick to lower-mass, more tenuous
structures (e.g. Kenyon Hartmann, 1995; Simon Prato, 1995), a fraction that is
recovered in more recent studies as well (e.g. Ercolano Pascucci, 2017).
Interestingly, the 8 identified transition disks by Strom et al. are all but
one marked as weak-line T Tauri star, i.e. a non-accreting young star, which
would be consistent with these disks being caught in the act of dissipation,
making them indeed ‘transitional’. However, a closer inspection of these disks
with _Spitzer_ and _Herschel_ photometry shows that the _IRAS_ photometric
points of several of these disks are confused with other emission due to the
large beam size and thus not actual transition disks as discovered in later
decades (e.g. Ribas ., 2016).

New infrared facilities such as _ISO_ and _Spitzer_ led to the discovery of
many more transition disks with an infrared deficit, e.g. a 4 au cavity in TW
Hya (Calvet ., 2002), and larger cavities $>$10 au in e.g. HD100546 (Bouwman
., 2003), DM Tau and GM Aur (Calvet ., 2005), CoKu Tau/4 (D’Alessio ., 2005),
T Cha, LkH$\alpha$330, SR 21 and HD 135344B111HD135344B is also referred to as
SAO 206462. (Brown ., 2007) and many others, where the cavity size was
inferred from radiative transfer modeling (see also Section 4). Follow-up
imaging with millimeter interferometry with the SubMillimeter Array (SMA),
CARMA and Plateau de Bure Interferometer (PdBI) confirmed the presence of
inner dust cavities in several of these disks (e.g. Piétu ., 2006; Brown .,
2009; Hughes ., 2009; Isella ., 2010; Andrews ., 2011), although the recovered
images had relatively low image fidelity due to the limited baselines and thus
sparse uv-coverage of these interferometers and the derivation of the cavity
size was primarily based on the location of the ‘null’ in the visibility curve
(Hughes ., 2007). The maximum baselines of these interferometers reached a few
100 meters, resulting in a beam size of 0.3" at best, resolving only the
largest inner cavities ($\gtrsim$30 au). Other observations in e.g. thermal
infrared imaging also revealed inner dust clearing (Geers ., 2007) and
deficits in the inner gas surface density through spectro-astrometry of near
infrared CO rovibrational lines (e.g. Pontoppidan ., 2008; van der Plas .,
2009; Brown, Herczeg ., 2012). However, it was not until the start of
operations of the ALMA telescope that the morphology of transition disks and
their cavities in dust and gas could be fully revealed and quantified (see
Section 2).

Figure 3: The four main mechanisms proposed to explain transition disks: grain
growth (Dullemond ., 2001), photoevaporation (R.D. Alexander ., 2006),
clearing by a companion (Artymowicz Lubow, 1996) and dead zones (Regály .,
2012). The latter two processes are accompanied by dust trapping in a pressure
bump (Birnstiel ., 2010). More information in the text.

Meanwhile, a large number of physical mechanisms had been proposed to explain
the inner cavities in transition disks, including the dynamical clearing by
companions (Artymowicz Lubow, 1996; Zhu ., 2011), where the latter could be a
stellar companion, i.e. the transition disk as circumbinary disk;
photoevaporative clearing at the end of the lifetime of the disk (R.D.
Alexander ., 2006; Owen ., 2011), or a combination thereof (R.D. Alexander
Armitage, 2009), dead zones (Regály ., 2012) or enhanced grain growth
(Dullemond ., 2001). The discovery of the 8 au stellar companion inside the 14
au cavity of CoKu Tau/4 (Ireland Kraus, 2008) initially led to the suggestion
that all transition disks might be circumbinary disks, and a binary companion
was also discovered for HD142527 (Lacour ., 2016; Claudi ., 2019), but firm
limits on companions in several transition disk cavities made this unlikely as
a general explanation (A.L. Kraus ., 2011).

Several Jovian companion candidates have been identified and dismissed (e.g.
Ligi ., 2018; Currie ., 2019; Wagner ., 2019), and it was not until the recent
confirmation of the protoplanets in the PDS 70 and AB Aur transition disks
(Keppler ., 2018; Haffert ., 2019; Currie, Lawson ., 2022) that planetary
companions are considered a viable explanation for at least some transition
disks, where many targets still lack firm enough limits due to the challenges
in high contrast imaging in young disks (Asensio-Torres ., 2021; van der
Marel, Birnstiel ., 2021; Currie, Biller ., 2022). Photoevaporative clearing
was initially dismissed for the transition disks with high accretion rates and
large ($>$20 au) inner cavities (Owen Clarke, 2012; Ercolano Pascucci, 2017)
leaving only the transition disks with smaller, SED-derived cavities as
possible candidates, although recent efforts in photoevaporative modeling show
that perhaps a wider range of cavities could be caused by photoevaporation as
well (Picogna ., 2019; Ercolano ., 2021). Also other mechanisms have been
proposed to explain cleared cavities (see Section 5.2.2). Overall, there may
not be a single mechanism responsible for all transition disks and, also
considering the range of definitions, it may be better not to consider
transition disks as a single population, in particular when using their
occurrence rates as a way to infer dissipation timescales. Specifically,
transition disks should not be considered as a separate stage of disk
evolution as long as it is not evident that all protoplanetary disks go
through an observable ’transitional’ phase. An empirical definition of a
transition disk could lead to biases depending on the used selection criteria
and this should be considered whenever transition disks are treated as a
population.

‘Complete’ transition disk samples remain extremely challenging to define,
both in SED studies and in millimeter imaging. In the latter, the spatial
resolution limits the cavities that can be sufficiently resolved, in
particular in the so-called snapshot surveys that target all Class II disks in
nearby star forming regions which are always relatively low resolution for
time efficiency (for an overview, see e.g. Manara ., 2022). In SED studies,
transition disk candidates are usually selected with color criteria (e.g.
Brown ., 2007; Muzerolle ., 2010; L.A. Cieza ., 2010; Merín ., 2010), followed
by radiative transfer modeling which is known to be highly degenerate and
identification can be affected by e.g. extinction, irradiation effects and
inclination (Ercolano ., 2009; Merín ., 2010; van der Marel ., 2022).
Furthermore, cavity sizes from SED modeling are known to differ from those
from millimeter imaging by a factor 2-3 (Brown ., 2009; van der Marel, Verhaar
., 2016), although the latter could also be the result of dust grain size
segregation (Rice ., 2006; Zhu ., 2011). The definition of the word
‘transition disk’ has become less definitive with the discovery of narrow-
gapped disks such as e.g. HL Tau, HD163296 and several disks in the DSHARP
survey (ALMA Partnership ., 2015; Isella ., 2016; Andrews, Huang ., 2018). The
inner cavities in transition disks could be considered as the most extreme
cases of such gaps in terms of their width and location (Andrews, 2020).
Considering their historic nature in both observational sense (deficit in the
SED, resolved gap in pre-ALMA images) and possible underlying mechanisms
(inside-out photoevaporative clearing or clearing by massive, Jovian planets),
as well as their potential for well-resolved imaging due to the large cavity
sizes compared to gapped disks, transition disks can still be considered as a
separate group of interest in disk studies as long as biases are considered.

In this review I will discuss the properties of known transition disks within
200 pc and primarily focus on the disks with cavity sizes $\gtrsim 15$ au to
be as complete and unbiased as possible. I will discuss the limitations of
this approach for our general understanding of transition disks and the steps
that are necessary for a truly complete picture. Section 2 describes the disk
properties as derived from (primarily) ALMA millimeter imaging, essentially
dismissing previous millimeter interferometry results due to the superb image
quality and increase in both resolution and sensitivity of ALMA. Section 3
reviews the various insights from infrared observations of transition disks,
although scattered light imaging is kept brief as this topic is largely
discussed in the PPVII chapter by Benisty . (2022). Section 4 summarizes the
insights from SED analysis and how these compare to the now known morphologies
seen in millimeter dust continuum. In Section 5, I will analyze a large number
of properties of the known transition disks to infer trends and possible
connections with the proposed mechanisms, followed by an outlook on future
studies of transition disks with new and upcoming observational facilities.

##  2 The millimeter perspective

###  2.1 Dust trapping

The millimeter images of transition disks have given evidence for the
existence of dust trapping in protoplanetary disks, as a way to retain
millimeter-sized dust particles in the outer part of the disk that would
otherwise be transported inwards as the result of radial drift
(Weidenschilling, 1977). Radial drift is the result of drag forces between the
gas and dust grains, depending on their Stokes number (which is proportional
to the grain size). As a smooth, axisymmetric protoplanetary disk naturally
has a negative radial pressure gradient in the gas, millimeter-sized dust
grains tend to drift inwards rapidly, essentially limiting the potential for
further growth to planetesimals and planets (e.g. Brauer ., 2008; Birnstiel .,
2010). The presence of local pressure maxima or pressure bumps in the outer
disk have long been proposed as possible explanation for the presence of
millimeter-sized dust grains in disks (e.g. Barge Sommeria, 1995; Pinilla,
Birnstiel ., 2012), as evidenced from strong millimeter fluxes and low
spectral indices (e.g. Andrews ., 2009; Ricci ., 2010). The pressure maxima
act as so-called _dust traps_ , which limit the radial drift. Dust can be
trapped in both the radial and azimuthal direction, leading to either ring-
like or asymmetric millimeter-dust concentrations (e.g. Pinilla, Birnstiel .,
2012; Birnstiel ., 2013), whereas the gas and small grains are still
distributed throughout the disk with more moderate changes in the surface
density. The segregation between millimeter-sized dust grains versus gas and
small grains provides evidence for dust trapping (see Figure 4 and Rice .
(2006)).

Figure 4: Two examples of observational evidence for dust trapping, both
radially (top) and azimuthally (bottom) for the J1604.3-2130 and Oph IRS 48
transition disks based on millimeter continuum emission, integrated 13CO
emission and infrared emission (originating from small dust grains). The white
dashed ellipse indicates the radius of the millimeter dust.The segregation
between the gas/small grains and millimeter dust provides evidence for the
existence of trapping. Images combined from van der Marel . (2013); van der
Marel, van Dishoeck . (2016); Dong . (2017).

Even before ALMA, transition disks had been proposed as promising candidates
of disks with dust traps in the context of dust filtration between large and
small grains to explain the presence of large inner cavities in combination
with the high accretion rates (Rice ., 2006; Pinilla, Benisty Birnstiel, 2012;
Zhu ., 2012). However, the highly asymmetric nature of the millimeter-dust
continuum emission in the Oph IRS 48 transition disk in contrast with the
ring-like distribution of the gas as traced by 12CO 6–5 and the small grains
as traced by VLT/VISIR 18 $\mu$m emission (van der Marel ., 2013) provided the
first observational evidence for the existence of dust traps. The smaller
inner cavity seen in the 12CO 6–5 with a radius of $\sim$20 au with a dust
trap at 60 au is consistent with a planet-gap model where the dust is trapped
at the very outer edge of the gap (Pinilla, Benisty Birnstiel, 2012). Radial
dust traps were identified soon after in e.g. J1604.3-2130 (Zhang ., 2014)
which showed a narrow continuum dust ring and gas disk as traced with 12CO 3–2
with a much smaller inner cavity. Also scattered light images provided
evidence for dust trapping, as the near infrared emission tracing the small
grains in the disk showed much smaller inner cavities or no cavity at all
(likely due to the coronagraphic mask) compared to the millimeter-dust
emission (Dong ., 2012; Garufi ., 2013), which could be reproduced by dust
evolution models with a dust trap at the edge of a planet-carved gap (de Juan
Ovelar ., 2013). Further developments in observations of gas and dust
distributions in transition disks are described in Section 2.4.

Dust substructures, in particular gaps and rings, are now commonly seen in
high-resolution ALMA continuum images of bright protoplanetary disks (e.g. the
DSHARP survey, Andrews, Huang ., 2018), and usually directly interpreted as
dust traps (Dullemond ., 2018). Transition disks thus form a subset of
protoplanetary disks with dust traps, and may be defined as a separate class
of objects due to historic reasons, as they contain the first resolvable
substructures on scales of $\gtrsim$30 au even with pre-ALMA interferometers.
Smaller scale substructures of $\lesssim$5 au remain unresolved unless
observed with the highest ALMA resolution (Bae ., 2022). Furthermore,
considering their potential origin of photoevaporative clearing, as well as
circumbinary and eccentric disks (see also Section 5.2), they remain
interesting to study separately from gapped disks which cannot be explained by
these mechanisms. Regardless of the scale and origin of the pressure bumps
causing the dust substructures, it is clear that they play an important role
in maintaining detectable millimeter dust in protoplanetary disks as radial
drift leads to a decrease in observable dust mass (Pinilla ., 2020). A recent
disk demographic study suggests that only a subset of protoplanetary disks is
able to maintain a high millimeter-dust mass due to the presence of dust
traps, whereas the majority of disks decreases in dust mass due to radial
drift in the absence of pressure bumps (van der Marel Mulders, 2021), although
this remains up to debate in lack of high resolution ALMA observations for the
majority of the fainter protoplanetary disks.

###  2.2 Millimeter-dust cavities

The key feature of a transition disk in millimeter images is the cleared inner
cavity in the dust continuum image. A ‘cleared’ inner cavity implies that the
inner part of disk is devoid of millimeter-size dust grains (except for the
very inner disk, see below), with a depletion factor of several orders of
magnitude based on radiative transfer calculations (Andrews ., 2011; van der
Marel, van Dishoeck ., 2015). Cavities have been measured with a large range
of sizes, ranging up to radii of $\sim$200 au in case of GG Tau (a
circumbinary disk), although more typically sizes of $\sim$30-50 au are found.
A lower limit to the cavity radius is difficult to assess as this is
essentially limited by the spatial resolution of the observations, but cavity
sizes as small as 5-10 au have been observed (e.g. Kurtovic ., 2021), as well
as a 2 au gap in TW Hya (Andrews ., 2016). Disks in nearby star forming
regions within 200 pc have been imaged with ALMA with a spatial resolution of
0.2-0.6" in disk survey studies (e.g. Ansdell ., 2016; Barenfeld ., 2016;
Pascucci ., 2016; F. Long ., 2018; L.A. Cieza ., 2019) whereas a subset of
interesting (usually brighter) targets have been imaged at higher resolution
of 0.05-0.1" as well, including many transition disks (e.g. Dong ., 2017;
Boehler ., 2018; Facchini ., 2019; Pinilla ., 2019; L.A. Cieza ., 2021; Maucó
., 2021; van der Marel ., 2022). For Lupus, the ALMA transition disk
population identified from the low-resolution snapshot survey was studied
separately (van der Marel ., 2018). Larger sample studies of transition disks
usually consist of a range of spatial resolutions as they are constructed from
ALMA archival data of multiple programs (Pinilla, Tazzari ., 2018; Francis van
der Marel, 2020). A handful of transition disks have been identified in
younger $<$1 Myr old systems as well, e.g. WL 17 in Ophiuchus with a cavity of
13 au (Sheehan Eisner, 2017; Gulick ., 2021) and 7 systems in Perseus with
cavity radii from 30-100 au (Sheehan ., 2020).

Figure 5: Different types of millimeter substructures in transition disks.

The range in spatial resolution of the disk surveys limits the detectable
transition disk cavities to a different minimum radius, depending on the
survey, typically 0.25" or detectable cavity radii $\gtrsim$20 au in Lupus,
Ophiuchus and Taurus (Ansdell ., 2016; L.A. Cieza ., 2019; F. Long ., 2019;
Akeson ., 2019), although Taurus has not been studied uniformly with ALMA yet
and some disks only have unresolved SMA observations (Andrews ., 2013).
Chamaeleon was imaged at 0.6" and only cavities $\gtrsim$50 au have been
resolved (Pascucci ., 2016), while Upper Sco was imaged at 0.35-0.55",
revealing only two transition disks with a cavity of $\sim$85 au, although
based on the resolution cavities $\gtrsim$25 au can be readily excluded in the
remaining sample (Barenfeld ., 2016). The detectable cavity radius can be a
factor of 2-3 smaller than the angular resolution as the resolution measures
the diameter, and modeling in the visibility plane can infer even smaller
cavities. Additional transition disks have been identified in targeted studies
at higher resolution down to cavity sizes of $\sim$10-15 au (e.g. Francis van
der Marel, 2020; Kurtovic ., 2021; van der Marel ., 2022). Overall, the sample
of transition disks with cavities $\gtrsim$25 au within 200 pc is more or less
complete based on these surveys, with possibly some additional unresolved
transition disks in Chamaeleon and Taurus. In Section 4.5 I will review the
known transition disks with large cavities from both SED and millimeter
imaging studies and estimate the completeness of the known transition disks
within 200 pc.

With the higher sensitivity of ALMA it has become evident that transition disk
cavities are not entirely cleared of millimeter-sized dust: centrally peaked
emission cospatial with the star is regularly detected in cavities of fully
resolved images of transition disks (e.g. Fukagawa ., 2013; Pinilla ., 2019,
2021; Francis van der Marel, 2020; Facchini ., 2020). This emission is usually
interpreted as thermal dust emission from the inner disk, although
contributions of free-free emission cannot be excluded when the emission is
unresolved, in particular at wavelengths $>$2 millimeter (Macías ., 2021), see
also Section 2.5. Inner dust disks have traditionally been inferred from near
infrared excess in the SED, where the disk is also classified as ‘pre-
transitional disk’ (Espaillat ., 2007). However, a lack of correlation between
near infrared excess and the millimeter flux of the inner disk has cast doubt
on the significance of this separate disk class (Francis van der Marel, 2020),
as further discussed in Section 4.3. The presence of inner dust rings also
makes it harder to have a clearly distinct definition of a transition disk
compared to so-called ‘ring disks’ such as the disks in the DSHARP survey
(Andrews, Huang ., 2018), for example in the cases of HD 143006, DM Tau and
WSB 60 (L.M. Pérez ., 2018; Kudo ., 2018; Francis van der Marel, 2020). It has
also become clear that not all transition disks are single dust rings (see
Figure 5): some disks with inner cavities also contain one or more additional
outer rings or asymmetries, which make them again similar to ring disks, for
example HD 97048, HD 135344B, HD 169142, AA Tau, GM Aur and V4046 Sgr (e.g.
Francis van der Marel, 2020). In this review paper these disks are not
considered separately as the inner clearing mechanism can still be compared to
that of other, ‘single-ring’ transition disks.

Cavity sizes in millimeter images can be derived using visibility modeling
(e.g. Berger Segransan, 2007), where the first ‘null’ (the Real component
crossing through zero) is a measure of the cavity size (Hughes ., 2007). This
was particularly relevant in pre-ALMA interferometric data due to the sparse
uv-coverage of early millimeter arrays, where the images could often not
directly reveal the presence of a cavity. Initially the emission profile was
usually estimated using a radiative transfer model, fitting the SED and the
millimeter data simultaneously using a power-law surface density profile with
a steep dropoff at the cavity edge (e.g. Brown ., 2009; Isella ., 2010;
Andrews ., 2011; Zhang ., 2014; van der Marel, van Dishoeck ., 2015; Dong .,
2017). As higher resolution ALMA images started to reveal more detailed radial
profiles (and asymmetric, see next section), the emission started to get fit
directly with a parameterized intensity profile, usually a Gaussian which
could be either symmetric or asymmetric around the peak radius (e.g. Pinilla
., 20171; Pinilla, Tazzari ., 2018) or even with multiple Gaussian components
to fit multiple rings (e.g. Facchini ., 2020). The radial peak is used as
equivalent to the cavity radius.

###  2.3 Asymmetries

About 20% of the observed transition disks in the sample study of Francis van
der Marel (2020) show striking asymmetric morphologies in their millimeter
dust rings, referred to as arcs, clumps, horseshoes or asymmetries (Andrews,
2020; van der Marel, Birnstiel ., 2021). These features are generally
interpreted as azimuthal dust traps, as they are clearly distinct from the gas
and small grain distributions. Asymmetries were tentatively detected pre-ALMA
in e.g. HD142527 and AB Aur (Ohashi, 2008), SR21 and HD135344B (Brown .,
2009), Oph IRS48 (Brown, Rosenfeld ., 2012) and LkH$\alpha$330 (Isella .,
2013) but not always recognized as such due to low SNR, image fidelity and
unknown stellar position.

The first significant asymmetric dust rings were discovered in ALMA images of
Oph IRS48 and HD142527 (van der Marel ., 2013; Casassus ., 2013), with
azimuthal contrasts of a factor 130 and 30, respectively, on radial scales of
50-100 au. Also AB Aur and LkH$\alpha$330 show a highly asymmetric dust ring
(Tang ., 2017; van der Marel, Birnstiel ., 2021; Pinilla ., 2022). Shallower
dust asymmetries with a contrast of a factor 1.5-2 were found in SR 21 and
HD135344B (L.M. Pérez ., 2014; Pinilla ., 2014), although in higher resolution
observations it turned out that these actually consisted of multiple dust
rings, some asymmetric, with higher contrasts (van der Marel, Cazzoletti .,
2016; Cazzoletti ., 2018; Muro-Arena ., 2020), as well as V1247 Ori, located
at 400 pc (S. Kraus ., 2017). These strong asymmetric features have been
interpreted as dust trapping in either long-lived vortices as the result of
the Rossby Wave Instability of a radial pressure bump (Barge Sommeria, 1995;
Birnstiel ., 2013; Zhu Stone, 2014) or in gas horseshoes due to a pile-up of
material in eccentric cavities in a circumbinary disk (Ragusa ., 2017; Miranda
., 2017; Price ., 2018). The ultimate distinction between these two mechanisms
is the $\alpha$-viscosity and the companion mass: vortices are only long-lived
when $\alpha_{t}\lesssim 10^{-4}$ but can be triggered by Jovian companions,
whereas the gas horseshoe is also formed at moderate $\alpha_{t}\sim 10^{-2}$
and requires a (sub)stellar companion with a mass ratio of at least 0.05
w.r.t. the host star (van der Marel, Birnstiel ., 2021). As current limits on
both the $\alpha$-viscosity and companions are inconclusive in most asymmetric
disks, the dominating mechanism remains unknown.

Figure 6: Comparison between asymmetric dust features seen with ALMA and the
spiral features seen in scattered light, data from (van der Marel, Birnstiel
., 2021). The top row shows large scale asymmetries which appear to be
unrelated, whereas the bottom row show striking similarities in the
morphologies, indicating that the asymmetry seen in millimeter wavelengths may
be part of the spiral arm itself (van der Marel, Birnstiel ., 2021).

Clumpy asymmetric features have been found in e.g. HD34282 (van der Plas .,
2017), HD143006 (Andrews, Huang ., 2018), TW Hya (Tsukagoshi ., 2019) and
MWC758 (Boehler ., 2018; Dong ., 2018), and spiral-like features in HD100453
(Rosotti ., 2019) and HD100546 (Norfolk ., 2022), suggesting that some
asymmetries could also be part of the spiral arms seen in scattered light
images of the disk. Figure 6 shows a couple of examples of asymmetries and
spiral arms seen in scattered light. Alternatively, it has been proposed that
the underlying vortex triggers the spiral density wave itself (Lovelace
Romanova, 2014; van der Marel, Cazzoletti ., 2016; Cazzoletti ., 2018),
although this has been debated by simulations (P. Huang ., 2019). More
tentative asymmetric features have been identified in CQ Tau (Ubeira Gabellini
., 2019), DM Tau (Hashimoto, Muto ., 2021) and ZZ Tau IRS (Hashimoto, Dong
Muto, 2021).

Asymmetric features can be identified in both the image plane and the
visibility plane: in the latter case, an asymmetric feature is apparent from
non-zero emission in the imaginary visibility curve, assuming that the data is
perfectly phase-centered on the center of the disk ring, properly deprojected
and intrinsically flat. An incorrect phase center can result in the apparent
presence of asymmetries, e.g. comparison of Pineda . (2014); Walsh . (2014).
Visibility modeling of an asymmetric disk can be performed with e.g. galario
(Tazzari ., 2018), providing a two-dimensional intensity profile rather than
the default one-dimensional radial profile, as successfully demonstrated by
e.g. (Cazzoletti ., 2018; van der Marel, Birnstiel ., 2021). Care should be
taken when fitting multiple components when they are not directly visible in
the image: Pinilla . (2014) found a best fit for a ring+inner vortex model to
ALMA visibility data of HD135344B, but re-imaging of the data revealed that
the vortex was actually located outside of the ring (van der Marel, Cazzoletti
., 2016).

It has been proposed that the appearance of dust asymmetries is related to the
local gas surface density, or more precise, the Stokes number of the emitting
millimeter grains (van der Marel, Birnstiel ., 2021). This implies that
asymmetric features may be more common at longer wavelengths, which can be
tested with future facilities such as the ngVLA and ALMA Band 1 receivers.
Another reason that more asymmetries may be detected at longer wavelengths is
because the emission at (sub)millimeter wavelengths is generally optically
thick (Andrews, Wilner ., 2018). Hints for asymmetric features at centimeter
wavelengths in disks that are axisymmetric in millimeter wavelengths are seen
in e.g. LkCa 15, SR 24S and J1604.3-2130 (Isella ., 2014; Norfolk ., 2021),
but a higher image fidelity is required to confirm this phenomenon.

Overall, the detection rate of asymmetric features in protoplanetary and
transition disks remains a matter of sensitivity, spatial resolution and
optical depth, and it is very likely that more disks turn out to be (somewhat)
asymmetric in nature. Higher resolution observations generally lead to higher
azimuthal contrast as dust rings remain radially unresolved at $>$0.1"
resolution. Large scale asymmetries such as seen in Oph IRS48 and HD142527 are
unlikely to remain undetected in the disk surveys, so at least for that high-
contrast asymmetry, the occurrence rate remains fairly low with less than 10%
of the known transition disks with large inner cavities ($>$20 au). Clearly,
(almost) axisymmetric dust rings dominate the dust morphology population at
large scales. Higher resolution observations of smaller disks are essential to
confirm this trend.

###  2.4 Gas in cavities

One of the key questions in transition disk studies is how much gas is still
present inside the cavity, and what its surface density profile looks like.
The gas is directly tracing the dynamics of the disk and thus may reveal the
mechanism that is responsible for the clearing of the cavity. The presence of
gas inside the cavity is evident from the observed high accretion rates in
transition disks (e.g. Najita ., 2007; Manara ., 2014), indicating that
whichever clearing mechanism is responsible for the cavity, it allows material
to be transported across the gap. Also kinematic analysis of CO rovibrational
line emission indicates the presence of gas well inside the dust cavity of a
small number of disks where the inner radius could be determined (e.g.
Pontoppidan ., 2008; Brown, Herczeg ., 2012; Banzatti ., 2018, and see Section
3.3).

Figure 7: Typical parametrized surface density profile of gas and dust in
thermal-chemical modeling of transition disks. The gas cavity is generally
smaller and less depleted than the dust cavity, whereas an inner dust disk is
generally included to reproduce near infrared emission, if present. Figure
based on van der Marel . (2018).

The relative gas surface density profile inside the dust cavity can be
determined using ALMA CO isotopologue data, with sufficient resolution to
resolve the cavity by at least one beam size. A combination of optically thick
and thin emission, i.e. multiple isotopologues, is crucial for an accurate
determination of the gas structure. Early ALMA observations revealed deep CO
cavities in e.g. Oph IRS48 (van der Marel ., 2013) and J1604.3-2130 (Zhang .,
2014) which were a factor 3 smaller than the dust cavity radius, where the
density dropped below the threshold of optically thick 12CO, at approximately
10-2 g cm-2. Considering self-shielding, it is unlikely that the CO itself
(rather than H2) is depleted in the inner part of the disk and the observed
profiles thus represent an actual surface density drop (Bruderer, 2013). Other
transition disks, such as HD142527, SR21 and HD135344B did not show a cavity
in the 12CO emission (Casassus ., 2013; L.M. Pérez ., 2014; van der Marel, van
Dishoeck ., 2015), implying that the cavity was either unresolved or the gas
surface density remained above the optically thick threshold.

Thermo-chemical modeling is required to quantify the gas surface density
inside the dust cavity from the spatially resolved CO emission, for multiple
reasons (Bruderer, 2013): the CO abundance is not constant w.r.t. H2
throughout the disk due to freeze-out and photodissociation, although the
latter can be diminished by self-shielding and dust shielding, in particular
inside a dust cavity with an inner dust disk; the gas temperature is decoupled
from the dust temperature, especially at the cavity edge and above the
midplane, where optically thick lines originate; chemical reactions that lead
to formation and destruction of CO vary throughout the disk depending on the
local conditions. Dedicated modeling codes such as DALI and ProDiMo (Bruderer
., 2012; Woitke ., 2009) can be used to calculate the expected emission cubes
of targeted molecular lines based on a given density structure and radiation
field.

Spatially resolved images of CO isotopologue emission (13CO and C18O) revealed
deep gaps inside the cavity of several transition disks, which was quantified
through both thermo-chemical modeling (Bruderer ., 2014; van der Marel, van
Dishoeck ., 2016; van der Marel ., 2018) as well as simpler, parameterized
radiative transfer modeling (L.M. Pérez ., 2015; Boehler ., 2017, 2018). The
model fitting procedure assumed a power-law surface density profile with drops
in surface density at different radii for the gas and the dust, respectively
(see Figure 7). It was found that the gas surface density dropped by several
orders of magnitude inside the cavity, which points towards the presence of
super-Jovian planets (Fung Chiang, 2016; van der Marel, van Dishoeck ., 2016).
However, this parametrized approach is physically unrealistic as the inward
extrapolated gas surface density at 1 au was well below the expected value
from the accretion rate, and because the gas surface density is not expected
to drop steeply as the result of planet-gap clearing (Pinilla, Benisty
Birnstiel, 2012).

Later studies revealed that the three main CO isotopologues 12CO, 13CO and
C18O show increasing gap sizes, indicating a gradual dropoff where each
isotopologue becomes optically thick at a different radius (seen e.g. in HD
169142, Fedele ., 2017), which was fit with a gradual slope for J1604.3-2130
and CQ Tau (Dong ., 2017; Ubeira Gabellini ., 2019). Generally, the 13CO
cavity radii are about a factor 2 smaller than the dust cavity radii (van der
Marel, Birnstiel ., 2021). Planet-disk interaction models were implemented
directly in the comparison with the observed CO profiles for PDS70 (Muley .,
2019) and in generic model predictions (Facchini, Pinilla ., 2018). The latter
study showed that stronger central 13CO emission is expected as the result of
planet clearing, and that this cannot only be explained by lack of spatial
resolution, as previously claimed (van der Marel ., 2018). More recent
observations fully resolve the cavity even in the CO emission, revealing some
central emission in both 12CO and HCO+ (e.g. Mayama ., 2018; Z.C. Long .,
2018; Tsukagoshi ., 2019) which allows further exploration of the link with
the high accretion rate, such as recently done for DM Tau (Hashimoto, Muto .,
2021; Francis ., 2022). CO observations of multiple line transitions are
required to fully unravel the gas temperature structure inside the cavity for
a proper interpretation of the gas surface density profile (Leemker ., 2022).
Also, for TW Hya, the gas surface density profile inside the cavity could be
determined from the direct measurement of pressure broadening in CO line wings
(Yoshida ., 2022).

Another aspect of gas studies in transition disks is the study of the
kinematics and the brightness temperature of 12CO, which have revealed spiral
signatures comparable to those seen in scattered light (e.g. Casassus ., 2021;
Wölfer ., 2021, 2022). Also warps are commonly seen in the velocity maps of
transition disks (Casassus ., 2013; Boehler ., 2017; Loomis ., 2017; Mayama .,
2018; van der Marel, Birnstiel ., 2021; Wölfer ., 2022) which are generally
interpreted as misalignment between the inner and outer disk as the result of
a companion (e.g. Rosenfeld ., 2014; Facchini, Juhász Lodato, 2018; Zhu,
2019), also revealed by azimuthally moving shadows (Pinilla, Benisty ., 2018;
Nealon ., 2020). Other kinematic signatures such as kinks, meridional flows
and circumplanetary disks (Teague ., 2018; Pinte ., 2018, 2019; S. Pérez .,
2019; Izquierdo ., 2021) are harder to detect in transition disk cavities due
to the drop in CO emission, and such signatures may only be found in gapped
disks where the CO emission is less depleted.

###  2.5 Multi-wavelength data

Multi-wavelength studies at millimeter and centimeter wavelengths (the latter
with e.g. VLA or ATCA) are considered a powerful tool to learn more about the
grain sizes in the disk, through the spectral index $\alpha_{mm}$ where values
between 2 and 3 are generally interpreted as evidence for grain growth (in
contrast with the $\alpha_{mm}$ ISM value of 3.7), although high optical depth
could also result in lower $\alpha_{mm}$ (Testi ., 2014). Disk-integrated
spectral indices of a sample of transition disks were found to be higher than
for regular disks, which can be understood as transition disks lack the inner
emission which is often very optically thick (Pinilla ., 2014). Spatially
resolved multi-wavelength analysis of transition disk dust rings shows that
the dust ring is narrower, and the peak is often slightly further out (Pinilla
., 2014, 20172; Cazzoletti ., 2018; Macías ., 2019; J. Huang ., 2020; Maucó .,
2021), although this can not always be confirmed, in particular with low
resolution ATCA images (Norfolk ., 2021), and in a few cases the dust ring is
even closer in at longer wavelengths (Macías ., 2018; Pinilla ., 2021),
although this could also be caused by contributions from the central emission
not included in the fit. Generally these data are consistent with predictions
from dust evolution models (Pinilla, Tazzari ., 2018).

Azimuthal asymmetries are in practice easier to study and compare in multi-
wavelength observations than axisymmetric disks, as variations can be measured
at larger scales and thus lower spatial resolution. Dust evolution models
predict that the azimuthal extent decreases with wavelength in azimuthal dust
traps (Birnstiel ., 2013), which was indeed measured in Oph IRS 48, HD142527
and HD135344B (van der Marel, Pinilla ., 2015; Casassus ., 2015; Cazzoletti .,
2018), while this remains inconclusive for SR21, MWC758 and AB Aur (Pinilla .,
2014; Muro-Arena ., 2020; Marino ., 2015; Fuente ., 2017).

At longer wavelengths, free-free emission originating from ionized gas close
to the star (Snell Bally, 1986) will start to contribute to the total flux as
well. This emission generally starts to dominate over the thermal dust
emission below 30 GHz, and $\alpha_{mm}$ generally ranges from 0 to 1 in this
regime (Rodmann ., 2006). Several transition disks have been observed in the
10-30 GHz range revealing that the spectral index indeed flattens, consistent
with free-free emission (Ubach ., 2012; Zapata ., 2017), although the emission
can be highly variable (Ubach ., 2017). In two cases, AB Aur and GM Aur, the
emission is sufficiently resolved to distinguish a jet morphology rather than
a disk (Rodríguez ., 2014; Macías ., 2016), but in most cases the origin of
the emission (thermal or free-free) remains unknown by lack of resolution. In
some cases, in particular due the azimuthal asymmetries, the central emission
can be spatially distinguished in the centimeter emission (van der Marel,
Pinilla ., 2015; Marino ., 2015; Casassus ., 2015), and also ALMA images
generally resolve the central emission inside dust cavities (Francis van der
Marel, 2020), which opens up new possibilities for the study of the role of
free-free emission in transition disks.

##  3 The infrared perspective

###  3.1 Infrared imaging

Transition disks are a prime target of interest of near infrared imaging. At
these wavelengths, observations probe the light scattered off by the surface
layer of the disk and is thus tracing an intrinsically different regime than
the millimeter images. Since the stellar emission dominates in the near-IR,
observations are carried out with adaptive optics-aided instruments such as
VLT/NACO, VLT/SPHERE, Gemini/GPI and Subaru/HICIAO and differential imaging
techniques such as the Polarization Differential Imaging (PDI) that probes the
polarized fraction of the scattered light.

Transition disks have been included in targeted surveys such as SEEDS (Uyama
., 2017), Gemini-LIGHTS (Rich ., 2022), DARTTS-S (Avenhaus ., 2018; Garufi .,
2020), and other SPHERE surveys (Garufi ., 2017, 2018). The reason for the
high occurrence of transition disks in these surveys is two-fold: first,
transition disks have always been chosen as key targets to look for wide-orbit
companions due to their inner cavities and second, transition disks tend to be
brighter in infrared wavelengths due to the irradiated cavity and due to being
more common around the more luminous Herbig stars than T Tauri stars (van der
Marel Mulders, 2021) which makes them more suitable for AO correction with an
optical wavefront sensor, although almost the entire parameter space of
stellar mass and age has been covered in near infrared imaging (see Fig. 3 in
Benisty ., 2022).

For a full overview and discussion of the techniques and observed morphologies
of near infrared imaging I refer to the PPVII chapter by Benisty (Benisty .,
2022), I will merely highlight a few aspects here. Companions will be
discussed in section 5.2.1. The near infrared images have revealed a wealth of
substructures, such as spiral arms, gaps, rings, cavities and broad and narrow
shadows as the result of misalignments, and in a few cases, companions and
companion candidates, as discussed in the review chapter. The emission is
usually located closer in than the millimeter dust ring, consistent with
predictions from dust trapping models (de Juan Ovelar ., 2013). Most
substructures are linked to the presence of companions in the disk, even when
those companions often remain undetected (Benisty ., 2022). In particular
spiral arms have received a lot of attention in connection with deriving
companion properties in transition disk cavities (Dong ., 2015; Fung Dong,
2015; Zhu ., 2015) but also wide outer companions. Alternatively, they have
been connected with the morphologies of millimeter dust asymmetries in
transition disks (see discussion in e.g. Garufi ., 2018; van der Marel,
Birnstiel ., 2021), but are not uniquely found in cavities alone. A possible
explanation for their high occurrence in transition disks was proposed by (van
der Marel, Birnstiel ., 2021) who noticed that spirals were predominantly
found in disks with larger gaps and more luminous stars, suggesting that
spiral arms are more easily detected for higher mass planets and warmer disks.

Mid-infrared thermal emission can also be used to trace small grains in disks.
Spatially resolving disks in infrared wavelengths requires ground-based
10m-class telescopes, and thus exceptional weather conditions to observe in
the infrared, which means that such images are relatively rare. Oph IRS48 was
imaged with VLT/VISIR, where a dust ring was spatially resolved at 19 $\mu$m,
(Geers ., 2007). Images with Subaru/COMICS revealed additional east-west
asymmetries in the Oph IRS48 disk (Honda ., 2018). In sample studies of Herbig
stars it was shown that mid infrared images of Group I disks are more often
extended, i.e. warmer by direct illumination due to the presence of an inner
hole, confirming that Group I Herbigs are transition disks with inner cavities
whereas Group II disks generally do not have a cavity (Maaskant ., 2013; Honda
., 2015), indicating that there is no evolutionary transition from Group I to
II disks (Garufi ., 2017, and see Section 4.2).

###  3.2 Near infrared interferometry

Transition disks with near infrared excess have been targeted in near infrared
interferometry studies in H and K band to gain more insights on their inner
dust disks. Early infrared interferometry with e.g. VLTI/AMBER, VLTI/MIDI and
the Keck Interferometer revealed gap-like structures as well as dust
composition in e.g. HD100546, T Cha and V1247 Ori, as well as HD179218 and
HD139614 which have so far not been resolved with millimeter interferometry
(Benisty ., 2010; Mulders ., 2013; Olofsson ., 2013; S. Kraus ., 2013; Matter
., 2016; Kluska ., 2018). A sample study with MIDI (Menu ., 2015) showed
evidence for dust gaps in a large number of transition disk candidates (Group
I disks, see Section 4.2).

More recently, larger samples of Herbig stars were studied with VLTI/PIONIER
and VLTI/GRAVITY including several transition disks down to scales of a few
tenths of an AU (Lazareff ., 2017; Gravity Collaboration ., 2019; Kluska .,
2020). Image reconstruction showed that inner disks contain both axisymmetric
and non-axisymmetric structures, and that the inner rim location scales with
the stellar luminosity. A targeted transition disk study with GRAVITY
(including some of the brighter T Tauri stars) confirmed the misalignment of
several of the inner dust disks with respect to the outer disk as traced by
ALMA line observations, where the shadows seen in scattered light could lift
the degeneracy in inclination angle (Bohn ., 2022).

Whereas near infrared interferometry is only sensitive to the inner rim of the
inner disk, the MATISSE instrument which operates between L and N band, will
be able to trace the dust emission out to larger radii of several au, which
will provide further insights in the inner dust disk structure.

###  3.3 Infrared line emission

Rovibrational line emission in the infrared traces the hot and warm gas at
radii within a few au of the host star, which is a valuable tracer of the
physical processes inside transition disk cavities even when spatially
unresolved. Kinematic analysis of rovibrational CO line profiles indicate the
presence of gas in the inner disk (Najita ., 2008), although many line
profiles are single-peaked and thus more consistent with disk winds rather
than Keplerian disk rotation (Salyk ., 2007, 2009; Najita ., 2009; Bast .,
2011). For some of the transition disks, the radial extent of the CO could be
determined using spectro-astrometry with VLT/CRIRES, for example in SR 21,
HD135344B, Oph IRS48, HD100546 and HD139614 (Pontoppidan ., 2008; Brown,
Herczeg ., 2012; Brittain ., 2009, 2013; Carmona ., 2014, 2017), showing that
the gas cavity radius is well inside the dust cavity radius. A broader sample
study shows that transition disks rarely have a hot CO component in
rovibrational line emission in comparison with full disks, indicating a
removal of inner gas (Banzatti Pontoppidan, 2015). Furthermore, Banzatti .
(2018) find a link between the amount of refractory elements, near infrared
excess and inner radius of the CO in a sample of Herbig transition disks,
which is interpreted as full depletion of the gas inner disk in the presence
of a planet and a dichotomy in the amount of near infrared excess. However,
such a dichotomy was not seen in the millimeter dust mass of the inner disks
in these transition disks (Francis van der Marel, 2020), challenging this
interpretation.

Mid infrared spectra with e.g. _Spitzer_ generally lack strong molecular line
emission in transition disks that is seen in full disks around T Tauri stars
(Najita ., 2010; Pontoppidan ., 2010). In particular, the H2O emission is
found to be anti-correlated with disk dust size for a large sample of disks,
which can be understood as icy pebbles are unable to drift inwards in the
presence of dust traps, such as those found in transition disks (Banzatti .,
2020; Kalyaan ., 2021). Also for TW Hya it has been suggested that the
presence of dust traps is the main reason for the low elemental abundances of
carbon and oxygen, considering infrared observations of CO and H2O (Bosman
Banzatti, 2019). Dust transport and trapping may play a crucial role in
setting the inner disk composition as traced by infrared emission, and
transition disks are clear laboratories for quantifying these mechanisms, in
particular in the coming years with the arrival of infrared spectra of inner
disks with the _James Webb Space Telescope_.

##  4 The SED perspective

###  4.1 SED of a transition disk

As transition disks were originally identified through a deficit in their SED
and millimeter interferometric imaging was not available until the 2000-2010s,
many early efforts in transition disk studies were entirely based on the
radiative transfer modeling of SEDs to derive properties of their dust
structure based on spatially unresolved photometry (see also the historic
overview by Espaillat ., 2014). _Spitzer_ played a crucial role here,
providing mid and far infrared photometry of thousands of YSOs in nearby star
forming regions, and targeted low-res spectroscopy with the IRS instrument in
the 5-35 $\mu$m regime, where the deficit is most easily recognized.
Unfortunately _Spitzer_ IRS spectra have only been taken for a limited number
of targets so a uniform study is not possible 222The reduced _Spitzer_ IRS
spectra are available online (Lebouteiller ., 2011).. Transition disks were
historically identified by comparison of the SED with the so-called ‘median’
SED of full disks in Taurus, the best studied star forming region at the time.
SEDs of transition disks usually show no excess emission in the near and mid
infrared (1-5 and 5-20 $\mu$m) above the stellar photosphere, and excess at
the level of the median Taurus disk at wavelengths $>$20 $\mu$m (Calvet .,
2005), indicating a lack of hot and warm dust close to the host star. As 80%
of the emission at 10 $\mu$m originates from within 1 au (D’Alessio ., 2006),
the mid infrared spectrum is particularly sensitive to small cavities, and
several transition disks that were imaged with millimeter interferometry
turned out to have much larger cavities than originally thought based on the
SED (e.g. Brown ., 2009; Calvet ., 2002; Andrews ., 2011). Some SEDs also show
excess at 1-5 $\mu$m, which are sometimes called pre-transition disks in the
literature (Espaillat ., 2007), but there are some issues with this
subclassification that will be discussed in the next section.

SEDs can be modelled using radiative transfer codes such as RADMC or MCFOST,
assuming an inwardly truncated optically thick disk, where the cavity
edge/wall of the outer disk is frontally illuminated by the star, which
dominates most of the emission at 20-30 $\mu$m (Brown ., 2007). The SED can
constrain the vertical and radial structure of the small dust grains, but due
to the illumination and shadowing of the inner disk, if present, the cavity
size parameters are degenerate (Dullemond Natta, 2003). The SED has also been
used to constrain the amount of settling of large grains in the disk
(Dullemond Dominik, 2004; L. Cieza ., 2007; Grant ., 2018). The total dust
mass is only reliable when millimeter fluxes are included. Other properties,
such as inclination and extinction, may also affect the outcome of the SED
modeling (Merín ., 2010). Finally, the 10 $\mu$m silicate feature is sometimes
used as indicator of small, optically thin dust inside the cavity (Espaillat
., 2007). With the advance of spatially resolved images of dust and gas, SEDs
are often fit together with the images for better constraints on the structure
(e.g. Andrews ., 2011; van der Marel, van Dishoeck ., 2016).

Transition disk candidates have been identified from SED photometry using
various color criteria indicating steep slopes in the infrared part of the SED
(e.g. Brown ., 2007; Furlan ., 2009; L.A. Cieza ., 2010; Merín ., 2010), that
were easy to calculate for the thousands of YSOs in _Spitzer_ photometric
catalogs of nearby star forming regions (e.g. Merín ., 2008; Luhman ., 2008;
Sicilia-Aguilar ., 2009). Dedicated surveys of transition disk candidates
using color criteria and subsequent SED analysis in the _Spitzer_ c2d and
Gould Belt catalogs resulted in many new candidates (L.A. Cieza ., 2010, 2012;
Romero ., 2012; van der Marel, Verhaar ., 2016), as well as contributions from
_Herschel_ photometry (Ribas ., 2013; Bustamante ., 2015; Rebollido ., 2015),
where van der Marel, Verhaar . (2016) is still the most complete catalog of
transition disk candidates to date, including estimates of cavity sizes and
comparison of various criteria.

###  4.2 Herbig disks

The study of Herbig disks and their SEDs has evolved almost completely
separate from the aforementioned SED studies around the later type T Tauri
stars which are much more common in the IMF. Herbigs are typically defined as
B, A and F type young stars and have been identified both isolated and in
clusters across their sky (for a full overview, see e.g. Vioque ., 2018). The
NIR images and the infrared color ($F_{30{\mu}m}/F_{13{\mu}m}$) divide the
Herbigs into two groups: extended, bright disks with spiral arms and high
$F_{30{\mu}m}/F_{13{\mu}m}$ value are called Group I, whereas faint, flat and
small disks with low $F_{30{\mu}m}/F_{13{\mu}m}$ value are called Group II
(Meeus ., 2001; Garufi ., 2017). Historically the two groups were considered
to follow an evolutionary sequence from flared (Group I) to flattened (Group
II) (Dullemond Dominik, 2004). Millimeter images of Group I disks all show
large inner dust cavities and massive outer dust disks (e.g. Garufi ., 2018),
whereas Group II disks are faint and appear to be compact in millimeter
emission (Stapper ., 2022). As the distinction appears to be about the
presence or absence of an inner cavity, the groups are recently considered as
following two separate evolutionary pathways (Maaskant ., 2013; Garufi .,
2018). It is evident that almost all Group I disks are essentially transition
disks, and they can be considered as the higher stellar mass
($M_{*}>1.5M_{\odot}$) equivalent of T Tauri stars with transition disks.

###  4.3 Near infrared excess

Near infrared excess in SEDs of transition disks is traditionally associated
with the presence of an inner dust disk with dust down to the sublimation
radius and a dust ‘gap’, where a lack of such excess indicates a fully cleared
cavity. A gapped disk is also called a pre-transition disk, as it was assumed
that the inner dust disk would disappear over time as the companion clears out
a wider gap and this is an evolutionary sequence (Espaillat ., 2007). However,
there are several issues with this interpretation: 1) The near infrared excess
is primarily sensitive to the optically thick inner wall and thus not
sensitive to the _amount_ of dust in the inner disk; 2) There is no
correlation between the presence of the inner dust disk at millimeter
wavelengths and the near infrared excess (Francis van der Marel, 2020); 3)
Some transition disks without near infrared excess have an inner dust ring
that is just outside the sublimation radius, e.g. DM Tau and TW Hya while the
companion responsible for the cavity is likely located further out (Andrews .,
2016; Hashimoto, Muto ., 2021; Francis van der Marel, 2020); 4) There is no
clear difference between accretion rates of transition disks and pre-
transition disks (Espaillat ., 2014); 5) The near infrared excess depends on
the inclination of the inner disk, such as demonstrated for e.g. RY Lup (van
der Marel ., 2018); 6) The near infrared excess can be variable with time, as
seen in a comparison of _Spitzer_ and _WISE_ photometry in J1604.3-2130 (Zhang
., 2014). Therefore, in this review the term pre-transition disk is not
further used.

Some transition disks show ‘seesaw variability’ in multi-epoch infrared
observations, i.e. a decrease in near infrared flux is accompanied with an
increase at longer wavelengths (Muzerolle ., 2009; Espaillat ., 2011; Flaherty
., 2012), where the flux changes by 10-50% over timescales of a few years.
This variability is interpreted as a change in the structure of the inner
disk, e.g. a change of the height of the inner disk with time, although the
underlying physical mechanism remains unclear (Espaillat ., 2014).
Unfortunately this type of study was only possible by combining the _Spitzer_
IRS spectra taken in multiple cycles, which has only been done for a small
number of targets. When comparing the seesaw sources from the variability
study by Espaillat . (2011) with spatially resolved ALMA observations (Francis
van der Marel, 2020), there is no consistent picture: both seesaw and non-
seesaw disks have a variety of inner disk dust masses or turn out not to have
inner cavities at all (e.g. WW Cha and CR Cha, Ribas ., 2013; Kanagawa .,
2021; Kim ., 2020). Understanding this variability behaviour thus requires
significantly more data and targets.

Another type of variability is seen in optical emission of so-called ‘dippers’
which can dim by a few tens of percent on timescales of approximately one day
(e.g. Alencar ., 2010; Cody ., 2014). This behaviour is thought to be caused
by occultation of circumstellar dust, in particular edge-on disks (Bouvier .,
1999). However, ALMA observations have revealed that dipper stars have a wide
range of inclinations, even completely face-on disks such as J1604.3-2130
(Ansdell ., 2016; Sicilia-Aguilar ., 2020), suggesting that misalignments
between inner and outer disks or warps may be common (Ansdell ., 2020), which
supported by numerous other studies of transition disks (e.g. Francis van der
Marel, 2020; Bohn ., 2022; Wölfer ., 2022). Due to the heterogeneous studies
of transition disks, it is not possible to assess the commonality of warps
statistically. Dippers are not restricted to transition disks only and thus
could be related to a range of phenomena (Ansdell ., 2020).

###  4.4 Evolved disks

Some SEDs of protoplanetary disks show decreased emission at all wavelengths
rather than a decrease only up to 20 $\mu$m like in transition disks. Such
disks are called ‘anemic’ (Lada ., 2006), ‘homologously depleted’ (Currie
Sicilia-Aguilar, 2011), ‘evolved’ (Hernández ., 2007) or ‘weak excess
transition’ (Muzerolle ., 2010) and are sometimes grouped together with
transition disks as evolved disks when calculating disk fractions and
lifetimes (e.g. Muzerolle ., 2010; Ribas ., 2015) whereas they may represent
completely different evolutionary processes in the disk. Such disks often also
classify as Class III disks in the Lada classification which are known to be
often mistaken in cluster membership in pre-Gaia studies (Oliveira ., 2009)
and are only sparsely studied with ALMA (Hardy ., 2015; Lovell ., 2021; Michel
., 2021). A systematic reassessment of these evolved Class III disks in both
membership and dust mass would be highly beneficial for our general
understanding of disk evolution, as well as their comparison with transition
disks to determine if disks evolve along separate evolutionary pathways.

###  4.5 Comparison between SEDs and millimeter imaging

A catalog of SED-derived transition disk candidates in van der Marel, Verhaar
. (2016) is constructed using a combination of photometric color criteria with
_Spitzer_ data and additional candidates from the literature. The main color
criteria were based on the _Spitzer_ photometry: 0$<$[3.6]-[8.0]$<$1.8 and
3.2$<$[8.0]-[24.0]$<$5.3 (Merín ., 2010). It thus provides the most complete
sample of transition disk candidates. In order to investigate the correlation
between SED selection and actually resolved millimeter cavities (and thus the
completeness of the current known transition disks with cavities in the
millimeter continuum) I have constructed a list of all transition disk
candidates from this work within 200 pc with an estimated cavity size of 15 au
or larger in Table 1. For the candidates with cavity sizes $<$15 au such an
assessment cannot be done as most disks have not been resolved at sufficient
spatial resolution. A handful of targets were excluded because they are
actually Class III, the _Spitzer_ photometry is contaminated (Ribas ., 2013)
or because they turn out not to be cluster members according to Gaia DR2
catalogs (Galli ., 2019). In addition, I have included all Group I Herbig
disks within 200 pc that were not already included from van Boekel . (2005) as
the study by van der Marel, Verhaar . (2016) was not complete w.r.t. Herbig
stars.

Figure 8: Three examples of SEDs of transition disks with large inner
cavities. A stellar spectrum is shown in dashed grey to show the amount of
infrared excess.

Table 1 contains 53 transition disk candidates, for which 34 have confirmed
millimeter cavities with sizes ranging from 11 to 185 au (32 disks have a
confirmed cavity size $>$15 au). For 13 disks there is no (sufficiently
resolved) millimeter data available to make an assessment about their cavity
size, for 6 disks a cavity $>$20 au can be excluded. In one case no dust
emission was detected at all (V410X-ray6), one disk was very compact (CIDA 7),
one disk was highly inclined (MY Lup) and the other 3 disks are located in
Ophiuchus where the SED suffers from high extinction which affects the SED
modeling (Merín ., 2010). Excluding the 13 disks with insufficient data, this
implies that 34/40 = 85% of the SED selected transition disk candidates with
$\gtrsim$15 au cavity indeed show a millimeter cavity. This could be as high
as 47/53 = 89% if the 13 disks without sufficient data will show a cavity as
well, and as high as 34/36 = 94% when highly inclined and highly extincted
sources are excluded.

Second, Table 2 contains a list of all published transition disks with
cavities $>$15 au discovered from millimeter imaging that were not recognized
as candidates by van der Marel, Verhaar . (2016) and references therein. This
is a total of 22 transition disks. Perhaps surprisingly, the famous PDS 70 and
AA Tau transition disks are part of this list as well. Inspection of their
SEDs and their classification in _Spitzer_ papers indicates that the majority
of these transition disks do not show a clear deficit in their SED, which is
the reason why they were not identified from color criteria. A handful of
targets were not (or incompletely) measured by _Spitzer_ so color criteria
could not have been applied. The reason for a lack of deficit could be caused
by for example a misaligned optically thick inner disk such as seen in RY Lup
(van der Marel ., 2018) and a handful of others (van der Marel ., 2022), but
it is also possible that the $\mu$m-sized grains are not as depleted as the
mm-sized grains in the cavity to decrease the infrared excess, which is often
seen in the comparison between scattered light and millimeter cavities
(Villenave ., 2019). Confusion with envelope emission or high inclination
could also affect the deficit in the SED.

Interestingly, the spectral type appears to play a role in the overlap between
SED deficit and resolved millimeter cavity. A total of 56 disks have a cavity
$\gtrsim$15 au, where the 34 disks that show both a millimeter cavity and a
SED deficit are dominated by early type stars with only a quarter of the
sample having spectral type K5 or later. The 22 disks that show a millimeter
cavity but no SED deficit are dominated by late type stars (K5 - M5), where
the only early type stars in this sample have highly inclined disks.
Essentially, the probability of missing a transition disk with a cavity $>$15
au based on their SED is 0% for A-type stars, which increases to 63% for
M-type stars considering the number of M stars with a millimeter cavity
without SED deficit (12) compared to the total number of M stars with a
millimeter cavity (17). As argued before, the current ALMA snapshot surveys
likely exclude additional transition disks with large cavities, except for the
regions studied at low or non-uniform resolution, Chamaeleon and Taurus. This
means that these statistics should be taken with caution as additional
cavities may still be discovered in these regions.

For smaller cavities a statistical study is not yet possible, as only a
handful of transition disk candidates with (predicted) small inner cavities
have been imaged at sufficiently high spatial resolution with ALMA: CX Tau
(Facchini ., 2019) and 6 transition disk candidates in Lupus (van der Marel .,
2022), but all but one of these disks do not show a small inner cavity but
instead, a highly inclined or a very compact dust disk, both of which can
explain the small deficit in the SED as well (Birnstiel ., 2012; van der Marel
., 2022). Small inner cavities of 1-2 au have been discovered by coincidence
in high-resolution ALMA images of XZ Tau B (Osorio ., 2016) and ISO-Oph2B
(González-Ruilova ., 2020) without deficits in their SED, but as these disks
are in multiple systems, the near infrared photometry is likely contaminated
by the companions. A handful of $\sim$5-10 au cavities have been resolved as
well, see Table 4.

The analysis above demonstrates the shortcomings of SED analysis in
identifying transition disks: even though SED identified transition disk
candidates have a high likelihood to be confirmed with millimeter imaging (for
large cavities), a large number of additional transition disks with large
cavities have been discovered without SED indicator, which appears to be even
more common for late type stars than for early type stars. For smaller
cavities ($<$15 au) this issues appears to be even more severe, but only very
small sample statistics are available at the moment. This realization is
particularly important for studies that use the fraction of SED identified
transition disks to derive parameters related to the underlying mechanism,
such as time scales of photoevaporative clearing (Ercolano Pascucci, 2017) and
impact of external vs internal photoevaporation (Coleman Haworth, 2022).
Essentially it is not recommended to use SED-derived statistics for such
comparisons, and more complete high-resolution millimeter studies are required
to investigate the overall presence of transition disk cavities, large and
small.

##  5 The role of transition disks

###  5.1 Transition disk demographics

All resolved transition disks with cavity size $>$15 au within 200 pc are
listed in Table 3 with their main disk and stellar properties using the
parallax information from Gaia DR2 (Gaia Collaboration ., 2018). Ages are
taken as the average cluster age for T Tauri stars (Michel .,
2021)(1.5$\pm$0.5 Myr for Ophiuchus and Taurus, 2$\pm$0.5 Myr for Chamaeleon,
2.5$\pm$0.5 Myr for Lupus, 5$\pm$0.5 Myr for CrA, 10$\pm$3 Myr for Upper Sco)
and individually derived ages for the Herbig stars (Vioque ., 2018). These
ages are somewhat higher than the average cluster age. All dust masses are
calculated using the default assumptions regarding the millimeter flux (Manara
., 2022). Cavity radii and spectral indices $\alpha_{\rm mm}$ are taken from
the literature (e.g. Pinilla ., 2014; Tazzari ., 2021) using a frequency range
between 100 and 345 GHz (ALMA Band 3/4 in combination with Band 6/7). As
argued above, this sample can only be considered as complete for cavities
$\gtrsim$25 au and possibly incomplete for cavity radii 15-25 au. In addition,
Table 4 lists a handful of resolved transition disks with cavity sizes $<$15
au with their properties, which is very likely incomplete due to lack of high-
resolution imaging on the bulk of the disk population, and these disks are not
included in the comparison plots.

Starting from the sample of Table 3, the transition disk demographics with
respect to the general protoplanetary disk population are evaluated in this
Section, using the data table by Manara . (2022) of disk and stellar
properties of Class II disks in nearby star forming regions (Taurus,
Ophiuchus, Lupus, Chamaeleon, CrA, Upper Sco), which have been corrected for
the Gaia DR2 distance as well.

Figure 9: Occurrence rates of transition disks. The right panels show the
occurrences when split up in sub-Solar ($<$0.5 $M_{\odot}$), Solar (0.5-1.5
$M_{\odot}$) and super-Solar ($>$1.5 $M_{\odot}$), showing that the occurrence
increases with stellar mass. For Ophiuchus this assessment cannot be made due
to the unknown stellar properties of the bulk of the disk population.

Figure 9 displays the general stellar mass distribution and occurrence rates
of transition disks per cluster. The stellar mass bins are split up in sub-
Solar ($<$0.5 $M_{\odot}$), Solar (0.5-1.5 $M_{\odot}$) and super-Solar
($>$1.5 $M_{\odot}$). The transition disk occurrence increases with stellar
mass, as previously derived by van der Marel Mulders (2021). The overall
fraction of transition disks per region is 10+/-3% for the 4 youngest clusters
(fractions are equal within 1$\sigma$), 8$\pm$5% for the intermediate age
cluster CrA and drops to 3$\pm$2% for Upper Sco. When split up in stellar mass
bins, the drop becomes insignificant due to the small number of transition
disks in Upper Sco. The overall drop indicates that transition disks, although
strong dust traps are thought to be long-lived (Pinilla ., 2020), eventually
dissipate their millimeter dust, which could be either a result of
planetesimal growth in the dust rings (Stammler ., 2019; Eriksson ., 2020;
Zormpas ., 2022) or general disk dissipation.

Figure 10: Correlation plots between the dust cavity radius and stellar and
disk properties: stellar mass, stellar luminosity, dust mass, accretion rate,
spectral slope of the millimeter emission and age. The known circumbinary
disks with wide orbit companions are encircled and not included in the
correlation fit.

In Figure 10 the cavity radius is compared with a number of stellar and disk
properties to check for correlations, expanding upon previous studies with
smaller samples of transition disks. For each plot, the correlation
coefficient is estimated using the linmix package (Kelly, 2007), excluding the
non-spectroscopic circumbinary disks as their cavity is likely caused by the
stellar companion (see Section 5.2.1). No correlation was found with stellar
mass, stellar luminosity, disk dust mass or accretion rate, similar to
previous work (Pinilla, Tazzari ., 2018). No trend of cavity sizes is seen
with age, although it should be noted that the targets with age $>$5 Myr are
dominated by Herbig disks which might distort this picture. A tentative
correlation was found with spectral index, consistent with findings by Pinilla
. (2014). As the spectral index is higher on average than for typical
protoplanetary disks (e.g. Testi ., 2014; Tazzari ., 2021) and it scales with
cavity radius, this correlation is interpreted as a lack of optically thick
central emission in transition disks (Pinilla ., 2014).

The lack of correlation with stellar parameters and disk dust mass implies
that the cavity radius (and its underlying clearing mechanism) may not be set
by the diversity in initial conditions, i.e. whatever is causing the cavity
acts in a similar way and scale across a range of stellar and disk properties,
and also remains constant with time considering the lack of trends in the age
plot. The implications are further discussed in Section 5.2.1. The lack of
correlation with stellar luminosity argues against any connection with
icelines, as previously shown for ring disks (J. Huang ., 2018; Z.C. Long .,
2018; van der Marel ., 2019).

Figure 11: Cumulative dust mass distributions of the transition disk sample of
this study and the general protoplanetary disk population using the data table
from Manara . (2022), indicating the fraction of disks above a given dust
mass. The $<$5 Myr group includes the protoplanetary disks from Taurus,
Ophiuchus, Lupus, Chamaeleon, whereas the $>$5 Myr group covers the older
Upper Sco region and several individual transition disks in e.g. Upper and
Lower Cen. Figure 12: Correlation plots between the disk dust mass, stellar
mass and accretion rate for both the transition disk sample and the general
protoplanetary disk population from (Manara ., 2022).

Figure 11 compares the transition disk dust masses with that of the general
protoplanetary disk population. It is immediately clear that transition disks
have significantly higher dust masses than the general protoplanetary disk
population and does not decrease with age, as previously seen (van der Marel
., 2018; van der Marel Mulders, 2021), whereas protoplanetary disk masses do
decrease between young and old regions. This plot is demonstrating a trend
predicted by Pinilla . (2020), that disks with strong dust traps (such as
those in transition disks) maintain their dust mass, whereas disks with no or
weak dust traps decrease in dust mass.

The correlation plots of Figure 12 are less straight-forward to interpret. It
has been suggested that transition disks do not follow the general
$M_{*}-M_{\rm dust}$ correlation, but have a much flatter linear slope
(Pinilla, Tazzari ., 2018), which would be explained by the presence of strong
dust traps in those disks (Pinilla ., 2020). However, their sample was heavily
biased towards higher mass stars, and the more extended transition sample in
the current work shows several transition disks around lower mass stars with
lower dust masses (e.g. van der Marel ., 2022). A linear fit through the
logarithmic datapoints gives a slope of 1.0$\pm$0.2 for $M_{\rm dust}$ vs
$M_{*}$ for the transition disks, similar to what is found for the overall
disk populations in young clusters (Ansdell ., 2017). This appears to be
consistent with the predictions by Pinilla . (2020) for disks with dust traps
(see their Figure 7): whereas the real dust mass remains constant and shows a
shallow relation with stellar mass, the correlation for the observable
millimeter dust mass steepens from 1 to 5 Myr due to more efficient boulder
growth in dust traps around low-mass stars (see also e.g. Zormpas ., 2022).
For disks without dust traps, the correlation is already steep at 1 Myr and
remains the same in slope and only drops in overall dust mass, due to rapid
radial drift. As individual ages remain too uncertain to measure this
difference in slope with age, the trends predicted by Pinilla . (2020) cannot
be directly observed, but the similarity of the two disk populations is
generally consistent.

Studies of accretion rates in transition disks show a mix of conclusions: some
studies find the accretion rates to be significantly lower than those of
regular protoplanetary disks (Najita ., 2007; Manara ., 2016) whereas other
studies find comparable rates (Manara ., 2014). This discrepancy is likely
caused by the different definitions of transition disks used in these studies.
The accretion rates of the transition disk sample with large cavities
presented in this work are not significantly different from that of the
protoplanetary disk population (Figure 12): a linear fit through the logarithm
of the data points results in $\log(M_{\rm acc}=(-7.9\pm 0.1)+(1.9\pm 0.1)\log
M_{*}$ for protoplanetary disks and $\log(M_{\rm acc}=(-8.4\pm 0.2)+(1.6\pm
0.4)\log M_{*}$ for transition disks. This implies that the underlying
clearing mechanism for these large cavity transition disks may not be related
to accretion. In particular, the mechanism that is opening the cavity is not
limiting the accretion rate through the cavity, or there is a massive and
long-lasting reservoir in their inner disks. This is further discussed in
Section 5.2.2. The third plot in Figure 12 demonstrates that transition disks
are on the high end of the disk mass distribution, but not significantly
different in terms of accretion rates.

###  5.2 Clearing mechanisms

####  5.2.1 Companions

The most promising explanation for transition disk cavities is the presence of
massive planetary companions that have cleared a gap along their orbit, in
particular after the discovery of (accreting) protoplanets of several Jovian
masses in PDS 70 and AB Aur (e.g. Keppler ., 2018; Haffert ., 2019; Currie,
Lawson ., 2022). The accretion rate can remain high in the presence of a giant
planet that has cleared a gap (Lubow D’Angelo, 2006), and this accretion has
been suggested to be wind-driven (Wang Goodman, 2017; Rosenthal ., 2020;
Martel Lesur, 2022). Furthermore, the stellar mass dependence of transition
disks corresponds to that of Jovian exoplanets at smaller orbital radii of 3-8
au, suggesting that the planets which potentially open the cavities would
eventually migrate inwards (van der Marel Mulders, 2021).

Before these discoveries, the idea of massive Jovian planets in cavities was
met with skepticism due to the large orbital radii of several tens of au that
were required, inconsistent with both planet formation models (e.g. Pollack .,
1996; Helled ., 2014; Drazkowska ., 2022), difficulties of opening a wide gap
with a single planet (Zhu ., 2011), and with direct imaging studies around
main sequence stars indicating that such companions were rare (see Nielsen .,
2019; Vigan ., 2021, for an occurrence analysis). This has led to the
possibility that multiple companions must be responsible for the cavities
(Dodson-Robinson Salyk, 2011). High contrast imaging studies resulted in
several debates on claims of companion candidates in transition disk cavities
(e.g. Huélamo ., 2011; Biller ., 2014; Sallum ., 2015; Currie ., 2015;
Thalmann ., 2016; Rameau ., 2017; Ligi ., 2018; Reggiani ., 2018; Currie .,
2019; Wagner ., 2019). Overviews of detection limits have been published for
several transition disk samples, although the detection limit varies strongly
with radius and from disk to disk, the conversion to planet mass is model-
dependent and dependent on the assumed circumplanetary disk (Zhu ., 2015;
Brittain ., 2020; van der Marel, Birnstiel ., 2021; Asensio-Torres ., 2021).
Also H$\alpha$ imaging has mostly resulted in upper limits (Zurlo ., 2020;
Close, 2020). Overall it has become clear that current high contrast imaging
instruments are limited in their detection possibilities down to several
Jovian masses, especially in the inner part of the cavity (Benisty ., 2022;
Currie, Lawson ., 2022). It has also been suggested that the detection of the
protoplanets in PDS70 may be the result of the protoplanets being in an early
stage of accretion, whereas protoplanets in other transition disks have
already finished their accretion and are naturally fainter (Francis van der
Marel, 2020; Brittain ., 2020). As the possibilities and limitations of high
contrast imaging are discussed in detail in these review chapters, and as
limits on companions are best treated on an individual basis, this is not
further discussed here.

Although circumbinary disks have historically been proposed to explain some
transition disks, such as for Coku/Tau 4 (Ireland Kraus, 2008), for many other
transition disks in Taurus and Ophiuchus a brown dwarf companion ($>$20
$M_{\rm Jup}$) in the inner $\sim$40 au has been excluded by sparse aperture
masking (A.L. Kraus ., 2008, 2011; Cheetham ., 2015; Ruíz-Rodríguez ., 2016)
and Keck interferometry (Pott ., 2010), as well as spectroscopic binary
searches (Kohn ., 2016).. It should be noted that Coku/Tau 4, despite being a
famous circumbinary transition disk, has actually not been spatially resolved
yet with millimeter interferometry. Other transition disks have been proposed
to be circumbinary based on modeling indirect signatures in the cavity such as
spirals and warps (Calcino ., 2019; Poblete ., 2020, 2022; Norfolk ., 2022).
Spectroscopic binaries have been discovered as well in some transition disk
cavities, but their separations are usually well within 1 au and thus
insufficient to explain a cavity $>$20 au in radius. Table 5 lists all
transition disks within 200 pc with known inner stellar companions and limits
on those.

The key contribution of ALMA in the question on the origin of cavities has
been the spatial mapping of the gas distribution in the cavities, as explained
in Section 2.4. The deep gas cavities that are often at least a factor 1.5
times smaller than the dust cavities point towards super-Jovian planets at
orbital radii $\sim$10-20 au inside dust cavities of 40-80 au (e.g. Pinilla,
Benisty Birnstiel, 2012; van der Marel, van Dishoeck ., 2016; Facchini,
Pinilla ., 2018; van der Marel, Birnstiel ., 2021). The resulting large dust
cavity radii can be explained by the presence of multiple planets such as seen
in PDS 70, with a derived gas cavity radius of 22 au (van der Marel, Birnstiel
., 2021), a dust ring radius of 74 au with an inner shoulder down to 45 au and
protoplanets at 20 and 35 au (Keppler ., 2018; Haffert ., 2019). The dust ring
radius can be considered as the peak of the pressure bump at the outer edge of
the gas gap. In AB Aur, the protoplanet was found at 93 au (Currie, Lawson .,
2022), with a gas cavity radius of 98 au (van der Marel, Birnstiel ., 2021)
and a dust ring radius at 170 au again with an inner shoulder. For other
transition disks, an inner dust shoulder is not always detected, possibly due
to lower sensitivity and resolution, and the dust ring would be equivalent to
the dust cavity radius. A large separation between protoplanet and dust ring
radius has also been suggested to be caused by eccentric cavities which
develop naturally for super-Jovian planets (Kley Dirksen, 2006; Muley .,
2019). An eccentric cavity has been measured for the transition disk in MWC
758 (Dong ., 2019) but eccentricity is generally unconstrained.

####  5.2.2 Other clearing mechanisms

Figure 13: Relation between cavity radius and accretion rate which can be
reproduced by various photoevaporation models (Ercolano ., 2018; Picogna .,
2019; Gárate ., 2021) with the data points of our transition disk sample plus
the resolved transition disks with small cavities. The cavities caused by the
protoplanets of PDS 70 and AB Aur are overplotted as well. Figure based on van
der Marel . (2022).

A second mechanism to explain the presence of transition disk cavities is
photoevaporative clearing at the end of the lifetime of the disk (R. Alexander
., 2014), when the accretion rate drops below the photoevaporative rate and a
deep gap is rapidly created, clearing the disk from the inside out (Clarke .,
2001). This explanation would connect naturally with the term ‘transition
disk’ as an evolutionary state and transition disk fractions have been used to
determine the expected timescale of the clearing (e.g. Ercolano Pascucci,
2017). However, the non-uniform definitions and uncertainties in SED-derived
transition disk classifications challenges this approach (van der Marel .,
2022). Whereas in initial models photoevaporation could only explain the
transition disks with small cavities $<$30 au and low accretion rates
$<10^{-9}M_{\odot}$ yr-1 (Owen Clarke, 2012; Ercolano Pascucci, 2017), recent
efforts in developing more advanced prescriptions of photoevaporation, e.g.
including X-rays, carbon depletion and dead zones in the inner disk (Ercolano
., 2018; Wölfer ., 2019; Picogna ., 2019; Gárate ., 2021) can reproduce a much
larger range of transition disk cavities, although not all of them (see Figure
13). In contrast to previous comparisons, no SED derived cavities are included
in this plot as their transition disk nature remains debatable (van der Marel
., 2022). On the other hand, the cavity of PDS 70 can be reproduced as well by
this model while it is known to contain two protoplanets. Whether
photoevaporation can indeed be a feasible explanation for observed transition
disks thus remains a matter of debate until more firm constraints on
protoplanet detections have been obtained. Also the direct detection of
photoevaporative winds would constrain such a mechanism (for a full discussion
on disk winds see Pascucci . (2022)).

Other mechanisms for transition disk cavities have been proposed as well,
although with limited observable predictions. So-called dead zones (regions of
low ionization where the magnetorotational instability is inhibited) are
capable of creating a millimeter dust ring or asymmetry (Regály ., 2012; Flock
., 2015) but are not consistent with the deep gas cavities in transition disks
(van der Marel, van Dishoeck ., 2016), unless combined with inner clearing by
a MHD disk wind (Pinilla ., 2016). Detailed modeling indicates that pressure
bumps at dead zone edge may not necessarily develop (Delage ., 2022). Enhanced
dust growth was shown to lead to a deficit in the SED (Dullemond Dominik,
2005), but a complete removal of the millimeter grains in the inner part of
the disk leading to a spatially resolved millimeter-dust cavity is unlikely
considering continuous fragmentation (Birnstiel ., 2012). However, this
mechanism could perhaps explain the very compact millimeter dusk disks
(Facchini ., 2019; van der Marel ., 2022). More generally, a range of
hydrodynamic instabilities have been proposed to explain the presence of gaps
and rings such as those seen in the DSHARP survey, which might be able to
explain some of the transition disk cavities as well (see discussion in
Andrews (2020)).

###  5.3 Disk chemistry

The disk chemical composition sets the atmospheric composition of planets
forming in these disks (Öberg Bergin, 2021). Transition disks are particularly
interesting in this context, due to their possible prevalence of giant planets
in these disks, and also because of their irradiated cavity wall, which may
lead to increased evaporation of complex organic molecules that are otherwise
locked up in ices (Cleeves ., 2011). Observationally, transition disks are
often included in samples of disk chemistry studies without showing
significant differences with other disks (e.g. Öberg ., 2011, 2021). However,
two transition disks stand out in this regard: both HD 100546 and Oph IRS48
show a rich complex organic chemistry (A.S. Booth, Walsh ., 2021; A.S. Booth,
van der Marel ., 2021; van der Marel ., 2014; van der Marel, Booth ., 2021;
Brunken ., 2022) with several molecular detections, in particular methanol,
CH3OH. The latter was surprising as both disks are Herbigs which are too warm
to contain a large amount of CO-ice, which is required to form significant
amounts of CH3OH in disks (Herbst van Dishoeck, 2009). This has been
interpreted as inheritance of ices from the dark cloud phase into the
protoplanetary disk phase where the ices can be released as pebbles drift
inwards (A.S. Booth, Walsh ., 2021). CH3OH has also been detected in TW Hya
(Walsh ., 2016) but in much smaller abundance and thus unlikely to be related
to ice traps.

Figure 14: Distribution of the outer dust rings in transition disks and the
stellar luminosity. The grey shaded area indicates the 22-30 K regime in the
midplane as expected from the stellar luminosity, which corresponds to the
typical CO freeze out temperature. Disks that are warm enough to evaporate
their ices and disks that have been included in disk chemistry surveys (Öberg
., 2011, 2021) are marked by name.

The question is why only these two transition disks show such a rich chemistry
compared to other disks. The answer may lie in the unique combination of the
cavity edge location and the temperature gradient in the disk: the cavity edge
happens to lie in the warmer $>$30 K regime of the disk, and gas temperatures
can reach $>$100 K in the layers above the midplane (van der Marel, Booth .,
2021). As dust pebbles drift inwards towards the nearest dust trap, they
transport their icy layers as well (R.A. Booth Ilee, 2019; Krijt ., 2020). If
that dust trap (cavity edge) happens to be warm enough, the icy layers can be
released. As the disk mid-plane temperature is set by the stellar luminosity
following $T(r)\propto\sqrt[4]{L_{*}/r}$, the expected 30 K line can be
compared with the dust trap locations in transition disks. However, as some
transition disks have outer dust rings as well, it is important to consider
the location of the most _outer_ dust trap, not necessarily the cavity edge.
Figure 14 shows this comparison for the transition disks considered in this
study. The majority of the transition disks (‘cold dust traps’) lie above the
expected CO iceline (22-30 K) (Bisschop ., 2006; Noble ., 2012), i.e. their
dust traps are likely too cold to have their icy content evaporated, even
above the midplane. This sample includes several disks that have been studied
for chemical complexity. On the other hand, the ‘warm dust traps’ (including
IRS48 and HD100546 where CH3OH has been detected) are likely warm enough for
evaporation and are thus key targets to search for chemical complexity.

The link between dust transport and disk composition has been suggested
independently in e.g. the anti-correlation of warm H2O emission with disk dust
size (Banzatti ., 2020; Kalyaan ., 2021), rovibrational emission in TW Hya
(Bosman Banzatti, 2019), chemical composition of accreting material onto the
star (McClure ., 2020) and in the bimodal distribution in C2H emission, a
tracer of the elemental C/O ratio (van der Marel, Bosman ., 2021). Transition
disks are part of these samples and the comparison of disks with and without
strong dust traps is key for our understanding of the influence of dust
transport on disk composition.

###  5.4 Fate of transition disks

It has been suggested that disks with dust traps, including transition disks,
are the progenitors of the debris disks (L.A. Cieza ., 2021; Michel ., 2021;
van der Marel Mulders, 2021), i.e. the planetesimal belts around main sequence
stars $>$10 Myr old, based on a dust evolution argument: only if sufficient
pebbles are trapped at large orbital radii they can grow to sufficiently
massive planetesimal belts that produce the debris observed at infrared and
submillimeter wavelengths (e.g. Hughes ., 2018). When unhindered by pressure
bumps, pebbles would drift inwards before having the time to grow
planetesimals (Pinilla ., 2020). Modeling work shows that this is indeed
quantitatively feasible considering debris and protoplanetary disk infrared
luminosities and time scales (Najita ., 2022).

Figure 15: Distribution of stellar masses and dust ring locations in the
transition disk sample and in the debris disk samples from the _Herschel_ SONS
survey (Holland ., 2017) and the millimeter ALMA sample (Matrà ., 2018).

Figure 15 shows the distribution of stellar masses and ring radii of
transition disks vs debris disks, using both the cold debris disk sample from
the _Herschel_ SONS survey (Holland ., 2017) and the millimeter debris disk
sample from the ALMA sample (Matrà ., 2018). Although transition disks appear
to be more often detected around lower mass stars than debris disks, this is
likely a selection effect, as the absolute number of low-mass stars is much
higher in the IMF. Both the debris disk and transition disk populations show
an increasing occurrence with stellar mass supporting the hypothesis that they
are connected (Sibthorpe ., 2018; van der Marel Mulders, 2021). It is possible
that even more transition disks around low-mass stars are to be discovered as
these disks are generally fainter and not yet imaged at high angular
resolution. The ring radii of both populations are comparable for the SONS
survey, whereas the ALMA debris disk survey distributions peaks at somewhat
larger dust ring radii, but this can be a selection bias as this sample was
not uniformly selected (Matrà ., 2018). Overall, this demonstrates that the
dust ring radii are indeed comparable. Also the discovery of so-called hybrid
disks is interesting in the evolutionary context: these disks, such as
HD141569, have a significant amount of gas compared to their dust content, and
are thought to be in transition between protoplanetary and debris disks (e.g.
Péricaud ., 2017; Miley ., 2018). The dust and gas morphology of HD141569 has
been suggested to be ring-like as well (Currie ., 2016; J.A. White Boley,
2018; Di Folco ., 2020), consistent with transition disks.

Figure 16: The $M_{*}-M_{\rm dust}$ correlation of protoplanetary disks,
transition disks and debris disks based on the data from Taurus, Ophiuchus,
Lupus and Chamaeleon from Manara . (2022) and the debris disk data from
Holland . (2017).

Debris disk millimeter dust masses are several orders of magnitude below that
of protoplanetary disks (Lovell ., 2021; Michel ., 2021). The millimeter dust
in debris disks is secondary, i.e. it is generated by a collisions of
planetesimals (Wyatt, 2008). Figure 16 compares the $M_{*}-M_{\rm dust}$
correlation of protoplanetary disks, transition disks and debris disks. If the
transition disks are the progenitors of debris disks, their observable dust
mass has to decrease by 2-3 orders of magnitude for super-Solar stars. For
sub-Solar masses the statistics of debris disk masses are insufficient. Dust
evolution models of protoplanetary disks including boulder formation already
predict a decrease of 1 orders of magnitude of obserable dust mass in disks
with dust traps (Pinilla ., 2020; Zormpas ., 2022). It remains a question
whether such faint protoplanetary disk rings exist, and what that tells about
the timescales for boulder and planetesimal formation. The evolved Class III
disks with moderate infrared excess are good candidates, but they have not
been studied uniformly so far (Michel ., 2021). One difficulty of studying
Class III disks is the cluster membership, which has only recently been
resolved with Gaia DR2: for example, the Lupus Class III survey by (Lovell .,
2021) is likely contaminated with a large number of objects from the much
older UCL region (Michel ., 2021). Also the inferred dust size distribution
and resulting opacity of protoplanetary and debris disks remains highly
uncertain (Manara ., 2022). More dedicated surveys of evolved disks are
required to bridge the gap between protoplanetary and debris disks.

##  6 Summary and outlook

The spatially resolved imaging of transition disks with large cavities has
revolutionized our understanding of the physical mechanisms behind them. The
direct and indirect evidence for protoplanets embedded in their cavities is
piling up, providing answers to one of the major questions of the previous
decade regarding transition disks: can massive planets at large orbits be
responsible for the cavities? At the same time photoevaporation and dead zone
models have continued to develop and the question whether some transition
disks may ben caused by something other than companions remains open. In
context of companions, the interest within the field has somewhat shifted from
transition disks to disks with gaps in general, but it is clear that
transition disks still provide great test beds for models of planet disk
interaction.

Further characterizations and detections (or deeper limits) of giant planets
in transition disk cavities are crucial for determining the general nature of
the cavity clearing. Direct imaging with new facilities such as _James Webb
Space Telescopes_ , the upgraded the VLT/SPHERE, Gemini/GPI and Subaru/SCExAO
instruments and in the future the 30m-class telescopes will provide much
deeper limits on embedded protoplanets, in particular at closer distances to
the host star. Also opportunities of circumplanetary disk detection at both
millimeter and infrared wavelengths are promising.

Clearly, the confirmation of planetary companions at wide orbits creates an
issue for giant planet formation models, as it requires an efficient (and
rapid) formation process. Considering the existence of gapped disks even at
1-2 Myr, this implies that giant planet cores with sufficient mass to create a
pressure bump form within that timescale, and have already accreted enough
gaseous material to clear a wide, deep gap in the gas. At the same time,
inward migration must be halted or slowed down sufficiently to explain how
large scale gaps and cavities exist for several Myr, and the accretion onto
the host star has to be maintained in the presence of deep gas gaps. Planet
formation and planet-disk interaction models will have to be able to reproduce
this to explain the observations. One important insight here is the finding
that transition disks tend to be massive and more often found around higher
mass stars, implying that high initial disk masses have to be used as input
for such formation models. However, this could be the result of an
observational bias towards brighter disks and cannot be confirmed until deep
high-resolution ALMA observations of the majority of the smaller, fainter
protoplanetary disks are obtained.

On the other hand, the efficiency of dust evolution, growth and transport in
pressure bumps remains an active topic of study. Are the lower dust masses
seen in transition disks around low-mass stars the result of dust growth to
planetesimals, or lower initial disk masses? Can transition disk rings indeed
be the progenitors of debris disks, and if so, how can we prove this? Can dust
transport indeed change the chemical composition across the disk? How does the
dust in the inner disk evolve and how is it related to the flow of material
through the gap? Is dust growth different in azimuthal dust traps than in
radial dust traps? Can the azimuthal nature of some dust traps indeed be
explained by their Stokes number? Considering optical depth issues at
millimeter wavelengths, spatially resolved centimeter observations tracing
optically thin larger grains will provide more insights, which will be
possible with the low frequency receivers of ALMA Band 1 and 2, and with the
future ngVLA. Chemical composition of the inner disk as measured with e.g.
_JWST_ in comparison with the outer disk as measured with ALMA will provide
more answers on the transport of ices and the consequences for exoplanet
atmosphere composition.

One of the major open questions in transition disk studies is whether smaller
cavities $<$10 au indeed exist as well in millimeter wavelengths, and if so,
whether these are simply scaled down versions of the spatially resolved large
cavities. The lack of high-resolution ALMA observations of smaller disks is a
clear limitation to our interpretation of the disk population and the role of
transition disks, in particular with respect to the small cavity transition
disk population identified by their SEDs. ALMA certainly has the capacity to
perform such imaging campaigns, and it is time to shift the focus from
brighter, larger disks to the smaller disks to fully reveal the smaller scale
substructures and their underlying mechanisms.

Acknowledgments I would like to thank the anonymous referee for their
constructive report and useful suggestions. Furthermore, I would like to thank
Jonathan Williams, Sean Andrews, Richard Alexander, Antonio Garufi and Stefan
Kraus for their suggestions to improve the manuscript.

##  Appendix A Data tables

This Appendix contains all tables used in the analysis of this work.

Table 1: SED transition disk candidates

_SED identified transition disks and their millimeter cavity status_
---
Region | Target |  $R_{\rm cav}$(au) | Statusa | SpT | Ref b | Notes
Chamaeleon | CSCha | 37 | Y | K2 | 1 |
Chamaeleon | HD97048 | 63 | Y | A0 | 1 |
Chamaeleon | SZCha | 72 | Y | K0 | 2 | cavity size corrected for Gaia DR2
Chamaeleon | CHXR22E | - | U | M3.5 | 3 | no cavity at 0.6” resolution
Chamaeleon | ESOH$\alpha$559 | - | U | M5 | 4 | no cavity at 0.6” resolution
Chamaeleon | ISO-ChaI-52 | - | U | M4.5 | 4 | no cavity at 0.6” resolution
Chamaeleon | T25 | - | U | M4 | - |
Chamaeleon | T35 | - | U | K5 | 4 | no cavity at 0.6” resolution
CrA | RXJ1842.9-3532 | 37 | Y | K2 | 1 |
CrA | RXJ1852.3-3700 | 49 | Y | K2 | 1 |
CrA | CrA-9 | - | U | M1 | 5 | no cavity at 0.3” resolution
EpsiCha | Tcha | 34 | Y | G8 | 1 |
Isolated | HD169142 | 26 | Y | A5 | 1 |
LowerCen | HD100453 | 30 | Y | F0 | 1 |
LowerCen | HD100546 | 27 | Y | A0 | 1 |
Lupus | J16083070-3828268 | 62 | Y | K2 | 6,7 |
Lupus | J16102955-3922144 | 11 | Y | M4.5 | 6,7 |
Lupus | Sz111 | 55 | Y | M1 | 6,7 |
Lupus | Sz84 | 12 | Y | M5 | 6,7 |
Lupus | Sz91 | 86 | Y | M1 | 1 |
Lupus | MYLup | - | N | K0 | 8 | no cavity at 0.04”, high inclination
Ophiuchus | DoAr44 | 40 | Y | K2 | 1 |
Ophiuchus | IRAS16201-2410 | 28 | Y | G0 | 9 |
Ophiuchus | IRS48 | 83 | Y | A0 | 1 |
Ophiuchus | RXJ1615.3-3255 | 17 | Y | K5 | 10 | cavity size corrected for Gaia DR2
Ophiuchus | RXJ1633.9-2442 | 36 | Y | K7 | 11 |
Ophiuchus | SR21 | 56 | Y | G4 | 1 |
Ophiuchus | WSB60 | 32 | Y | M6 | 1 |
Ophiuchus | DoAr28 | - | U | K5 | - |
Ophiuchus | J16281385-2456113 | - | U | K7 | - |
Ophiuchus | J162740.3-242204 | - | N | K5 | 12 | no cavity at 0.25” resolution
Ophiuchus | J163023.4-245416 | - | N | - | 12 | no cavity at 0.25” resolution
Ophiuchus | J163115.7-243402 | - | N | - | 12 | no cavity at 0.25” resolution
Taurus | ABAur | 156 | Y | A0 | 1 |
Taurus | CQTau | 50 | Y | F2 | 1 |
Taurus | DMTau | 25 | Y | M2 | 1 |
Taurus | GMAur | 40 | Y | K5 | 1 |
Taurus | IPTau | 25 | Y | M0 | 1 |
Taurus | LkCa15 | 76 | Y | K2 | 1 |
Taurus | MWC758 | 62 | Y | A7 | 1 |
Taurus | UXTauA | 31 | Y | G8 | 1 |
Taurus | V892Tau | 27 | Y | A0 | 13 |
Taurus | DETau | - | U | M3 | - |
Taurus | IRAS04125+2902 | - | U | M0 | - |
Taurus | J04184133+2827250 | - | U | K7 | - |
Taurus | J04324911+2253027 | - | U | K5.5 | - |
Taurus | CIDA7 | - | N | M5 | 14 | no cavity at 0.1” resolution
Taurus | V410X-ray6 | - | N | M4.5 | 15 | non-detection
Upper Sco | HD143006 | 41 | Y | G5 | 8 |
Upper Sco | J160421.7-213028 | 87 | Y | K3 | 1 |
UpperCen | HD135344B | 52 | Y | F5 | 1 |
UpperCen | HD142527 | 185 | Y | F6 | 1 |
UpperCen | HD139614 | - | U | A9 | 16 | no cavity at 0.5” resolution

a Y = cavity confirmed by millimeter imaging; N = no cavity found at
millimeter imaging (see last column for resolution); U = unknown, millimeter
imaging insufficient to confirm or non-existent.
b References millimeter cavity size. 1) Francis van der Marel (2020); 2)
Pinilla, Tazzari . (2018); 3) Z.C. Long . (2018); 4) Pascucci . (2016); 5)
Cazzoletti . (2019); 6) van der Marel . (2018); 7) van der Marel . (2022); 8)
Andrews, Huang . (2018); 9) L.A. Cieza . (2021); 10) van der Marel, van
Dishoeck . (2015); 11) L.A. Cieza . (2021); 12) L.A. Cieza . (2019); 13) F.
Long . (2021); 14) Kurtovic . (2021); 15) Ward-Duong . (2018); 16) Stapper .
(2022).

Table 2: New millimeter resolved transition disks

_Disks with millimeter cavity without previous SED identification_
---
Region | Target |  $R_{\rm cav}$(au) |  | SpT | Ref a
Chamaeleon | HPCha | 50 |  | K7 | 1
Chamaeleon | SYCha | 35 |  | K5 | 2
CrA | PDS99 | 56 |  | K6 | 1
Isolated | V4046Sgr | 31 |  | K7 | 1
Lupus | J16070384 | 52 |  | M4.5 | 3
Lupus | J16070854-3914075 | 40 |  | M5 | 3
Lupus | J16090141 | 64 |  | M4 | 3
Lupus | RYLup | 67 |  | K2 | 3
Lupus | Sz100 | 26 |  | M5.5 | 3
Lupus | Sz118 | 64 |  | K5 | 3
Lupus | Sz123A | 39 |  | M1 | 3
Lupus | Sz129 | 10 |  | K7 | 4
Ophiuchus | ROXRA3/rho-Oph2 | 69 |  | M0 | 5
Ophiuchus | SR24S | 35 |  | K6 | 1
Ophiuchus | WSB82/rho-Oph38 | 50 |  | K0 | 5
Taurus | AATau | 44 |  | K5 | 1
Taurus | CIDA1 | 21 |  | M4.5 | 6
Taurus | CIDA9A | 29 |  | M2 | 1
Taurus | GGTau | 224 |  | K7 | 1
Taurus | MHO2 | 28 |  | M3 | 1
Taurus | MHO6 | 10 |  | M5 | 6
Taurus | RYTau | 27 |  | G2 | 1
Taurus | UZTau | 10 |  | M2 | 7
Taurus | ZZTauIRS | 50 |  | M5 | 8
UpperCen | PDS70 | 74 |  | K7 | 1

a References millimeter cavity size. 1) Francis van der Marel (2020); 2)
Orihara . (2022); 3) van der Marel . (2022); 4) Andrews, Huang . (2018); 5)
L.A. Cieza . (2021); 6) Kurtovic . (2021); 7) Z.C. Long . (2018); 8)
Hashimoto, Dong Muto (2021).

Table 3: Full sample of transition disks within 200 pc with cavities $>$15 au

Association | Target | $R_{\rm cav}$ | $M_{d}$ | $d$ | SpT | $M_{*}$ | $\log\dot{M}$ | $L_{*}$ | Ageb | SED?c | Ref |
---|---|---|---|---|---|---|---|---|---|---|---|---
|  | (au) | ($M_{\oplus}$) | (pc) |  | ($M_{\odot}$) | ($M_{\odot}$ | ($L_{\odot}$) | (Myr) |  |  |
|  |  |  |  |  |  | yr-1) |  |  |  |  |
Cham | CSCha | 37 | 29 | 176 | K2 | 1.5 | -8.3 | 1.9 | 2 $\pm$ 0.5 | Y | 1 |
Cham | HD97048 | 63 | 322 | 185 | A0 | 2.4 | -8.2 | 30 | 4.4 $\pm$ 1.1 | Y | 1 |
Cham | SYCha | 35 | 16 | 183 | K7 | 0.8 | -9.4 | 0.5 | 2 $\pm$ 0.5 | NS | 2 |
Cham | SZCha | 72a | 47 | 190 | K2 | 1.5 | -7.8 | 1.2 | 2 $\pm$ 0.5 | Y | 3 |
Cham | HPCha | 50 | 32 | 160 | K0 | 1.4 | -9 | 2.4 | 2 $\pm$ 0.5 | NS | 1 |
CrA | PDS99 | 56 | 23 | 155 | K6 | 0.9 | - | 1.1 | 5.5 $\pm$ 0.5 | NS | 1 |
CrA | RXJ1842.9-3532 | 37 | 12 | 154 | K2 | 0.9 | -8.8 | 0.8 | 5.5 $\pm$ 0.5 | Y | 1 |
CrA | RXJ1852.3-3700 | 49 | 13 | 146 | K2 | 1 | -8.7 | 0.6 | 5.5 $\pm$ 0.5 | Y | 1 |
EpsiCha | TCha | 34 | 13 | 110 | K0 | 1.2 | -8.4 | 1.3 | 5 $\pm$ 3 | Y | 1 |
Isolated | HD169142 | 24 | 27 | 114 | A5 | 2 | -8.7 | 8 | 9 $\pm$ 5 | Y | 1 |
Isolated | V4046Sgr | 31 | 14 | 72 | K7 | 0.8 | -9.3 | 0.5 | 12 $\pm$ 2 | NS | 1 |
LowerCen | HD100453 | 30 | 20 | 104 | A9 | 1.5 | <-8.3 | 6.2 | 6.5 $\pm$ 0.5 | Y | 1 |
LowerCen | HD100546 | 25 | 48 | 110 | B9 | 2.2 | -7 | 25 | 5.5 $\pm$ 1.4 | Y | 1 |
Lupus | J16070384-3911113 | 52 | 1 | 159 | M4.5 | 0.2 | -12.5 | 0 | 2.5 $\pm$ 0.5 | N | 4 |
Lupus | J16070854-3914075 | 40 | 10 | 160 | - | - | - | - | 2.5 $\pm$ 0.5 | N | 4 |
Lupus | J16083070-3828268 | 62 | 10 | 156 | K2 | 1.5 | -9.1 | 1.8 | 2.5 $\pm$ 0.5 | Y | 4 |
Lupus | J16090141-3925119 | 64 | 1 | 164 | M4 | 0.2 | -9.6 | 0.2 | 2.5 $\pm$ 0.5 | N | 4 |
Lupus | RYLup | 67 | 23 | 159 | K2 | 1.5 | -8.2 | 1.9 | 2.5 $\pm$ 0.5 | N | 4 |
Lupus | Sz100 | 26 | 4 | 137 | M5.5 | 0.1 | -9.9 | 0.1 | 2.5 $\pm$ 0.5 | N | 4 |
Lupus | Sz111 | 55 | 16 | 158 | M1 | 0.5 | -9.6 | 0.2 | 2.5 $\pm$ 0.5 | Y | 4 |
Lupus | Sz118 | 64 | 7 | 164 | K5 | 1 | -9.2 | 0.7 | 2.5 $\pm$ 0.5 | N | 4 |
Lupus | Sz123A | 39 | 4 | 163 | M1 | 0.6 | -9.2 | 0.1 | 2.5 $\pm$ 0.5 | N | 4 |
Lupus | Sz91 | 67 | 3 | 159 | M1 | 0.5 | -8.7 | 0.2 | 2.5 $\pm$ 0.5 | Y | 4 |
Oph | DoAr44 | 40 | 19 | 146 | K3 | 1 | -8.2 | 1.9 | 1.5 $\pm$ 0.5 | Y | 1 |
Oph | IRAS16201-2410 | 32a | 12 | 157 | M | - | - | - | 1.5 $\pm$ 0.5 | Y | 2 |
Oph | IRS48 | 70 | 13 | 134 | A0 | 2 | -8.4 | 17.8 | 1.5 $\pm$ 0.5 | Y | 5 |
Oph | ROXRA3 | 49 | 16 | 144 | M0 | 0.5 | -8.5 | 0.7 | 1.5 $\pm$ 0.5 | N | 6 |
Oph | RXJ1615.3-3255 | 17a | 65 | 155 | K7 | 1.2 | -8.5 | 0.9 | 1.5 $\pm$ 0.5 | Y | 7 |
Oph | RXJ1633.9-2442 | 36 | 17 | 141 | K7 | 0.8 | -10 | 1 | 1.5 $\pm$ 0.5 | Y | 6 |
Oph | SR21 | 56 | 19 | 138 | G3 | 2 | -7.9 | 11 | 1.5 $\pm$ 0.5 | Y | 1 |
Oph | SR24S | 35 | 24 | 114 | K1 | 1.5 | -7.2 | 2.5 | 1.5 $\pm$ 0.5 | N | 1 |
Oph | WSB60 | 32 | 19 | 137 | M6 | 0.2 | -8.9 | 0.2 | 1.5 $\pm$ 0.5 | Y | 1 |
Oph | WSB82 | 50 | 49 | 156 | - | - | - | 5.1 | 1.5 $\pm$ 0.5 | N | 6 |
Taurus | AATau | 44 | 12 | 129 | M0.6 | 0.5 | -8.4 | 1.1 | 1.5 $\pm$ 0.5 | N | 1 |
Taurus | ABAur | 156 | 29 | 163 | A0 | 2.2 | -6.8 | 65 | 4 $\pm$ 1.4 | Y | 1 |
Taurus | CIDA1 | 21 | 3 | 135 | M4.5 | 0.2 | -7.9 | 0.2 | 1.5 $\pm$ 0.5 | N | 8 |
Taurus | CIDA9A | 29 | 11 | 171 | M1.8 | 0.4 | - | 0.1 | 1.5 $\pm$ 0.5 | NS | 1 |
Taurus | CQTau | 50 | 41 | 163 | F2 | 1.6 | -7 | 10 | 8.9 $\pm$ 2.8 | Y | 1 |
Taurus | DMTau | 18 | 20 | 145 | M3.0 | 0.3 | -8.3 | 0.2 | 1.5 $\pm$ 0.5 | Y | 1 |
Taurus | GGTau | 224 | 191 | 149 | K7 | 0.7 | -7.3 | 1.6 | 1.5 $\pm$ 0.5 | NS | 1 |
Taurus | GMAur | 40 | 43 | 160 | K5 | 1 | -8.3 | 1 | 1.5 $\pm$ 0.5 | Y | 1 |
Taurus | IPTau | 25 | 3 | 130 | M0.6 | 0.5 | -8.1 | 0.6 | 1.5 $\pm$ 0.5 | Y | 1 |
Taurus | LkCa15 | 76 | 33 | 158 | K5.5 | 0.8 | -8.4 | 1.3 | 1.5 $\pm$ 0.5 | Y | 1 |
Taurus | MHO2 | 28 | 40 | 133 | M3 | 0.4 | - | 1 | 1.5 $\pm$ 0.5 | N | 1 |
Taurus | MWC758 | 62 | 23 | 160 | A7 | 1.6 | -7.4 | 14 | 8.3 $\pm$ 0.5 | Y | 1 |
Taurus | RYTau | 27 | 39 | 133 | G0 | 1.7 | -7.1 | 15 | 1.5 $\pm$ 0.5 | N | 1 |
Taurus | UXTauA | 31 | 16 | 139 | G8 | 2.3 | -8 | 2.5 | 1.5 $\pm$ 0.5 | Y | 1 |
Taurus | V892Tau | 29a | 41 | 118 | B8 | 2 | - | 2 | 1.5 $\pm$ 0.5 | Y | 3 |
Taurus | ZZTauIRS | 58 | 19 | 131 | M4.5 | 0.2 | -8.9 | 0.2 | 1.5 $\pm$ 0.5 | N | 9 |
UpperCen | HD135344B | 52 | 49 | 136 | F5 | 1.6 | -7.4 | 6.7 | 5.7 $\pm$ 2.8 | Y | 1 |
UpperCen | HD142527 | 185 | 306 | 157 | F6 | 2.3 | -7.5 | 9.9 | 6.6 $\pm$ 0.3 | Y | 1 |
UpperCen | PDS70 | 74 | 12 | 113 | K7 | 0.8 | -10.2 | 0.3 | 5 $\pm$ 1 | NS | 1 |
UpperSco | HD143006 | 41 | 20 | 166 | K0 | 1.6 | -8.1 | 3.8 | 3.8 $\pm$ 1.7 | Y | 10 |
UpperSco | J160421.7-213028 | 87 | 21 | 150 | K3 | 1.2 | -10.5 | 0.7 | 10 $\pm$ 0.5 | Y | 1 |

References. 1) Francis van der Marel (2020); 2) Orihara . (2022); 3) Pinilla,
Tazzari . (2018); 4) van der Marel . (2022); 5) van der Marel, Pinilla .
(2015); 6) L.A. Cieza . (2021); 7) van der Marel, van Dishoeck . (2015); 8)
Kurtovic . (2021); 9) Hashimoto, Dong Muto (2021); 10) J. Huang . (2018).
a Cavity size corrected for new Gaia DR2 distance.
b Average age of the cluster following Michel . (2021), individual ages for
the Herbig stars from Vioque . (2018). c Indicator whether the SED was already
indicating the presence of an inner cavity: Y = yes, N = no, NS =
no/insufficient _Spitzer_ photometry available to make the ass.

Table 4: Resolved transition disks within 200 pc with cavities $<$15 au

Association | Target | $R_{\rm cav}$ | $d$ | SpT | $M_{*}$ | $\dot{M}$ | SED?a | Ref
---|---|---|---|---|---|---|---|---
|  | (au) | (pc) |  | ($M_{\odot}$) | ($M_{\odot}$ yr-1) |  |
Lupus | Sz129 | 10 | 162 | K7 | 0.78 | -8.4 | N | 1
Lupus | Sz76 | 4 | 160 | M4 | 0.23 | -9.3 | Y | 1
Lupus | J16102955-3922144 | 11 | 160 | M4.5 | 0.22 | -9.8 | Y | 1
Lupus | Sz84 | 12 | 153 | M5 | 0.17 | -9.23 | Y | 1
Taurus | UZTauE | 11 | 134 | M1 | 0.89 | - | N | 2
Taurus | MHO6 | 10 | 141 | M5 | 0.12 | -9.3 | NS | 3
Taurus | XZTauBb | 1.3 | 146 | M2 | 0.37 | -7.9 | N | 4
TWHya | TWHya | 3 | 60 | M0 | 0.6 | -8.9 | Y | 5
Oph | ISO-Oph2B | 2.2 | 134 | - | - | - | N | 6

References. 1) van der Marel . (2022); 2) F. Long . (2019); 3) Kurtovic .
(2021); 4) Osorio . (2016); 5) Andrews . (2016); 6) González-Ruilova . (2020);
a Indicator whether the SED was already indicating the presence of an inner
cavity: Y = yes, N = no, NS = no/insufficient _Spitzer_ photometry available
to make the ass.
b Tentative: not recovered in Ichikawa . (2021).

Table 5: Limits on (sub)stellar companions in transition disks

Circumbinary disks in transition disk sample
---
Target | $R_{\rm cav}$ | Binary sep. | Binary | Ref
| (au) | (au) | sufficient? |
CS Cha | 37 |  $\sim 4$ (RV) | N? | 1
RXJ1633.9-2442∗ | 36 | 3.3 | N? | 12 (not detected in 7)
DM Tau∗ | 18 | 6.5 | Y? | 2
GG Tau | 224 | 35 | Y | 3
HD142527 | 185 | 13 | Y | 4
RY Tau | 27 | ? (RV) | ? | 1
V892 Tau | 34 | 8 | Y | 5
V4046Sgr | 31 | 0.045 | N | 6
Transition disks for which a inner (sub)stellar companion ($>20M_{\rm Jup}$)
has been excluded
Target | $R_{\rm cav}$ | Range |  | Ref
| (au) | (au) |  |
DoAr44 | 40 | 1-47 |  | 7
SR21 | 56 | 1-44 |  | 7,12
SR24S | 35 | 1-36 |  | 7,12
RXJ1604.3-2130 | 87 | 1-48 |  | 8
CIDA9 | 29 | 1-55 |  | 9
DM Tau | 18 | 1-46 |  | 9,10
GM Aur | 40 | 1-51 |  | 9,10
IP Tau | 25 | 1-42 |  | 9
LkCa15 | 76 | 1-15 |  | 9,10
RY Tau | 27 | 1-43 |  | 9,10
UX Tau A | 31 | 1-45 |  | 9,10
ZZ Tau IRS | 58 | 1-42 |  | 9
RXJ1615.3-3255 | 17 | 2-31 |  | 2
RXJ1842.9-3532 | 37 | 2-31 |  | 2
Transition disks for which a spectroscopic binary has been excluded
Target | $R_{\rm cav}$ |  |  | Ref
| (au) |  |  |
SZ Cha | 72 |  |  | 11
RXJ1852.3-3700 | 49 |  |  | 11
SR21 | 56 |  |  | 11
IRAS 16201-2410 | 32 |  |  | 11
RXJ1633.9-2442 | 36 |  |  | 11

∗ Tentative
References 1. Nguyen . (2012); 2) Willson . (2016); 3) R.J. White . (1999); 4)
Lacour . (2016); 5) Leinert . (1997); 6) Stempels Gahm (2004); 7) Cheetham .
(2015); 8) A.L. Kraus . (2008); 9) A.L. Kraus . (2011); 10) Pott . (2010); 11)
Kohn . (2016); 12) Ruíz-Rodríguez . (2016).

##  2 Data availability statement

All data generated or analysed during this study are included in this
published article (and its supplementary information files).

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