# Molecules with ALMA at Planet-forming Scales (MAPS). A Circumplanetary Disk
Candidate in Molecular Line Emission in the AS 209 Disk

Jaehan Bae Department of Astronomy, University of Florida, Gainesville, FL 32611, USA  Richard Teague Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA  Sean M. Andrews Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  Myriam Benisty Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France  Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France  Stefano Facchini Universitá degli Studi di Milano, via Celoria 16, 20133 Milano, Italy  Maria Galloway-Sprietsma Department of Astronomy, University of Florida, Gainesville, FL 32611, USA  Ryan A. Loomis National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903, USA  Yuri Aikawa Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan  Felipe Alarcón  Department of Astronomy, University of Michigan, 323 West Hall, 1085 S. University Avenue, Ann Arbor, MI 48109, USA  Edwin Bergin Department of Astronomy, University of Michigan, 323 West Hall, 1085 S. University Avenue, Ann Arbor, MI 48109, USA  Jennifer B. Bergner NASA Hubble Fellowship Program Sagan Fellow  University of Chicago Department of the Geophysical Sciences, Chicago, IL 60637, USA  Alice S. Booth Leiden Observatory, Leiden University, 2300 RA Leiden, the Netherlands  Gianni Cataldi National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan  Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan  L. Ilsedore Cleeves Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA  Ian Czekala Department of Astronomy and Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA  Center for Exoplanets and Habitable Worlds, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA  Center for Astrostatistics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA  Institute for Computational & Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA  Viviana V. Guzmán Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile  Jane Huang NASA Hubble Fellowship Program Sagan Fellow  Department of Astronomy, University of Michigan, 323 West Hall, 1085 S. University Avenue, Ann Arbor, MI 48109, USA  John D. Ilee School of Physics and Astronomy, University of Leeds, Leeds, UK, LS2 9JT  Nicolas T. Kurtovic Max-Planck-Institut für Astronomie, Königstuhl 17, 69117, Heidelberg, Germany  Charles J. Law Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  Romane Le Gal Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université Grenoble Alpes, CNRS, F-38000 Grenoble, France  Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la piscine, F-38406 Saint-Martin d’Hères, France  Yao Liu Purple Mountain Observatory & Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, Nanjing 210023, China  Feng Long Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  François Ménard Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France  Karin I. Öberg Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  Laura M. Pérez Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile  Núcleo Milenio de Formación Planetaria (NPF), Chile  Chunhua Qi Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  Kamber R. Schwarz Max Planck Institute for Astronomy, Königstuhl 17, Heidelberg, Germany  Anibal Sierra Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile  Catherine Walsh School of Physics and Astronomy, University of Leeds, Leeds, UK, LS2 9JT  David J. Wilner Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  Ke Zhang Department of Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI 53706, USA

###### Abstract

We report the discovery of a circumplanetary disk (CPD) candidate embedded in
the circumstellar disk of the T Tauri star AS 209 at a radial distance of
about 200 au (on-sky separation of 1.”4 from the star at a position angle of
$161^{\circ}$), isolated via 13CO $J=2-1$ emission. This is the first instance
of CPD detection via gaseous emission capable of tracing the overall CPD mass.
The CPD is spatially unresolved with a $117\times 82$ mas beam and manifests
as a point source in 13CO, indicating that its diameter is $\lesssim 14$ au.
The CPD is embedded within an annular gap in the circumstellar disk previously
identified using 12CO and near-infrared scattered light observations, and is
associated with localized velocity perturbations in 12CO. The coincidence of
these features suggests that they have a common origin: an embedded giant
planet. We use the 13CO intensity to constrain the CPD gas temperature and
mass. We find that the CPD temperature is $\gtrsim 35$ K, higher than the
circumstellar disk temperature at the radial location of the CPD, 22 K,
suggesting that heating sources localized to the CPD must be present. The CPD
gas mass is $\gtrsim 0.095M_{\rm Jup}\simeq 30M_{\rm Earth}$ adopting a
standard 13CO abundance. From the non-detection of millimeter continuum
emission at the location of the CPD ($3\sigma$ flux density $\lesssim
26.4~{}\mu$Jy), we infer that the CPD dust mass is $\lesssim 0.027M_{\rm
Earth}\simeq 2.2$ lunar masses, indicating a low dust-to-gas mass ratio of
$\lesssim 9\times 10^{-4}$. We discuss the formation mechanism of the CPD-
hosting giant planet on a wide orbit in the framework of gravitational
instability and pebble accretion.

††facilities: ALMA††software: CASA (McMullin et al., 2007), bettermoments
(Teague & Foreman-Mackey, 2018), GoFish (Teague, 2019), Matplotlib (Hunter,
2007), NumPy (van der Walt et al., 2011), TRIVIA (Bae, 2022)

##  1 Introduction

One of the best and most direct ways to study planet formation processes is to
observe young planets while they are forming. Although this task has long been
challenging, the situation is rapidly changing. In particular, recent
observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have
demonstrated the possibility of detecting young planets by spatially and
kinematically resolving the characteristic gas flows around forming planets
(see review by Pinte et al., 2022), including localized velocity perturbations
associated with planet-driven spirals (Pinte et al., 2018; Casassus & Pérez,
2019) and large-scale meridional flows falling on to planet-hosting gaps
(Teague et al., 2019a; Yu et al., 2021).

As a giant planet forms, it opens a radial gap in its natal circumstellar disk
(Lin & Papaloizou, 1986). Within the Hill (or Bondi) sphere of the planet, a
circumplanetary disk (CPD) forms in order to conserve the angular momentum of
the material accreted from the circumstellar disk (Korycansky et al., 1991;
Ward & Canup, 2010). The growth of a planet after gap opening is controlled by
gas accretion through its CPD (Lubow et al., 1999). CPDs also play a crucial
role in satellite formation. Indeed, the prograde, nearly circular and co-
planar orbits of regular satellites of Jupiter and Saturn suggest that their
formation must have happened within CPDs. Despite their importance, however,
many attempts to search for CPDs have yielded non-detections (Isella et al.,
2014; Pérez et al., 2019; Pineda et al., 2019; Andrews et al., 2021), and the
detections made to date are limited in millimeter continuum observations
(Isella et al., 2019; Benisty et al., 2021; Wu et al., 2022). Consequently,
fundamental properties of CPDs, such as their sizes (see discussion in
Paardekooper et al. 2022) and whether they are indeed rotationally-supported
“disks” or instead pressure-supported envelopes (Szulágyi et al., 2016; Fung
et al., 2019) are not fully understood.

In this Letter, we report the discovery of a CPD candidate embedded in the
circumstellar disk of AS 209, identified using 13CO $J=2-1$ emission. AS 209
is a T Tauri star located in the outskirts of the Ophiuchus star-forming
region at a distance of 121 pc (Gaia Collaboration et al., 2020), with a
stellar mass of $1.2~{}M_{\odot}$ and an age of 1–2 Myr (Andrews et al., 2009;
Öberg et al., 2021, and references therein). In (sub-)millimeter continuum
emission, the AS 209 disk is $\simeq 140$ au in size and has pronounced
multiple concentric rings and gaps (Fedele et al., 2018; Huang et al., 2018;
Guzmán et al., 2018; Sierra et al., 2021). Hydrodynamic planet-disk
interaction simulations showed that the continuum gaps can be explained by one
or more planets embedded in the disk (Fedele et al., 2018; Zhang et al.,
2018). The gas disk probed by 12CO emission extends to $\simeq 300$ au, much
further out than the continuum disk (Guzmán et al., 2018). In 12CO an annular
gap is seen centered at a radius of 200 au, sandwiched by two bright rings
whose intensities peak at 168 and 245 au (Guzmán et al., 2018; Law et al.,
2021a). Near-infrared scattered light observations also revealed this gap
around 200 au (Avenhaus et al., 2018). This 12CO and near-infrared gap at 200
au is within which the CPD candidate is found in this study.

This Letter is organized as follows. In Section 2, we summarize the
observations. In Section 3, we present the 12CO data, which reveals an annular
gap and localized kinematic perturbations, and the 13CO data, which shows a
CPD candidate. We also present a null detection of C18O and continuum emission
at the location of the CPD candidate. In Section 4, we place constraints on
the size, temperature, and mass of the CPD, and discuss the potential
formation pathway of the planet. We summarize our findings and state our
conclusions in Section 5.

##  2 Observations

The 12CO, 13CO, and C18O data used in this study were obtained as part of the
ALMA Large Program Molecules with ALMA at Planet-forming Scales (MAPS;
2018.1.01055.L; Öberg et al. 2021). We refer the reader to Öberg et al. (2021)
for the observational setup and calibration process, and Czekala et al. (2021)
for the imaging process including the Jorsater & van Moorsel (1995, hereafter
JvM) correction, which is used to properly scale the CLEAN residual map into
consistent units of Janskys per CLEAN beam (instead of Janskys per dirty beam)
and is crucial for correctly estimating the flux density of faint extended
emission. Throughout this paper, we use the robust = 0.5 weighted, JvM-
corrected images111Publicly available from https://alma-maps.info/data.html
unless stated otherwise. The 12CO, 13CO, and C18O images have synthesized beam
sizes of $134\times 100$ mas, $140\times 104$ mas, and $141\times 105$ mas
with position angles of $-89.2^{\circ}$, $-89.1^{\circ}$, and $-88.6^{\circ}$,
respectively. The rms noise measured in line-free channels is $0.605~{}{\rm
mJy~{}beam}^{-1}$, $0.447~{}{\rm mJy~{}beam}^{-1}$, and $0.337~{}{\rm
mJy~{}beam}^{-1}$, respectively.

While the JvM correction allows the CLEAN model and residual image to have
consistent units, Casassus & Carcamo (2022) cautioned that it may exaggerate
the peak signal-to-noise ratio of restored images. As such, we carry out our
analyses using both JvM-corrected images (presented in the main Sections) and
JvM-uncorrected images (presented in the Appendix) in order to ensure that the
CPD detection is statistically significant regardless of which image we use.

For the analysis of continuum data, we combined the continuum data from ALMA
Large Programs MAPS and Disk Substructures at High Angular Resolution
Project222In Andrews et al. (2018), the continuum data obtained as part of the
DSHARP program was combined with archival data from programs 2013.1.00226.S
(Huang et al., 2016) and 2015.1.00486.S (Fedele et al., 2018). (DSHARP;
2016.1.00484.L; Andrews et al. 2018), using CASA version 6.4. We refer the
reader to Sierra et al. (2021) for the imaging of the MAPS continuum data and
Andrews et al. (2018) for the observational setup and calibration process of
the DSHARP continuum data. To account for any source proper motions or
atmospheric/instrumental effects between observations, we aligned each dataset
to a common phase center using the fixvis and fixplanets tasks to apply the
necessary phase shifts. We then made a composite image at 240 GHz, the mean
frequency of the DSHARP and MAPS continuum data. The continuum image was
generated with a Briggs weighting scheme with robust = 0.5 and a $65\times 20$
mas FWHM Gaussian taper (at PA = 0°) in order to enhance the point source
sensitivity near the location of the CPD candidate. The synthesized beam has
FWHM of $106\times 94$ mas at a position angle of $-69.2^{\circ}$ degrees. We
find that the PSF of the combined data is complex due to the combined $u,v$
coverage of the datasets. For that reason, we do not perform the JvM
correction on the continuum data. The rms noise within an elliptical annulus
$\pm 0\farcs 1$ around the radial location of the CPD candidate (i.e., between
$1\farcs 6$ and $1\farcs 8$ in the deprojected plane) is $8.8~{}\mu{\rm
Jy}~{}{\rm beam}^{-1}$.

Figure 1: Selected 12CO $J=2-1$ channel maps. Upper panels shows an annular
gap. The black dashed ellipses present the projected circular orbit with a
radius of 206 au. Lower panels show a large velocity perturbation in the
southern side of the disk (highlighted with black circles). A similar
perturbation is not present in the northern side of the disk, emphasizing the
localized nature of the perturbation. The synthesized beam is presented in the
lower left corner of each panel. The color map starts at $5\sigma\simeq
3.3~{}{\rm mJy~{}beam}^{-1}$.

The DSHARP and MAPS observations were taken about two years apart. Between the
two observations, the CPD candidate is expected to have moved about 8 mas on
the sky, less than $10\%$ of the synthesized beam of the combined continuum
data. We thus opt not to account for the orbital motion of the CPD candidate.

##  3 Results

###  3.1 An Annular Gap and Localized Kinematic Perturbations in 12CO

We present selected 12CO channel maps in Figure 1 (see Figure 7 in Appendix A
for additional channel maps). The 12CO emission reveals a 78 au-wide gap
(determined by the distance between the adjacent peaks in the radial intensity
profile; Law et al. 2021a) around $\simeq 200$ au, which is most clearly seen
along the semi-major axis of the disk (see $5.6-6.0~{}{\rm km~{}s}^{-1}$ in
Figure 1). The radial location of the gap is consistent with what is
previously reported in Guzmán et al. (2018) and Teague et al. (2018) using
independent 12CO datasets, and in Avenhaus et al. (2018) using near-infrared
scattered light images. Teague et al. (2018) showed that the 12CO gap is
associated with deviations in the rotational motion of the gas with variations
of up to $5\%$ relative to the background Keplerian rotation, which arises
from the steep radial gas pressure gradient at the edges of the gap. While we
defer more comprehensive kinematic studies to a forthcoming paper (Galloway-
Sprietsma et al., in prep.), comparable rotational velocity deviations are
found around the 12CO gap using the MAPS 12CO data.

In addition to the annular gap, 12CO reveals a strong localized (both in space
and velocity) perturbation on the southern side (see $4.6-5.0~{}{\rm
km~{}s}^{-1}$ in Figure 1), reminiscent of a so-called velocity kink induced
by planets (Pinte et al., 2018). Similar velocity perturbations are found at
the same location and velocity in independent archival 12CO datasets presented
in Guzmán et al. (2018) and Teague et al. (2018). The velocity perturbation is
seen near the semi-minor axis of the disk, suggesting that the perturbation is
associated with radial and/or vertical motions rather than rotational motions
(Teague et al., 2019b). We note that a similar velocity perturbation is not
seen on the northern side of the disk, highlighting the localized nature of
the perturbation.

###  3.2 Point Source Emission in 13CO

Figure 2 presents selected 13CO channel maps (see Figures 8 and 9 in Appendix
A for additional channel maps). In addition to the characteristic butterfly
pattern arising from Keplerian rotation of the AS 209 disk, we find a point
source emission at a projected distance of $1\farcs 4$ and a position angle of
$161^{\circ}$ (east from north). The offset from the star (assumed to be at
the center of the concentric continuum rings) is $(\Delta{\rm RA},\Delta{\rm
Dec})=(0\farcs 450\pm 0.004,-1\farcs 332\pm 0.003)$. Assuming that the point
source is embedded in the midplane of the AS 209 disk, the deprojected
distance between the point source and the central star is $1\farcs 7$ ($\simeq
206$ au). It is worth pointing out that the 13CO point source is located at
the center of the 12CO annular gap and is coincident in both radius and
azimuth with the localized velocity perturbations seen in 12CO. In Section
4.1, we will discuss the potential (common) origin of these features.

The 13CO data is limited by the $200~{}{\rm m~{}s}^{-1}$ velocity resolution
set by the MAPS program. In the fiducial image cube, the point source is found
in two adjacent channels, $4.7~{}{\rm km~{}s}^{-1}$ (Figure 2 left) and
$4.9~{}{\rm km~{}s}^{-1}$ (Figure 2 right). The peak intensities at the two
channels are 5.75 and $5.85~{}{\rm mJy~{}beam}^{-1}$, respectively. While we
cannot image at a finer channel spacing than the velocity resolution, we
produce an additional image cube with the same channel spacing of $200~{}{\rm
m~{}s}^{-1}$, but with the starting velocity offset by half of the velocity
resolution ($100~{}{\rm m~{}s}^{-1}$). The middle panel in Figure 2 shows the
channel map at $4.8~{}{\rm km~{}s}^{-1}$ from the half-channel-shifted image
cube. The point source is more clearly seen, with the peak intensity of
$9.62~{}{\rm mJy~{}beam}^{-1}$ (rms noise $=0.543~{}{\rm mJy~{}beam}^{-1}$).
Simply comparing the peak intensity with the rms noise, the point source in
the three velocity channels is detected above 12, 18, and $13\sigma$ levels,
respectively.

Figure 2: Selected 13CO $J=2-1$ channel maps. A point source is seen at a
projected distance of $1\farcs 4$ with a position angle of $161^{\circ}$ (east
from north). The dashed ellipses show the projected circular orbit of an
object with a orbital semi-major axis of 206 au. Near the center of the AS 209
disk, the inner disk appears as a point source because of an annular gap
around $\simeq 0\farcs 5$ (Law et al. 2021a, see also Figure 3). The
synthesized beam is presented on the lower left corner of each panel. The
color map starts at $5\sigma\simeq 2.3~{}{\rm mJy~{}beam}^{-1}$. Figure 3:
(Red) Radial profile of the 13CO integrated intensity along the location of
the point source, averaged over $\pm 2^{\circ}$ in position angle around the
point source. (Black) Azimuthally-averaged radial profile of the 13CO
integrated intensity excluding the point source. The gray and red shaded
regions show the standard deviation of the integrated intensity in each
$0\farcs 05$ radial bin. The point source is seen well above the azimuthally-
averaged integrated intensity. The gap at $\simeq 0\farcs 5-0\farcs 6$ is
consistent with the 13CO gap previously reported by Law et al. (2021a). The
size of the major axis of the synthesized beam is shown with the black
horizontal bar.

Although the 13CO emission from the AS 209 disk is less extended than the 12CO
emission, 13CO emission is detected out to $\simeq 2\arcsec$ (in deprojected
distance), beyond the radial location of the point source. If the point source
is embedded in the midplane within the 12CO gap, it is thus possible that some
of the emission toward the point source originates from the residual gas
within the gap. To assess the potential contamination from the gas in the gap,
in Figure 3 we present a radial profile of the integrated intensity averaged
over the azimuthal region $\pm 2^{\circ}$ around the point source. We compare
that to an azimuthally-averaged, radial integrated intensity profile excluding
the $\pm 2^{\circ}$ region around the point source. At the deprojected radial
distance of $1\farcs 7$, the integrated intensity of the point source is
$2.54~{}{\rm mJy~{}beam}^{-1}~{}{\rm km~{}s}^{-1}$, while the integrated
intensity excluding the point source is $1.15~{}{\rm mJy~{}beam}^{-1}~{}{\rm
km~{}s}^{-1}$ with a standard deviation of $0.33~{}{\rm
mJy~{}beam}^{-1}~{}{\rm km~{}s}^{-1}$.

Figure 4: (Left) C18O $J=2-1$ channel map at $4.8~{}{\rm km~{}s}^{-1}$, the
velocity the 13CO point source has the largest intensity. (Right) The combined
MAPS + DSHARP continuum image. The color table is saturated toward the disk
center and stretched to better show faint emission at large radii. In both
panels, the white contours show the 13CO $J=2-1$ emission at $4.8~{}{\rm
km~{}s}^{-1}$ at 5 and $10\sigma$ levels. In the lower-left corner of each
panel, synthesized beams for the C18O and continuum data are shown with yellow
ellipses while the synthesized beam for the 13CO data is shown with white
ellipses.

The brightness temperature of the 13CO emission within the gap (excluding the
point source) is $\lesssim 8$ K. For a reasonable gas temperature of $\gtrsim
20$ K (so that CO does not freeze), the gap must be optically thin ($\tau_{\rm
gap}\lesssim 0.5$) because the brightness temperature is lower than the gas
temperature. The fact that the integrated intensity toward the point source
does not increase with the beam size (see Appendix B) further supports that
the gap is optically thin in 13CO and the point source dominates the emission.
If we assume that a similar amount of gas is present around the point source
to elsewhere within the gap, the contamination from the gas in the gap is thus
expected to be $1.15/2=0.575~{}{\rm mJy~{}beam}^{-1}~{}{\rm km~{}s}^{-1}$
(because only half of the gas in front of the point source would contribute).
Subtracting this from the integrated intensity toward the point source, we
find that the point source is detected at a $6.0\sigma$ significance. We carry
out the same analysis using the JvM-uncorrected image and find that the point
source is detected at a $4.4\sigma$ significance (see Appendix C).

In order to determine whether or not the point source is spatially resolved,
we follow Isella et al. (2019) and Benisty et al. (2021) and image the data
using various robust parameters ranging from $-1$ to 2. The synthesized beam
size, rms noise, Gaussian fit to the point source emission are summarized in
Appendix B and Tables 1 and 2. We find that the peak and integrated
intensities are constant within uncertainties when the semi-minor axis of the
beam is $\lesssim 120$ mas, indicating that the point source is spatially
unresolved at those scales. For larger beam sizes, the peak and integrated
intensities increase since the point source is no longer spatially separated
from the AS 209 disk.

###  3.3 Null Detection in C18O and Continuum

In Figure 4, we present C18O $J=2-1$ emission at $4.8~{}{\rm km~{}s}^{-1}$ and
the continuum image. No C18O emission at a $>3\sigma$ significance is detected
at the location of the 13CO point source. Similarly, no emission coincident
with the 13CO point source is detected at a $>3\sigma$ significance in the
continuum image. We further inspected the continuum data by modeling the
continuum emission in the visibility plane following Andrews et al. (2021),
but no point source emission coincident with the 13CO point source (and
elsewhere) is found. In Section 4.2, we place upper limits on the CPD gas and
dust masses based on the null detection of C18O and continuum emission.

Figure 5: A summary figure showing the coincidence of the CPD candidate with
the 12CO/scattered light gap and 12CO velocity kink. (a) Radial profiles of
the normalized (black) 12CO integrated intensity and (red) the near-infrared
scattered light (from Avenhaus et al. 2018; scaled by $r^{2}$ to account for
the drop in the incident stellar photons). The shaded region shows $1\farcs
4-2\farcs 0$, an approximate radial location of the 12CO gap. (b) Radial
profile of the normalized 13CO integrated intensity along the azimuthal
location of the CPD (see Figure 3). (c) 12CO channel map at $4.8~{}{\rm
km~{}s}^{-1}$. The location of the 12CO gap is shown with the black dashed
ellipses. (d) A zoom-in of the blue box in panel (c), showing the velocity
kink in 12CO (background color map) within the 12CO gap (black dashed curves).
The blue contours show 13CO emission at 10 and $15\sigma$ levels. Note that
the CPD candidate locates at the center of the 12CO gap and coincides with the
velocity kink in 12CO, highly suggestive of the presence of an embedded
planet. The synthesized beams for 12CO and 13CO data are shown in the lower-
left and lower-right corner of the panel, respectively.

##  4 Discussion

###  4.1 Origin of the 13CO Point Source

We find that the 13CO point source lies at the center of the 12CO gap and that
it is colocated with the 12CO velocity kink (Figure 5). The coincidence
suggests that the most likely explanation is that we are witnessing a planet
and its CPD embedded in the AS 209 disk. In fact, the compact nature of the
13CO emission separated from the circumstellar disk bears a striking
resemblance to what is predicted in numerical simulations of a CPD embedded in
a circumstellar disk presented in Perez et al. (2015). There is no obvious
counterpart of the 13CO point source in 12CO, but this is likely because the
annular gap is optically thick in 12CO. Similarly, we do not find a
counterpart of the 12CO velocity kink in 13CO or C18O due to their low optical
depth within and beyond the circumstellar disk gap (see e.g., Figure 5d).

Recent numerical simulations have shown that circumplanetary materials form a
rotationally-supported disk when the cooling is efficient, but when cooling is
inefficient, a pressure-supported envelope is instead formed (Szulágyi et al.,
2016; Fung et al., 2019). The point source is spectrally unresolved in the
existing dataset, and whether the emission is associated with a disk or an
envelope has to be confirmed in the future using high velocity resolution
molecular line observations that can probe the kinematics of the point source.
For the rest of this paper, we opt not to distinguish between circumplanetary
disk and envelope, but simply use the term CPD to refer to the two
collectively.

What is the mass of the planet candidate? We can estimate the planet mass
using the empirical relation between the gap width and planet mass derived
from numerical simulations. The width of the gap, determined by the distance
between the adjacent peaks in the 12CO radial intensity profile, is 78 au (Law
et al., 2021a). The scaling relation between the gap width normalized by the
planet’s orbital semi-major axis $\Delta_{\rm gap}/R_{p}$, disk aspect ratio
$(h/r)_{p}$, disk viscosity $\alpha$, and planet mass $M_{p}$ suggests
$\Delta_{\rm
gap}/R_{p}=0.41(M_{p}/M_{*})^{0.5}(h/r)_{p}^{-0.75}\alpha^{-0.25}$ (Kanagawa
et al. 2016, see also similar relations in e.g., Zhang et al. 2018; Yun et al.
2019). Adopting the disk aspect ratio at the midplane $(h/r)_{p}=0.118$ from
the circumstellar disk temperature constrained using CO isotopologues (Law et
al., 2021b), $\Delta_{\rm gap}/R_{p}=0.38$ converts to a planet mass of
$M_{p}=1.3~{}M_{\rm Jup}\cdot(\alpha/10^{-3})^{1/2}$. The inferred mass is
comparable to those suggested to explain velocity kinks in other circumstellar
disks (e.g., Pinte et al., 2018, 2019).

###  4.2 Properties of the CPD

####  4.2.1 CPD size and temperature

As shown in Appendix B, the CPD is spatially unresolved. When the CPD is
optically thick and the disk gas is in local thermodynamic equilibrium (LTE),
the actual CPD gas temperature, $T_{\rm CPD}$ (assumed to be uniform across
the CPD for simplicity), and the observed brightness temperature of the CPD,
$T_{\rm obs}$, are related as $T_{\rm obs}=\epsilon T_{\rm CPD}$ where
$\epsilon$ is the beam dilution factor333Here, we assume that the gap is
optically thin in 13CO (see Section 3.2) and ignore attenuation of the CPD
emission by the residual gas in the circumstellar disk gap. The inferred CPD
temperature thus offers a lower limit to the actual CPD temperature.. Adopting
a thin disk geometry, the beam dilution factor is given by

$\epsilon={fD^{2}_{\rm CPD}\cos i\over\theta_{\rm maj}\theta_{\rm
min}/\ln{2}},$ (1)

where $f$ is a dimensionless parameter introduced to account for the fraction
of the emitting area in a given velocity channel compared to the full
geometric area of the CPD (see below), $D_{\rm CPD}$ is the diameter of the
CPD, $i$ is the inclination of the CPD, $\theta_{\rm maj}$ and $\theta_{\rm
min}$ are the semi-major and semi-minor axis of the synthesized beam,
respectively. We can then relate the temperature and diameter of the CPD as

$T_{\rm CPD}=T_{\rm obs}{\theta_{\rm maj}\theta_{\rm min}/\ln{2}\over
fD^{2}_{\rm CPD}\cos i}.$ (2)

Without the knowledge of the emitting area we opt to adopt $f=1$, although it
is reasonable to expect $f<1$ because the channel spacing of the data
($200~{}{\rm m~{}s}^{-1}$) is smaller than the FWHM of the line emission
arising from the CPD ($300-400~{}{\rm m~{}s}^{-1}$; see Tables 1 and 2). With
$f=1$, our analysis thus offers a lower limit to the CPD temperature. We use
the brightness temperature in the $4.8~{}{\rm km~{}s}^{-1}$ channel ($\simeq
19$ K) since it is likely that the channel contains the largest emitting area.
We adopt $i=35^{\circ}$ (Öberg et al., 2021) assuming that the CPD and AS
209’s circumstellar disk are co-planar.

Figure 6a shows the CPD temperature required to explain the observed 13CO
emission as a function of the CPD diameter, calculated with Equation (2). We
note again that the CPD temperature in Figure 6a presents a lower limit
because a higher CPD temperature is needed to explain the observed 13CO
emission (1) when attenuation by the gas in the circumstellar disk gap is not
negligible, (2) when the emitting area in the central channel is smaller than
the full geometric area of the CPD ($f<1$), and/or (3) when the CPD is
optically thin. We find that the CPD temperature is $\gtrsim 35$ K, higher
than the circumstellar disk gas temperature at the radial location of the CPD,
22 K (Law et al., 2021b). This suggests that additional heating sources
localized to the CPD are required to explain the observed 13CO emission, such
as planet’s thermal/accretion heating, the CPD’s internal viscous/turbulent
heating, or shock/compressional heating by infalling circumstellar disk
material (Szulágyi et al., 2016; Szulágyi & Mordasini, 2017).

In the discussion above (and also below in Section 4.2.2), we assume that the
CPD and AS 209’s circumstellar disk are co-planar. This is a reasonable
assumption to start, but numerical simulations show that it is possible that
CPDs and their parent circumstellar disk can be misaligned if the planet is
formed in a turbulent environment (Jennings & Chiang, 2021). In fact, based on
the observationally constrained large obliquity of a planetary mass
($M_{p}=12-27~{}M_{\rm Jup}$) companion 2M0122b, $48^{+28}_{-21}$ degrees,
Bryan et al. (2020) proposed that the CPD of 2M0122b would have to be tilted
out of the circumstellar disk plane (see also Bryan et al., 2021). For our
analysis, the CPD temperature would be minimized when the CPD is face-on
($i=0^{\circ}$; see Equation 2). However, even when the CPD is face-on we
infer the CPD temperature is $\gtrsim 29$ K, higher than the circumstellar
disk temperature, suggesting that additional heating sources localized to the
CPD are still required. Future high spatial/velocity resolution observations
would constrain the CPD geometry from its rotation, allowing us to place more
accurate constraints on the CPD temperature.

Figure 6: (a) The CPD temperature required to explain the observed 13CO
emission, as a function of the CPD diameter. The upper horizontal axis shows
the planet mass corresponding to the CPD diameter assuming that the CPD radius
is 1/3 of the Hill radius: $R_{\rm CPD}=(1/3)R_{\rm
Hill}=(1/3)(M_{p}/3M_{*})^{1/3}$. The dashed horizontal line shows AS 209’s
circumstellar disk temperature at 206 au, 22 K (Law et al., 2021b). The gray
shaded region shows the planet mass constrained by width of the 12CO gap,
$0.42-4.2~{}M_{\rm Jup}$, adopting a range of disk viscosity
$\alpha=10^{-4}-10^{-2}$ (see Section 4.1). (b) A lower limit to the CPD gas
mass inferred from the 13CO emission (blue) and an upper limit to the CPD gas
mass inferred from the null detection of C18O (red). The dashed curves show
$M_{\rm CPD}=1.0,0.3,0.1$, and $0.03~{}M_{p}$ (from left to right). (c) The
CPD dust mass inferred from the non-detection of the continuum emission. The
solid curve assumes a maximum grain size of 1 mm, while the dashed curve
assumes a maximum grain size of $1~{}\mu$m. (d) The CPD dust-to-gas mass ratio
obtained from the inferred CPD gas and dust masses. In this figure, the black
solid/dashed curves show lower (temperature in panel a) or upper (dust mass in
panel c) limits, while the gray hatched regions present the allowed region in
the parameter space bounded by limits from the 13CO (blue) and C18O (red)
data.

####  4.2.2 CPD mass

For a given CPD size and the corresponding lower limit to the CPD temperature
(black curve in Figure 6a), we can place a lower limit to the 13CO mass. This
is possible because the upper state energy of the 13CO $J=2-1$ transition
(15.8 K) is below the lower limit of the CPD temperature; the population in
the $J=2$ rotational level would always drop if the CPD temperature is higher
than the lower limit, requiring a larger CPD mass than for cooler
temperatures.

To obtain the total 13CO number density in the CPD, we first calculate the
number density of 13CO molecules in the $J=2$ level as

$n_{{\rm{}^{13}CO},J=2}={4\pi\over hcA_{21}}\int{I}{\rm d}v\int d^{2}{\rm
d}\Omega,$ (3)

where $h$ is the Planck constant, $A_{21}=6.04\times 10^{-7}~{}{\rm s}^{-1}$
is the spontaneous emission coefficient from molecular data from the LAMDA
database (Schöier et al., 2005), $\int{I}{\rm d}v=2.50~{}{\rm
mJy~{}beam}^{-1}~{}{\rm km~{}s}^{-1}$ is the integrated line intensity, $d$ is
the distance to AS 209, 121 pc (Gaia Collaboration et al., 2020), and $\Omega$
is the solid angle of the beam. We then calculate the total 13CO number
density under the assumption that all energy levels are populated under LTE:

$n_{\rm{}^{13}CO,total}=n_{{\rm{}^{13}CO},J}{Z\over
2J+1}\exp\left[{hB_{e}J(J+1)\over kT}\right],$ (4)

where $Z=\sum^{\infty}_{J=0}(2J+1)\exp[-hB_{e}J(J+1)/kT]$ and $B_{e}=55.10101$
GHz is the rotational constant (Müller et al., 2005). We convert the 13CO
number density to the total gas mass enclosed in the CPD adopting interstellar
CO abundance $[^{12}{\rm CO}]/[{\rm H}_{2}]=10^{-4}$ and 13CO-to-12CO
abundance ratio $[^{13}{\rm CO}]/[^{12}{\rm CO}]=1/60$ (Wilson & Rood, 1994).
The resulting CPD mass is presented in Figure 6b. The CPD gas mass is $\simeq
0.095~{}M_{\rm Jup}~{}(\simeq 30~{}M_{\earth})$ when $D_{\rm CPD}=14$ au, and
increases with decreasing CPD size because the population in the $J=2$ level
decreases with higher CPD temperature. The CPD-to-planet mass ratio is $M_{\rm
CPD}/M_{p}\gtrsim 0.02$.

In the discussion above, we assumed a CO-to-H2 ratio inferred from the local
interstellar medium (ISM). However, near the radial location of the CPD
candidate, CO in the AS 209’s circumstellar disk is inferred to be depleted in
the gas phase by about an order of magnitude compared with the local ISM
(Zhang et al., 2021). If the CO in the CPD is depleted at a comparable level
the CPD-to-planet mass ratio would be $M_{\rm CPD}/M_{p}\gtrsim 0.2$, a level
at which the CPD may be gravitationally unstable.

We can place upper limits to the CPD gas mass from the null detection of C18O
emission ($3\sigma\simeq 1.011~{}{\rm mJy~{}beam}^{-1}$) following the same
approach to 13CO. To do so, we assume that 13CO and C18O share the same line
width ($\simeq 400~{}{\rm m~{}s}^{-1}$) and temperature (presented in Figure
6a). We adopt $A_{21}=6.01\times 10^{-7}~{}{\rm s}^{-1}$ (Schöier et al.,
2005), $B_{e}=54.89142$ GHz (Müller et al., 2005), and $[{\rm C}^{18}{\rm
O}]/[^{12}{\rm CO}]=1/560$ (Wilson & Rood, 1994). The resulting upper limit to
the CPD gas mass is presented in Figure 6b. Interestingly, the null detection
of C18O places a tight upper limit that is only about $50\%$ larger than the
lower limit obtained with 13CO. When the CPD temperature is larger than the
lower limit presented in Figure 6a, both 13CO lower limit and C18O upper limit
would increase. However, because the rotational constant of the two lines are
comparable, the ratio between the upper limit inferred by C18O and the lower
limit inferred by 13CO would remain nearly constant (see Equation 4) and the
null detection of C18O would still provide tight constraints.

We can also place constraints on the CPD dust mass from the non-detection of
continuum emission ($3\sigma\simeq 26.4~{}\mu{\rm Jy}~{}{\rm beam}^{-1}$).
Adopting the DSHARP dust opacity (Birnstiel et al., 2018) with a maximum grain
size of 1 mm ($\kappa_{\nu}=2.0~{}{\rm cm^{2}~{}g^{-1}}$ at 240 GHz) and the
CPD temperature constrained as in Figure 6a, the CPD dust mass in the
optically thin regime is $\lesssim 0.006~{}M_{\earth}\simeq 0.47~{}{\rm
lunar~{}masses}$. If instead the CPD has only small, $\mu$m-sized grains,
$\kappa_{\nu}=0.42~{}{\rm cm^{2}~{}g^{-1}}$ (Birnstiel et al., 2018) and the
CPD dust mass is $\lesssim 0.027~{}M_{\earth}\simeq 2.2~{}{\rm
lunar~{}masses}$. These results are presented in Figure 6c.

Using the constraints on the CPD gas and dust mass, we infer the dust-to-gas
mass ratio to be $\lesssim 2\times 10^{-4}$ when the maximum grain size in the
CPD is 1 mm, and $\lesssim 9\times 10^{-4}$ when the maximum grain size in the
CPD is 1 $\mu$m (Figure 6d). This suggests that the CPD is likely lacking dust
at a level below that in the typical ISM environment, presumably due to a
limited dust supply (recall that AS 209’s continuum disk is confined within
the inner $\simeq 140$ au) and/or rapid radial drift of dust within the CPD.

###  4.3 How Did the Planet Form?

Based on the width of the 12CO gap, we infer that the planet mass is around a
Jupiter-mass. How did a giant planet form at an orbital radius of 200 au?

One possibility is that the AS 209 disk was gravitationally unstable in the
past and the planet formed via gravitational instability (GI; Boss 1997).
Limitations of the GI scenario often mentioned in the literature are that the
masses of GI-induced fragments are generally large, often in the regime of
brown dwarfs, and that GI-induced fragments suffer from tidal disruption
and/or rapid radial migration (Baruteau et al., 2011; Zhu et al., 2012).
However, recent magnetohydrodynamic simulations including the disk’s self-
gravity have shown that magnetic fields can limit the mass of the GI-induced
fragments to a fraction to a few Jupiter masses, and also can prevent
fragments from being tidally disrupted (Deng et al., 2021). It is also shown
that orbital migration can stall when the planet actively accretes and carves
a deep gap (Fletcher et al., 2019).

Despite recent theoretical developments, one challenge to the GI scenario is
that the AS 209 disk has a very small mass of $0.003-0.0045~{}M_{\odot}\simeq
3.1-4.7~{}M_{\rm Jup}$ (Favre et al., 2019; Zhang et al., 2021). According to
the inferred gas surface density profile from Zhang et al. (2021) and the
midplane temperature constrained by CO isotopologues from Law et al. (2021b),
the Toomre $Q$ parameter (Toomre & Toomre, 1972) is $>100$ at all radii in the
AS 209 disk with $Q\simeq 300$ at 200 au, indicating that the disk must be
gravitationally stable presently. While it is not impossible that the disk was
sufficiently massive in the past, the small present-day disk mass implies that
the disk should have lost its mass very efficiently since then.

An alternative to the GI scenario is pebble accretion (Johansen & Lacerda,
2010; Ormel & Klahr, 2010). In order for the pebble accretion scenario to
work, a few things have to be reconciled. First, the continuum emission of the
AS 209 disk is confined within the inner $\simeq 140$ au presently. How did
the core grow and how did the planet not trap millimeter grains beyond its
orbit? We propose that in this scenario, the core of the planet never reached
the pebble isolation mass (Morbidelli & Nesvorny, 2012) and pebbles could
freely drift inward crossing the planet’s orbit. Based on scaling relations
from hydrodynamic simulations (e.g., Lambrechts et al., 2014; Bitsch et al.,
2018), the pebble isolation mass at 200 au in the AS 209 disk ($h/r=0.118$;
Law et al. 2021b) is expected to be $\gtrsim 100~{}M_{\earth}$, much more
massive than the critical core mass of $M_{\rm crit}=10-20~{}M_{\earth}$ at
which point runaway gas accretion can start (Mizuno, 1980). We can thus
envision a scenario in which the pebbles that existed beyond the orbit of the
core migrated inward, leaving the core behind.

Second, we need to check if there existed sufficient pebbles to form a core of
the giant planet. To do so, we first estimate the total mass of pebbles
necessary to drift toward the core to grow to $10~{}M_{\earth}$ following
Lambrechts & Johansen (2014). We obtain $M_{\rm peb}\approx
130~{}M_{\earth}\cdot(M_{c}/20M_{\earth})^{1/3}(r/5~{}{\rm au})^{1/2}({\rm
St}/0.05)^{1/3}\approx 650~{}M_{\earth}$ assuming that the dominant pebble
size has a Stokes number of 0.05. The total dust mass in the AS 209 disk is
currently about $300~{}M_{\earth}$ (Sierra et al., 2021), but given that it is
very unlikely that all the millimeter grains currently inward of the planet’s
orbit were once beyond 200 au, more efficient pebble accretion than the
standard pebble accretion model of Lambrechts & Johansen (2014) is likely
required. A few possibilities include the presence of pressure bumps (Pinilla
et al., 2012) or a vortex that might have formed early on from the infalling
flows from the protostellar envelope (Bae et al., 2015), and changes of
sticking properties of grains that lead to the traffic jam effect around snow
lines (Drkazkowska & Alibert, 2017), which can slow down or even halt the
radial drift of grains. In fact, it is interesting to note that the midplane
gas temperature at the current radial location of the planet ($T\simeq 22$ K;
Law et al. 2021b) is close to the expected CO and N2 freezeout temperature
($T_{\rm frz,CO}=19-24$ K, $T_{\rm frz,N_{2}}=17-21$ K; Huang et al. 2018).

Third, we need to make sure that there had been sufficient time for a core to
form via pebble accretion. The timescale for pebble accretion to form a core
is given by $t_{\rm PA}\approx 4\times 10^{4}$ years $\cdot(M_{\rm
crit}/10M_{\earth})^{1/3}(r/5~{}{\rm au})$ (Lambrechts & Johansen, 2012).
Adopting a critical core mass of $10~{}M_{\earth}$, at 200 au the pebble
accretion time scale is $t_{\rm PA}\approx 1.6$ Myr although, as mentioned
earlier, slower radial drift or particle trapping can enhance the pebble
accretion efficiency and shorten the required time to form the core. Given
that the estimated age of AS 209 is $1-2$ Myr (Andrews et al., 2009; Öberg et
al., 2021), the long core accretion timescale might indicate that the planet
could have entered the runaway accretion phase in the last $\lesssim 1$ Myr.
If the planet has been accreting $\simeq 1~{}M_{\rm Jup}$ over the last
million year or so, the average accretion rate would be $\simeq
10^{-6}~{}M_{\rm Jup}~{}{\rm yr}^{-1}$, orders of magnitude larger than the
accretion rate of PDS 70b and c measured from H$\alpha$ line emission
($10^{-8\pm 1}~{}M_{\rm Jup}~{}{\rm yr}^{-1}$; Wagner et al. 2018; Haffert et
al. 2019). Observations of H$\alpha$ line can confirm if the planet is indeed
undergoing a runaway accretion, although long-term monitoring observations are
likely required to distinguish steadily high accretion from episodic burst-
type accretion (Lubow & Martin, 2012).

###  4.4 Future Observational Direction

We conclude the discussion by listing a few future directions from the
observational perspective. While we could only place upper/lower limits from
the existing data, future higher-level transition observations of 13CO
(together with the existing 13CO $J=2-1$ data) will allow us to determine
whether 13CO is optically thick or thin, and thus to place stronger
constraints on the CPD temperature. As shown in Figure 6b, the null detection
of C18O already placed a tight upper limit to the CPD gas mass that is only
about $50~{}\%$ larger than the lower limit inferred by 13CO. Deeper C18O
observations (by a factor of $\simeq 2$ in sensitivity assuming that isotope
selective photodissociation is not important) should thus detect the CPD and
place stringent constraints on the CPD gas mass. The existing data is limited
by a $200~{}{\rm m~{}s}^{-1}$ velocity resolution and we presently cannot
conclude whether the point source emission is consistent with a rotationally-
supported disk or a pressure-supported envelope that is potentially heated by
an embedded planet (e.g., Alves et al., 2020). Future observations with higher
velocity resolution will enable us to distinguish the two scenarios and to
dynamically constrain the planet mass from the rotation of the CPD. High
spatial/velocity resolution observations will also enable us to constrain the
geometry of the CPD (e.g., position angle, inclination) from the disk
rotation. Knowing the CPD geometry will allow us to place more accurate
constraints on the CPD temperature and mass. In addition, estimating the
obliquity of the CPD can help us to better understand the formation mechanism
and environment of the CPD-hosting planet, in particular whether the planet
had formed in a turbulent environment that would induce large obliquities (see
e.g., Bryan et al., 2020; Jennings & Chiang, 2021). The inferred CPD
temperature suggests that heating sources localized to the CPD must be
present. Such planetary/CPD heating can produce chemical asymmetries in the
circumstellar disk, which are shown to be detectable with high-sensitivity
molecular line observations using ALMA (Cleeves et al., 2015). Besides
additional ALMA observations, future observations in the infrared wavelengths
using the James Webb Space Telescope and ground-based telescopes would
independently confirm the planet/CPD and allow us to probe the thermal
emission from the planet which can help to constrain the mass of the planet.
In addition, searching for accretion signatures (e.g., H$\alpha$ line
emission) would yield invaluable constraints on the current position of the
planet in its evolutionary stages.

##  5 Conclusion

We report the discovery of a CPD candidate in the 13CO $J=2-1$ emission,
embedded in the AS 209 disk at a radial distance of about 200 au. This is the
first instance of CPD detection via gaseous emission, allowing us to probe the
overall CPD mass. The CPD candidate is located in the middle of an annular gap
identified in 12CO and near-infrared scattered light observations, and is
associated with localized velocity perturbations in 12CO (see Figure 5). The
coincidence of these features strongly suggest that we are witnessing the
signature of a giant planet and its CPD embedded in the AS 209 disk.

The CPD is spatially unresolved with a $117\times 82$ mas beam, indicating
that its diameter is $\lesssim 14$ au. Based on the 13CO intensity, we were
able to constrain the CPD temperature and gas mass. The CPD temperature is
$\gtrsim 35$ K, greater than the circumstellar disk temperature at the
midplane of the radial location of the CPD, 22 K (Law et al., 2021b). This
suggests that there must be heating sources localized to the CPD in order to
maintain the high temperature. Potential sources include planet’s
thermal/accretion heating, CPD’s internal viscous/turbulent heating, and
shock/compressional heating by infalling circumstellar disk material. The CPD
gas mass is $\gtrsim 0.095~{}M_{\rm Jup}\simeq 30~{}M_{\earth}$, adopting a
standard 13CO abundance. Based on the non-detection of continuum emission
toward the CPD location, we found that the CPD has $\lesssim
0.027~{}M_{\earth}\simeq 2.2~{}{\rm lunar~{}masses}$ of dust. Together with
the inferred gas mass, this indicates a low dust-to-gas mass ratio of
$\lesssim 9\times 10^{-4}$ within the CPD, presumably due to a limited dust
supply to the CPD and/or rapid radial drift of dust within the CPD.

The estimated age of the system is only $1-2$ Myr (Andrews et al., 2009; Öberg
et al., 2021). If confirmed, this CPD-hosting planet would be one of the
youngest exoplanets detected to date. Observing planets at this young age
allows us to place strong constraints on the mechanism and timescale of planet
formation, crucial to gaining new insights into the formation and evolution of
giant planets.

## Acknowledgments

We thank the anonymous referee for providing us with a helpful report that
improved the initial manuscript. M.B. acknowledges support from the European
Research Council (ERC) under the European Union’s Horizon 2020 research and
innovation programme (PROTOPLANETS, grant agreement No. 101002188). Y.A.
acknowledges support by NAOJ ALMA Scientific Research Grant code 2019-13B,
Grant-in-Aid for Scientific Research (S) 18H05222, and Grant-in-Aid for
Transformative Research Areas (A) 20H05847. J.B.B. acknowledges support from
NASA through the NASA Hubble Fellowship grant #HST-HF2-51429.001-A awarded by
the Space Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Incorporated, under NASA contract
NAS5-26555. G.C. acknowledges support by NAOJ ALMA Scientific Research Grant
code 2019-13B. L.I.C. acknowledges support from the David and Lucille Packard
Foundation, NSF AAG AST-1910106, and NASA ATP 80NSSC20K0529. V.V.G. gratefully
acknowledges support from ANID BASAL projects ACE210002 and FB210003, and by
ANID, – Millennium Science Initiative Program – NCN19_171. Support for J.H.
was provided by NASA through the NASA Hubble Fellowship grant #HST-
HF2-51460.001-A awarded by the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy, Inc.,
for NASA, under contract NAS5-26555. J.D.I. acknowledges supports from the
Science and Technology Facilities Council of the United Kingdom (STFC) under
ST/ T000287/1 and an STFC Ernest Rutherford Fellowship (ST/W004119/1) and a
University Academic Fellowship from the University of Leeds. N.K. acknowledges
support provided by the Alexander von Humboldt Foundation in the framework of
the Sofja Kovalevskaja Award endowed by the Federal Ministry of Education and
Research. C.J.L. acknowledges funding from the National Science Foundation
Graduate Research Fellowship under Grant No. DGE1745303. K.I.Ö. acknowledges
support from the Simons Foundation (SCOL #321183) and an NSF AAG Grant
(#1907653). L.P. gratefully acknowledges support by the ANID BASAL projects
ACE210002 and FB210003, and by ANID, – Millennium Science Initiative Program –
NCN19_171. K.S. acknowledges support from the European Research Council under
the European Union’s Horizon 2020 research and innovation program under grant
agreement No. 832428-Origins. A.S. acknowledges support from ANID FONDECYT
project No. 3220495. C.W. acknowledges financial support from the University
of Leeds, the Science and Technology Facilities Council, and UK Research and
Innovation (grant numbers ST/T000287/1 and MR/T040726/1). This paper makes use
of the following ALMA data:

  * 1.

https://almascience.nrao.edu/aq/?project_code=2013.1.00226.S (catalog
ADS/JAO.ALMA#2013.1.00226.S)

  * 2.

https://almascience.nrao.edu/aq/?project_code=2015.1.00486.S (catalog
ADS/JAO.ALMA#2015.1.00486.S)

  * 3.

https://almascience.nrao.edu/aq/?project_code=2016.1.00484.L (catalog
ADS/JAO.ALMA#2016.1.00484.L)

  * 4.

https://almascience.nrao.edu/aq/?project_code=2018.1.01055.L (catalog
ADS/JAO.ALMA#2018.1.01055.L)

ALMA is a partnership of ESO (representing its member states), NSF (USA) and
NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI
(Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA
Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio
Astronomy Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc.

##  Appendix A Additional channel maps

Figure 7 presents the 12CO channel maps for the image cube with a half-channel
velocity shift. Figures 8 and 9 present the 13CO channel maps for the fiducial
image cube and the cube with a half-channel velocity shift.

Figure 7: Channel maps showing the 12CO $J=2-1$ emission from the half-
channel-shifted image cube. The color map starts at $5\sigma$. The synthesized
beam is shown in the lower left corner of each panel. An interactive version
of the figure made with TRIVIA is available online. Controls at the top allow
the user to zoom, pan, and rescale the images. The controls at the bottom
include a slider and play/stop buttons to cycle through the channel maps.
Placing the cursor in the image will display positional offset and intensity.
Figure 8: Channel maps showing the 13CO $J=2-1$ emission. The color map starts
at $5\sigma$. The synthesized beam is shown in the lower left corner of each
panel. An interactive version of the figure made with TRIVIA is available
online. The controls at the bottom include a slider and play/stop buttons to
cycle through the channel maps. Placing the cursor in the image will display
positional offset and intensity. Figure 9: Same as Figure 8, but for the half-
channel-shifted image cube. An interactive version of the figure made with
TRIVIA is available online. The controls at the bottom include a slider and
play/stop buttons to cycle through the channel maps. Placing the cursor in the
image will display positional offset and intensity.

##  Appendix B 13CO emission with various imaging parameters

Tables 1 and 2 present a summary of the 13CO point source in the images
adopting various robust parameters from $-1$ to 2. The rms noise is measured
within the synthesized beam around the CPD over the first 20 line-free
channels of the cube ($v_{\rm LSR}=14.7-10.9~{}{\rm km~{}s}^{-1}$). The
central velocity, FWHM, peak intensity, and integrated intensity are obtained
from Gaussian fitting of the emission within the synthesized beam around the
CPD, using the CASA task specfit.

In Figure 10, we present the peak and integrated intensities as a function of
the beam minor axis. The peak and integrated intensities remain constant
within uncertainties when the beam minor axis is $\lesssim 120$ mas,
indicating that the point source is spatially unresolved at those scales. The
peak and integrated intensities increase for larger beam sizes because the
beam contains emission from the AS 209 disk.

Table 1: Summary of CPD Properties in Various JvM-corrected Images  Briggs | Beam | PA | rms Noise | Central Velocity | FWHM | Peak Intensity | Integrated Intensity
---|---|---|---|---|---|---|---
parameter | (mas $\times$ mas) | (deg) | (mJy beam-1) | (km s-1) | (km s-1) | (mJy beam-1) | (mJy beam-1 km s-1)
-1 | $118\times 83$ | $-84.561$ | 1.184 | $4.813\pm 0.031$ | $0.388\pm 0.079$ | $5.41\pm 0.92$ | $2.23\pm 0.59$
-0.75 | $118\times 83$ | $-84.943$ | 1.098 | $4.809\pm 0.029$ | $0.384\pm 0.075$ | $5.52\pm 0.90$ | $2.26\pm 0.57$
-0.5 | $119\times 84$ | $-85.306$ | 1.015 | $4.813\pm 0.027$ | $0.381\pm 0.071$ | $5.71\pm 0.88$ | $2.32\pm 0.56$
-0.25 | $120\times 85$ | $-85.660$ | 0.950 | $4.815\pm 0.026$ | $0.385\pm 0.067$ | $5.77\pm 0.84$ | $2.36\pm 0.54$
0 | $123\times 88$ | $-86.495$ | 0.788 | $4.810\pm 0.021$ | $0.381\pm 0.056$ | $5.90\pm 0.71$ | $2.39\pm 0.45$
0.25 | $130\times 94$ | $-87.737$ | 0.624 | $4.812\pm 0.017$ | $0.393\pm 0.043$ | $5.75\pm 0.52$ | $2.40\pm 0.34$
0.5 | $140\times 104$ | $-89.062$ | 0.457 | $4.812\pm 0.012$ | $0.404\pm 0.029$ | $5.65\pm 0.35$ | $2.43\pm 0.23$
0.75 | $155\times 118$ | $+89.836$ | 0.327 | $4.815\pm 0.080$ | $0.418\pm 0.020$ | $6.04\pm 0.24$ | $2.69\pm 0.16$
1 | $172\times 133$ | $-89.898$ | 0.243 | $4.819\pm 0.061$ | $0.439\pm 0.015$ | $6.56\pm 0.19$ | $3.07\pm 0.14$
1.25 | $187\times 146$ | $-88.779$ | 0.208 | $4.824\pm 0.005$ | $0.447\pm 0.012$ | $7.09\pm 0.17$ | $3.37\pm 0.12$
1.5 | $195\times 154$ | $-88.138$ | 0.195 | $4.827\pm 0.005$ | $0.453\pm 0.012$ | $7.60\pm 0.17$ | $3.67\pm 0.12$
1.75 | $198\times 157$ | $-86.890$ | 0.191 | $4.823\pm 0.005$ | $0.455\pm 0.011$ | $7.79\pm 0.17$ | $3.77\pm 0.12$
2 | $200\times 158$ | $-86.819$ | 0.190 | $4.828\pm 0.005$ | $0.453\pm 0.011$ | $7.89\pm 0.17$ | $3.80\pm 0.13$
Table 2: Summary of CPD Properties in Various JvM-corrected Images with a Half-channel Velocity Shift  Briggs | Beam | PA | rms Noise | Central Velocity | FWHM | Peak Intensity | Integrated Intensity
---|---|---|---|---|---|---|---
parameter | (mas $\times$ mas) | (deg) | (mJy beam-1) | (km s-1) | (km s-1) | (mJy beam-1) | (mJy beam-1 km s-1)
-1 | $117\times 82$ | $-85.632$ | 1.268 | $4.806\pm 0.026$ | $0.301\pm 0.042$ | $7.70\pm 1.00$ | $2.45\pm 0.47$
-0.75 | $117\times 83$ | $-85.692$ | 1.256 | $4.806\pm 0.025$ | $0.293\pm 0.039$ | $8.00\pm 1.00$ | $2.50\pm 0.46$
-0.5 | $118\times 83$ | $-85.828$ | 1.221 | $4.807\pm 0.024$ | $0.294\pm 0.038$ | $8.05\pm 0.98$ | $2.52\pm 0.45$
-0.25 | $119\times 84$ | $-86.245$ | 1.105 | $4.808\pm 0.022$ | $0.286\pm 0.033$ | $8.33\pm 0.90$ | $2.54\pm 0.40$
0 | $122\times 87$ | $-86.972$ | 0.977 | $4.809\pm 0.019$ | $0.281\pm 0.028$ | $8.64\pm 0.80$ | $2.59\pm 0.35$
0.25 | $128\times 93$ | $-88.135$ | 0.727 | $4.815\pm 0.014$ | $0.284\pm 0.022$ | $8.32\pm 0.59$ | $2.52\pm 0.27$
0.5 | $138\times 103$ | $-89.390$ | 0.543 | $4.819\pm 0.010$ | $0.292\pm 0.017$ | $8.03\pm 0.42$ | $2.50\pm 0.20$
0.75 | $153\times 116$ | $+89.917$ | 0.370 | $4.825\pm 0.006$ | $0.300\pm 0.011$ | $8.32\pm 0.29$ | $2.66\pm 0.14$
1 | $170\times 132$ | $-89.857$ | 0.272 | $4.828\pm 0.005$ | $0.319\pm 0.009$ | $9.00\pm 0.23$ | $3.05\pm 0.12$
1.25 | $185\times 144$ | $-89.430$ | 0.233 | $4.834\pm 0.004$ | $0.335\pm 0.008$ | $9.54\pm 0.21$ | $3.40\pm 0.11$
1.5 | $195\times 153$ | $-87.999$ | 0.218 | $4.836\pm 0.004$ | $0.346\pm 0.008$ | $9.85\pm 0.21$ | $3.62\pm 0.12$
1.75 | $198\times 157$ | $-86.536$ | 0.213 | $4.834\pm 0.004$ | $0.344\pm 0.008$ | $10.31\pm 0.21$ | $3.78\pm 0.11$
2 | $200\times 157$ | $-87.365$ | 0.212 | $4.836\pm 0.004$ | $0.345\pm 0.008$ | $10.45\pm 0.21$ | $3.84\pm 0.12$
Figure 10: (Left) Peak intensity as a function of the beam minor axis. (Right)
Integrated intensity as a function of the beam minor axis. Note that both peak
intensity and integrated intensity stay constant when the beam is $\lesssim
120$ mas, suggesting that the CPD is spatially unresolved. When the beam is
$>120$ mas the peak intensity and integrated intensity increase due to the
contamination from the circumstellar disk. In both panels, blue points present
intensities in the fiducial image cube whereas red points present intensities
in the image cube where half-channel shift is applied.

##  Appendix C Analysis with JvM-uncorrected images

Figure 11 presents 13CO channel maps without JvM correction. Note that the CPD
is clearly seen in all three channels even before the JvM correction is
applied, with the peak intensity of 8.80, 13.5, and $8.47~{}{\rm
mJy~{}beam}^{-1}$, respectively. The peak intensity-to-noise ratio is about 6,
9, and 6 in the three channels, indicating that the detection is statistically
significant in the JvM-uncorrected image.

In Figure 12, we present radial integrated intensity profiles. At $1\farcs 7$,
the integrated intensity of the point source is $4.37~{}{\rm
mJy~{}beam}^{-1}~{}{\rm km~{}s}^{-1}$, while the integrated intensity
excluding the point source is $2.02~{}{\rm mJy~{}beam}^{-1}~{}{\rm
km~{}s}^{-1}$ with a standard deviation of $\sigma=0.77~{}{\rm
mJy~{}beam}^{-1}~{}{\rm km~{}s}^{-1}$. After subtracting the contribution from
the gap, the point source is detected at a $4.4\sigma$ significance.

Figure 11: Same as Figure 2, but without the JvM correction. Note that the CPD
is clearly seen in the JvM-uncorrected image. Figure 12: Same as Figure 3, but
the radial integrated intensity profiles are derived using the JvM-uncorrected
image.

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