∎

11institutetext: J.M. Alcalá 22institutetext: INAF-Osservatorio Astronomico di
Capodimonte, via Moiariello 16, 80131 Napoli, Italy
22email<EMAIL_ADDRESS>33institutetext: G. Cupani 44institutetext: INAF-
Osservatorio Astronomico di Trieste, Via G.B. Tiepolo, 11, I-34143 Trieste,
Italy
IFPU–Institute for Fundamental Physics of the Universe, Via Beirut, 2, I-34151
Trieste, Italy 55institutetext: C. J. Evans 66institutetext: UK Astronomy
Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
77institutetext: M. Franchini 88institutetext: INAF-Osservatorio Astronomico
di Trieste, Via G.B. Tiepolo, 11, I-34143 Trieste, Italy 99institutetext: B.
Nisini 1010institutetext: INAF-Osservatorio Astronomico di Roma, Via di
Frascati 33, 00078 Monte Porzio Catone, Italy

# Accretion and Outflows in Young Stars with CUBES††thanks: Based on
observations made with the VLT under programs 089.C-0143(A) and 097.C-0741(A)

J.M. Alcalá  G. Cupani  C. J. Evans  M. Franchini  B. Nisini

(Received: date / Accepted: date)

###### Abstract

The science case on studies of accretion and outflows in low-mass
($<$1.5$M_{\odot}$) young stellar objects (YSOs) with the new CUBES instrument
is presented. We show the need for a high-sensitivity, near-ultraviolet (NUV)
spectrograph like CUBES, with a resolving power at least four times that of
X-Shooter and combined with UVES via a fibrelink for simultaneous
observations. Simulations with the CUBES exposure time calculator and the end-
to-end software show that a significant gain in signal-to-noise can be
achieved compared to current instruments, for both the spectral continuum and
emission lines, including for relatively embedded YSOs. Our simulations also
show that the low-resolution mode of CUBES will be able to observe much
fainter YSOs (V $\sim$22 mag) in the NUV than we can today, allowing us extend
studies to YSOs with background-limited magnitudes. The performance of CUBES
in terms of sensitivity in the NUV will provide important new insights into
the evolution of circumstellar disks, by studying the accretion, jets/winds
and photo-evaporation processes, down to the low-mass brown dwarf regime.
CUBES will also open-up new science as it will be able to observe targets that
are several magnitudes fainter than those reachable with current instruments,
facilitating studies of YSOs at distances of $\sim$ kpc scale. This means a
step-change in the field of low-mass star formation, as it will be possible to
expand the science case from relatively local star-forming regions to a large
swathe of distances within the Milky Way.

###### Keywords:

Stars: pre-main sequence, low-mass – Accretion, accretion disks –
protoplanetary disks stars: variables: T Tauri CUBES instrument

##  1 Introduction

The study of protoplanetary disks is a rapidly growing research field.
Protoplanetary disks play a pivotal role in determining the initial conditions
for planet formation, and many of their characteristics are only now being
unveiled by new observational facilities at high angular and spectral
resolution, over a wide range of wavelengths, from X-ray to radio.

The way in which circumstellar disks evolve and form protoplanets is strongly
influenced by the processes of mass accretion onto the star, ejection of
outflows and photo-evaporation in winds of the disk material (hartmann16 ,
ercolanopascucci17 ). A proper understanding of the impact of these phenomena
requires comprehensive study of different physical processes throughout the
first 107 yr of the star-disk evolution.

Observationally, the study of these processes is conducted via multi-
wavelength investigations, in particular by studying classical T Tauri stars
(CTTS), which are young (a few 106 yr), very low- to solar-mass stars that are
actively accreting mass from planet-forming disks. In the current
magnetospheric-accretion paradigm for CTTS, the strong stellar magnetic fields
truncate the inner disk at a few stellar radii (DonatiLandstreet09 , johns-
krull13 ). Gas flows from this location onto the star along the stellar
magnetic field lines, forming an accretion shock at the stellar surface. The
heated (T$\sim$104K) optically-thick, postshock gas and optically-thin, pre-
shock gas emits in the Balmer and Paschen continua and in many spectral lines
(hartmann16 ). At the same time, magnetically driven winds carry away the
angular momentum of the accreting gas, thereby preventing the star from
spinning up. Meanwhile, the accretion shocks produce strong UV and X-ray
emission (e.g. bonito14 ) that irradiates and photo-evaporates the disk.

Through spectroscopic surveys of young stellar objects (YSOs) in nearby (d $<$
500pc) star-forming regions, the mutual relationships between accretion, jets
and disk structure have been addressed (e.g. alcala17 , giannini15  frasca17
manara17  nisini18 ), but further aspects remain unexplored, mainly because of
the low sensitivity and, in some cases, the limited spectral resolution
currently available in the near-ultraviolet (NUV). For instance, YSOs in
distant ($\sim$kpc scale) star-forming regions, where low metallicity effects
may have an important impact on the accretion process, have been poorly
studied.

Development of the new Cassegrain U-Band Efficient Spectrograph (CUBES) for
the Very Large Telescope (VLT) zanutta22  may enable important progress in the
investigation of accretion and winds-outflows in solar-type young stars.

In this contribution we present the CUBES science case on accretion and wind-
outflows in YSOs. Henceforth we use the terms CTTS or YSOs to refer to solar-
type young stellar objects. In Sect. 2 relevant previous work and the need for
a NUV instrument with high spectral resolution and sensitivity for studies of
YSOs is highlighted. In Sect. 3 the expected performances of CUBES on typical
solar-type YSOs are presented, taking into account the possibility of a
fibrelink to the Ultraviolet and Visible Echelle Spectrograph (UVES) on the
VLT. For background context, Sect. 4 introduces relevant ongoing YSO projects
that will influence CUBES operations, and in Sect. 5 we summarise some of the
key topics in the field that CUBES will address. Finally, a summary and
conclusions are presented in Sect. 6.

##  2 Previous work and the need for sensitivity and spectral resolution

###  2.1 Measurements of accretion rates

The mass accretion rate, $\dot{M}_{\rm acc}$, can be derived from the energy
released per unit time in the accretion shock (accretion luminosity $L_{\rm
acc}$; see gullbring98 , hart98 ) given the stellar properties, in particular
stellar mass $M_{\star}$ and radius $R_{\star}$. Observationally, this
requires measurements of excess flux in the continuum and lines with respect
to similar non-accreting template stars. Such measurements are best performed
at ultraviolet (UV) wavelengths ($\lambda$ $<$ 4000 Å) with the Balmer
continuum excess emission and the Balmer jump (see HH08 , ingleby13 , alcala14
, alcala17 , manara17 , and references therein).

On the other hand, accretion is a highly variable process (basribatalha90 ,
jayawardhana06 , cody10 , venuti14 ) which leads to a range of $\dot{M}_{\rm
acc}$ values for a given object when measured at different epochs (see
costigan12 , costigan14 , biazzo12 ). Variability in YSOs induces dispersion
in $\dot{M}_{\rm acc}$ hence, in the observed $\dot{M}_{\rm acc}$–$M_{\star}$
and $\dot{M}_{\rm acc}$–$M_{\rm disk}$ scaling relationships, but cannot
explain the large scatter of more than 2 dex in $\log{}$$\dot{M}_{\rm acc}$ at
a given YSO mass. Such scaling relationships are predicted by the theory of
viscous disk evolution (lynden-bell74 , hartmann16 , rosotti17  and references
therein) but the $\dot{M}_{\rm acc}$–$M_{\rm disk}$ relationship has been
confirmed observationally only recently by spectroscopic surveys in strong
synergy with ALMA surveys of disks in star-forming regions (ansdell16 ,
manara16 , pascucci16 , mulders17 ).

####  2.1.1 The Balmer continuum

The continuum excess emission in YSOs is most easily detected as Balmer
continuum emission (see valenti93 , gullbring98  and references therein). Such
continuum excess emission has been used in the past to derive the accretion
luminosity, $L_{\rm acc}$, by fitting the YSO spectra with the sum of the
photospheric spectrum of a non-accreting template and the emission of a slab
of hydrogen (see HH08 , rigliaco12 , manara17 , alcala14 , alcala17  and
references therein). The accretion luminosity is given by the luminosity
emitted by the slab. Figure 1 shows an example of the procedure in the region
of the Balmer jump, applied to the X-Shooter spectrum of the T Tauri star GQ
Lup.

Figure 1: Example X-Shooter spectrum of the classical T Tauri star GQ Lup in
the region of the Balmer jump (in red). The spectrum of a non-accreting YSO
template of the same spectral type as GQ Lup is shown in green. The continuum
emission from a hydrogen slab is shown by the black continuous line. The best
fit to the data with the emission predicted by the slab model is shown in
blue. Figure adapted from alcala17 .

In the case of X-Shooter spectra, the wide wavelength range (3000-25000Å)
allows the stellar and accretion parameters to be self-consistently derived by
finding the best fit among a grid of slab models and using the continuum UV-
excess emission, while constraining both the spectral type of the target and
the interstellar extinction towards it. The best fit is found by minimizing a
$\chi^{2}_{\rm like}$ distribution (see Fig. 1 in manara17a ). The procedure
requires a good absolute flux calibration (better than $\sim$15%) throughout
the widest possible spectral range.

High sensitivity and spectral resolution in the NUV is needed for a better
definition of the Balmer jump, and to improve the fit to the continuum. This
is particularly important in cases of low accretion rates and YSOs with
spectral types earlier than about K3, where the contrast is low between
continuum excess emission and the photospheric + chromospheric emission
(alcala19  and see Sect. 5.1.2). A sensitivity greater than e.g. X-Shooter is
needed to detect the NUV excess emission in low-luminosity and slightly
embedded YSOs.

####  2.1.2 $L_{\rm acc}$–$L_{\rm line}$ relationships: issues with high
Balmer lines

It is well known that $L_{\rm acc}$, and therefore $\dot{M}_{\rm acc}$, is
correlated with the line luminosity, $L_{\rm line}$, of H i, He i and Ca ii
lines (e.g. muzerolle98 , calvet04 , HH08 , rigliaco12 , alcala14 , alcala17
and references therein). These latter works provide $L_{\rm acc}$–$L_{\rm
line}$ correlations simultaneously and homogeneously derived from the UV to
the near-infrared (NIR), underlying the importance of these emission features
as accretion diagnostics. These accretion tracers are key diagnostics with
which to estimate $L_{\rm acc}$ via the correlations mentioned above when
flux-calibrated spectra below $\lambda$$\sim$3700 Å are not available.
Examples of these correlations, drawn from measurements in X-Shooter spectra,
are shown in Fig. 2.

Figure 2: Examples of $L_{\rm acc}$–$L_{\rm line}$ correlations for Balmer
lines as labelled on each panel. Each point represents the $L_{\rm acc}$ and
$L_{\rm line}$ measurement from the X-Shooter spectrum of an individual YSO.
$L_{\rm acc}$ was determined from the slab modelling as explained in the text
(see also Fig. 1), while $L_{\rm line}$ is calculated from flux measurements
of the lines and adopting a distance to the individual YSO. Leftward arrows
represent upper limits on $L_{\rm line}$. The red lines represent the best
linear fit to the data. Figure adapted from alcala17 .

At low $L_{\rm acc}$ values, detection of the high Balmer lines is challenging
due to limited sensitivity. In fact, most of the lower-left points in Fig. 2
for the H 15 line are represented by upper limits on $L_{\rm line}$. As
concluded in previous works (e.g. rigliaco12 , alcala14 , alcala21 ), the
average $L_{\rm acc}$ and $\dot{M}_{\rm acc}$ derived from several
diagnostics, measured simultaneously, has a significantly reduced error and
can also be used to simultaneously check the extinction. This argues for
simultaneous spectroscopy from the UV to the longest practicable wavelengths,
at the highest possible sensitivity.

Note that difficulties at the high Balmer lines may arise because of line
blending, in particular when low low-resolution is used. Fig. 1 shows that the
crowding of the high Balmer lines leads to blending that effectively shifts
the Balmer limit from 3646Å to the apparent jump at $\sim$3700Å. Balmer lines
in strongly accreting YSOs are intrinsically broad ($\Delta$V $\gtrapprox$ 250
km/s) hence, the blending issues in such objects cannot be circumvented, even
at the highest available resolution. A high resolution, however, may help to
both better study the line profiles and disentangle narrow lines superimposed
on the H i profile (see e.g. Fig. 5).

####  2.1.3 X-Shooter–UVES comparison

In this practical example we use data of the YSO Sz 88 A, a T Tauri star in
the Lupus star-forming region. With an estimated $\dot{M}_{\rm
acc}$$\sim$3.2$\times$10-9$M_{\odot}$/yr (alcala17 , alcala19 ), this star is
an active accretor, hence its spectrum is rich in emission lines and exhibits
strong continuum excess emission in the UV. The interstellar extinction toward
the object is low ($A_{\rm V}$$=$0.25 mag, alcala19 ) and the source is
sufficiently bright (V$=$13.2 mag) for high-resolution spectroscopy with e.g.
UVES.

A comparison of X-Shooter and UVES data111Unpublished data from ESO programme
089.C-0143(A), PI. B. Nisini. for this object, in the spectral region around
the Balmer jump, is shown in Fig. 3. For comparison purposes, we have
normalised the UVES data to the X-Shooter flux at 3660 Å.

Figure 3: Portions of the X-Shooter (red) and UVES (blue) spectra of the YSO
Sz88A (large panel). The upper left insert shows the corresponding exposure
times, resolutions and S/N ratios. A zoom of the region around the Balmer jump
is shown in the lower panel. Balmer emission lines up to H 25 are labelled.
The UVES data were normalised to the X-Shooter flux at 3660 Å.

Whereas UVES provides a spectral resolution that is more than a factor of ten
larger than that from X-Shooter, the S/N ratio is seven times lower, despite
the much longer exposure time, even in an object as bright as Sz 88 A. Lines
up to H 25 can be detected in the X-Shooter spectrum, but they are not
apparent in the UVES data because of the low S/N. For the same reason,
structure is not seen in the line profiles in the UVES data. Note, however,
that the X-Shooter spectrum was acquired more than four years before the UVES
observation. Therefore, the non-detection of the high Balmer lines with UVES
could be a consequence of lower accretion activity in Sz 88 A during the UVES
observations. All this suggests the need for much higher sensitivity in the
NUV than that of UVES when aiming at detection of emission diagnostics during
low-accretion phases or in very weak accretors.

####  2.1.4 Physical conditions of the emitting gas

An important diagnostic for deriving the physical conditions (e.g. density and
temperature) of the accreting gas is to use the H i line ratios, in particular
by comparing the Balmer decrements with model predictions (e.g. KuanFischer11
). Based on the X-Shooter data by alcala17 , antoniucci17  presented a
detailed study of the Balmer, Paschen and Brackett decrements. These authors
analysed the Balmer decrements up to the H 15 line, using H$\beta$ ($=$H 4) as
the reference value.

As shown in Fig. 3, lines up to H 25 were detected in the X-Shooter spectrum
of Sz 88 A. Hence, we use these data here to exemplify the behaviour of the
decrements in the high Balmer lines. Line fluxes were measured on the same
X-Shooter spectrum, following the methodologies described in alcala17 . The
fluxes were then corrected for interstellar extinction ($A_{\rm V}$$=$0.25
mag, alcala17 ) adopting the extinction curve from fitzpatrick19 . The
resulting Balmer decrements are shown in Fig. 4. For the same reasons as
discussed in antoniucci17  we also used the H$\beta$ line as the reference
value. At the X-Shooter resolution, the H$\epsilon$ ($=$H 7) line is not
resolved from the Ca ii$\lambda$3969 Å line.

Figure 4: Balmer decrements as a function of quantum number for the star Sz 88
A (blue dots), where the H$\beta$ line (n=4) has been used as the reference
value. The line ratio corresponding to H$\epsilon$ (n$=$7) is shown as an
upper limit because of problems of blending with the Ca ii$\lambda$3969 Å
line. The green and red continuous and dashed lines show models from
KuanFischer11  with densities and temperatures as labelled.

For lines up to about H 15 the Balmer decrements in Sz 88 A follow the type-4,
or L-shape, trend as defined in antoniucci17 . As demonstrated by these
authors, this L-shape decrement is very well described by models
(KuanFischer11 ) of optically thick H i emitting gas with a temperature in the
range 7500–8500 K and densities 11.4$<\log$(NH)$<$12 (see also the two lower-
right panels in Fig. 11 of antoniucci17 ). However, the trend of the
decrements for lines with quantum number higher than $\sim$15 inverts the
slope, possibly indicating optically-thin emission from gas at lower
temperature. This behaviour was also observed by rigliaco11  in the hydrogen
spectrum of the brown dwarf 2MASS J05382543$-$0242412 (or $\sigma$-Ori 500)
based on X-Shooter data. The authors concluded that the physical conditions of
the emitting gas inferred from the hydrogen spectrum of this brown dwarf are
in contrast with the predictions of the magnetospheric accretion models, which
predict much higher temperatures. This kind of analysis can only be performed
for bright and highly accreting objects, where the high-n Balmer lines can be
detected. However, the understanding of discrepancies with models requires
accurate measurements of the Balmer decrement up to high n-number in samples
of objects with different accretion activity, which requires a greater
sensitivity than so far available and the highest possible spectral
resolution.

###  2.2 Wind/outflow tracers

Mass loss through highly collimated jets and slow disk winds is a key
mechanism driving accretion and contributing to disk dissipation. Previous
X-Shooter surveys of YSOs (nisini18 , nisini19 ) have studied the rate of
occurrence of collimated jets, deriving a tight link with the properties of
slow winds, and establishing a relationship between the rate of mass loss in
the outflow and the mass accretion rate (nisini19 ).

While the [O i] 6300 Å line has provided the historical foundation for
identifying and measuring jets and slow disk winds, for insights into the
physical conditions of the different outflow manifestations one needs to
observe several forbidden lines, probing different excitation regimes. In this
respect, a key line in the NUV spectral range is the [O ii] line at 3726 Å, as
the [O ii]3726 / [O i]6300 flux ratio can be directly used to infer the degree
of plasma ionisation. Knowledge of this is fundamental to infer the total gas
density (once the electron density is known from other line ratios, such as
the [O i]5577/6300 ratio) and thus to have a correct measurement of the mass-
loss rate, which is a critical parameter to constrain the different models for
disk dissipation and thus understand the protoplanetary disk evolution and
planet formation (e.g. Kunitomo2020 )

Gas ionization is low in T Tauri outflows (e.g. a fraction of 0.1-0.3) so the
[O ii] 3726 Å line is usually weak and therefore requires sensitive NUV
observations to detect it in different sources. In addition, and as
illustrated in Fig. 5, a spectral resolution higher than that provided by
X-shooter is also needed to resolve the line into components at different
velocities and thus measure the ionization fraction in each of them. This
example effectively shows that the [O ii] can be resolved into two components,
possibly of low and high velocity, that cannot be resolved at the X-Shooter
resolution. We note that the [O ii] is a doublet at 3726.0/3728.8 Å and what
we are seeing in Fig. 5 are two velocity components of the [O ii]3726.0 Å
line, separated by 0.7 Å i.e. about 56 km/s which is a typical velocity
separation in T Tauri outflows. The other component of the doublet at 3728.8 Å
(at 2.8 Å from the 3726.0 Å line) is barely visible above the noise in the
UVES spectrum, as expected having a radiative rate a factor of 10 smaller than
the 3726.0 Å line.

Notably, the intensity of the forbidden line in Fig. 5 remains approximately
the same, whereas the intensity of permitted lines, such as H 13 and H 14, is
clearly changing. As mentioned before, the latter may be a consequence of
variable accretion, as the UVES spectrum was acquired more than four years
after the X-Shooter observation.

Figure 5: Portions of the X-Shooter (red) and UVES (blue) spectra of Sz 88 A
in a spectral range around the [O ii] 3726 Å line. The spectra are the same as
in Fig. 3. The UVES data were normalised to the X-Shooter flux at 3660 Å.

##  3 CUBES performance for YSO studies

The current topics in YSO studies and limits of observations outlined in Sect.
2 argue for a high-sensitivity spectrograph such as CUBES, with a resolving
power at least four times that of X-Shooter, and possibly linked to UVES for
simultaneous observations with both instruments. We now present results from
simulations to quantify the potential performance of CUBES in the context of
YSO studies.

The primary operational mode of CUBES is expected to have a resolving power
R$\geq$20,000 over the spectral range from 3000 Å to 4050 Å. Taking advantage
of the high-sensitivity provided by CUBES in the near-UV, a second operational
mode with R$\approx$7,000 over the same spectral range is foreseen to allow
the investigation of background-limited objects for science cases not
requiring the high-resolution mode. In addition, a fibrelink to UVES for
simultaneous observations at longer wavelengths is included in the conceptual
design zanutta22 .

To investigate the instrument performance for the YSO cases we used the
exposure time calculator (ETC)222http://archives.ia2.inaf.it/cubes/#/etc and
the end-to-end simulator (E2E) software333https://cubes.inaf.it/end-to-end-
simulator developed for CUBES genoni21 . To apply these tools to our science
case, accurately flux-calibrated spectral templates of YSOs are needed, with
good S/N ($>$90) in the NUV, and with a resolution similar or higher than that
expected for CUBES. Real spectra, simultaneously satisfying such
characteristics, are not available, while reliable models of spectra
reproducing the complete YSOs phenomenology do not yet exist. Real data that
most closely satisfy these requirements are the X-Shooter spectra. Despite
their lower resolution, the general spectral shape can be used to predict the
S/N of the continuum (and the higher S/N expected for the emission lines). We
therefore used these as our input templates, in particular those of the YSOs
discussed in Sect. 2, namely GQ Lup and Sz 88 A. A shortcoming of the
X-Shooter data is that the S/N drops significantly for $\lambda$ $<$ 3250 Å
hence, for simplicity, we set the flux to zero below this wavelength.

###  3.1 High-resolution mode

##### Brighter objects (V$<$19 mag):

We used the X-Shooter observation of GQ Lup (Sz 75; SpT$=$K6, alcala19 ) as
our input template, assuming $A_{\rm V}$$=$0 mag. We used the CUBES ETC to
estimate the performance for V$=$10 mag and 19 mag, in adopted conditions of
seeing$=$1 arcsec and airmass=1.2, with predicted S/N ratios (at
$\lambda$=3600 Å) of $\sim$320 and 10, in 5 min and 1 hr, respectively. For an
input template with a weak continuum Balmer emission, and with an earlier
spectral type than GQ Lup (e.g. MY Lup, SpT$=$K0, alcala19 ), slightly lower
values of S/N are predicted, with S/N = 300 in 5 min and 9 in 1 hr for V$=$10
mag and 19 mag, respectively. Adopting V$=$18.5 mag (for the MY Lup template)
we find S/N$=$14 in 1 hr; this magnitude is typical of YSOs in the low-
metallicity region Sh2-284 cusano11 .

Figure 6: Results of the CUBES ETC for the faint case discussed in the text.
The template spectra for visual extinctions of $A_{\rm V}$$=$0 and 3 mag are
shown in the upper and lower left-hand panels, respectively. Note that the
unreddened flux of the template corresponds to the actual magnitude of Sz 88 A
(V$=$13.2 mag), but it is internally normalised to V$=$20 mag in the ETC. The
ETC results for 1.5hr exposures for the two $A_{\rm V}$ cases are shown in the
right-hand panels. The blue curves with dots in the right-hand panels show the
S/N ratio as a function of exposure time for $\lambda=$3600 Å. The resulting
S/N ratios at 3600 Å are labelled in red. All the plots were extracted from
the ETC.

Adopting a later M6 template (2MASS J16095628$-$ 3859518 or Lup818s in
alcala14 ) with V$=$19 mag and the same conditions in the ETC yielded
S/N$\approx$10 in 1 hr. This example highlights the interesting prospect of
systematically studying accreting brown dwarfs at relatively high resolution
in the UV for the first time.

##### Faint objects (V$>$19 mag):

For objects as faint as V$=$ 20 mag, we use the X-Shooter data of Sz 88 A as
the input template, which also has a very good S/N and was used to exemplify
the behaviour of the Balmer decrement in Sect. 2.1.4. This template is shown
in the left-hand panels of Fig. 6 for the cases of $A_{\rm V}$$=$0 and 3 mag
of visual extinction. Using the CUBES ETC and adopting conditions of
seeing$=$0.8 arcsec and airmass=1.2, we estimate a S/N of 12 and 6 at
$\lambda=$3600 Å for the two $A_{\rm V}$ values, respectively, in 1.5 hrs of
exposure and with binning $\times$2 in the spatial direction. The S/N ratio at
3600 Å as a function of exposure time is shown in the right-hand panels of
Fig. 6 for the two $A_{\rm V}$ values. We also used the E2E simulator with the
same template and observing assumptions and recovered an estimated S/N that
was in very good agreement with the ETC.

For comparison with the above results, we obtained S/N$\approx$5 at $\sim$3600
Å in our X-Shooter spectrum of the young brown dwarf 2MASS J05382543$-$
0242412 ($\sigma$-Ori 500 with V$=$20 mag, $A_{\rm V}$$=$0 mag, rigliaco11 ).
This 1.5 hr exposure was done under similar conditions as those assumed for
the ETC predictions above, meaning that in the faint example (with $A_{\rm
V}$$=$0) CUBES would provide data with twice the S/N and about four times
better resolution than X-Shooter. In addition, the quality of the CUBES data
for moderately embedded objects will be sufficient for the studies discussed
in Sec. 5.

It is worth mentioning that the S/N ratio per pixel provided by the ETC does
not take into account any optimal extraction of the 1-dimensional spectra and
therefore the predicted S/N value is expected to improve in case of optimal
extraction from the 2-dimensional images. Also, the S/N of the emission lines
will be higher than for the continuum.

Figure 7: Results from the CUBES E2E simulator for the low-resolution mode and
the faint case (V$=$20 mag) discussed in the text. The left-hand panels show
the resulting S/N as a function of wavelength for visual extinctions $A_{\rm
V}$$=$0 and 3 mag as indicated. Note the higher S/N of the emission lines with
respect to the continuum and the zero S/N for $\lambda<$3250 Å due to the cut
of flux in the input X-Shooter data. The right-hand panels show the noise
breakdown for the two corresponding $A_{\rm V}$ values. The two histograms in
each panel are the photons/pixel of each component as coloured and labelled in
the legend, for the two instrument arms (0 or 1 on the x-axis, the ‘blue’ and
‘red’ arms, respectively). Plots extracted from the E2E simulator.

###  3.2 Low-resolution mode

We make use of the E2E software to simulate the low-resolution mode of CUBES,
adopting the first configuration described in Sect. 4.2 of genoni21 , which
uses six slices of 1 arcsec each. With the same input parameters as in the
previous subsection (Sz 88 A template, V$=$20 mag, seeing $=$ 0.8 arcsec,
airmass$=$1.2, T${}_{\rm exp}=$ 1.5 hr, $\lambda=$3600 Å) we obtained the
results shown in Fig. 7, namely S/N values of 20 and 10 for the $A_{\rm
V}$$=$0 and 3 mag models, respectively, with a spatial binning $\times$4 and
wavelength binning $\times$3 (which still yields a sampling well above the
Nyquist limit). The noise breakdown histograms are also shown in the right-
hand panels of the figure. Note the limited contribution of the target on the
spectrograph arm ‘0’, which is due to the zero flux at $\lambda<$3250 Å in the
input template.

This simple exercise shows that a significant gain in S/N, both in continuum
and lines, can be achieved with the CUBES low-res. mode, and suggests that
observations of even fainter YSOs can be attempted if we are able to trade
spectral resolution for sensitivity. Hence, pushing to lower flux limits, for
V$=$22 mag and $A_{\rm V}$$=$0 mag, with T${}_{\rm exp}=$2 hr and adopting the
same parameters as before, the E2E simulator yields the results shown in Fig.
8. The predicted S/N is $\sim$5 at $\lambda=$3600 Å, but it is $>$10 for many
emission lines, which opens-up the possibility to extend studies done with
X-Shooter to YSOs with background-limited magnitudes.

We note that the result from the E2E simulator is the combination of the
extracted spectra from all the instrument slices (see genoni21 ); in our
simulations we only use three slices but most of the object’s flux will go
through the central slice, hence we expect an optimal extraction of this slice
would further improve the final S/N, compared to the estimates shown here.

Figure 8: Upper panel: the E2E input spectrum of Sz 88 A with and without
taking into account airmass (black dashed and continuous lines, respectively)
for the case of a V$=$22 mag YSO and $A_{\rm V}$$=$0 mag.; the background
contribution is shown with the blue line. Lower panel: the E2E output S/N as a
function of wavelength (dashed line) for a 2 hour exposure. Note the zero S/N
for $\lambda<$3250 Å due to the cut of flux of the input X-Shooter data. Plots
extracted from the E2E simulator.

###  3.3 Link with UVES

The simultaneous fit of the Balmer continuum and physical parameters of YSOs
is best when the widest possible spectral window is available, which requires
observations that extend redwards far beyond the CUBES range (see Sect. 2). A
fibrelink to UVES for observations with both instruments simultaneously would
make this possible. As accretion is a highly variable process, the
simultaneous link is necessary for the fitting procedure described in Sect. 2.

Introducing a fibrelink entails a reduction in the UVES throughput, so this is
only an efficient option if the performance of UVES with the fibrelink is
sufficiently good to deliver the required observations within the time needed
for CUBES observations. The conceptual design provides a 40% throughput for
the fibre-link at $>$5000 Å. We estimated the exposure times required to reach
a S/N=10, adopting the unreddened spectral template shown in Fig. 6 (in the
reddened case, the NUV performance will be the more limiting factor); for
airmass$=$1.2 and seeing$=$0.8 arcsec for:

  * $\bullet$

CUBES ETC: continuum near the [O ii] 3726 Å line, V$=$19.5 mag, with the high-
resolution option and binning $\times$2 spatially, gives S/N$\approx$10 in 1
hr.

  * $\bullet$

UVES ETC: continuum near the [O i] 6300 Å line, V$=$20.5 mag (scaled to mimic
the reduced throughput of the fibrelink compared to normal UVES operations),
with a 1.0” slit, and the red-arm (580 nm) setting. The latter combination
gives a resolution R$\approx$40,000 with sampling of $>$5 pixels. With
2$\times$2 binning we recover a S/N$\approx$5.5 in 1 hr. Binning this by a
further factor of three (oversampling, degrading resolution to match CUBES),
gives a comparable S/N in a similar exposure time; i.e. the UVES exposure time
fits within the CUBES observation.

Without the fibrelink, a red-arm UVES exposure of $\sim$1200s gives comparable
S/N (ignoring the preset and separate acquisition time), i.e. each combined
(CUBES + UVES) observation effectively saves half an hour of otherwise
separate UVES observations in this example.

##  4 Synergies with other projects

For background context of future programmes in this field, we now briefly
introduce some on-going large programmes on YSOs that will have strong
synergies with CUBES observations.

###  4.1 ALMA disk surveys

The Atacama Large Millimeter Array (ALMA) provides sufficient sensitivity and
resolution at sub-mm wavelengths to detect and measure the mass, $M_{\rm
disk}$, of dusty protoplanetary disks around YSOs with a mass down to 0.1
$M_{\odot}$ (ansdell16 , ansdell18 , pascucci16 , barenfeld16 , cazzoletti19
). Such surveys have provided the demographics of disks in nearby star-forming
regions. The synergy of these surveys with the studies of the properties of
the central objects, such as stellar mass, $M_{\star}$, and accretion rate,
$\dot{M}_{\rm acc}$, have provided important constraints on models of
viscously evolving disks by confirming the correlations of $M_{\rm disk}$ with
$M_{\star}$ and $\dot{M}_{\rm acc}$ (see Sect. 5.1.3), although with
significant scatter. However, these synergies did not include statistically
significant samples of YSOs with $M_{\star}$$\leq$ 0.1$M_{\odot}$, preventing
investigation of whether the scaling relationships are different for low-mass
substellar objects to those for stars. Future sensitive ALMA surveys of
compact and small disks in very low-mass YSOs, will allow investigation of the
correlations down to the low-mass brown dwarf regime.

###  4.2 ULLYSES & ODYSSEUS

The public Hubble Space Telescope UV Legacy Library of Young Stars as
Essential Standards (ULLYSES) survey will enable significant advances in the
study of astrophysical disk accretion processes. A total of 500 orbits of this
Directors Discretionary Time programme has been dedicated to far-UV
spectroscopy with the Cosmic Origins Spectrograph (at R$\approx$18,000) and
low-resolution NUV/optical spectroscopy with the Space Telescope Imaging
Spectrograph of tens of solar-type YSOs. The targets cover a range in stellar
mass, accretion rate, and age (e.g., alcala17 , manara17 ).

A large, international team has initiated the Outflows and disks around Young
Stars: Synergies for the Exploration of ULLYSES Spectra (ODYSSEUS) project to
bring together the broad expertise required to resolve the fundamental
problems on accretion and wind on YSOs through the investigations enabled by
ULLYSES espaillat22 , and to coordinate the ambitious datasets of simultaneous
and contemporaneous observations that will enhance the impact of the public
data (e.g. PENELLOPE manara21 ). We anticipate strong synergies of future
CUBES observations of YSOs with ODYSSEUS science products.

###  4.3 PENELLOPE

The ESO Large Programme (106.20Z8) PENELLOPE (manara21 , frasca21 ) at the VLT
is providing contemporaneous high-resolution (UVES, ESPRESSO) and UV-NIR mid–
resolution (X-Shooter) spectra of the ULLYSES targets to provide their
accretion/wind and stellar physical and kinematical properties. The project
involves more than 50 researchers and, together with ODYSSEUS, will provide
well characterized targets in terms of accretion/wind and stellar properties.
These will then serve as a bright benchmark sample for CUBES programmes.

###  4.4 GHOsT

The GIARPS High-resolution Observations of T Tauri stars (GHOsT) (giannini19 ,
gangi20 , alcala21 ) is a GIARPS@TNG optical/IR high-resolution spectroscopic
survey of a flux-limited, complete sample of T Tauri stars in the Taurus star-
forming region. This ongoing survey will homogeneously derive (i.e. avoiding
systematics due to non-simultaneous observations) stellar and
accretion/outflow parameters of sources in Taurus, and constrain the
properties of the gas in the inner disk regions. This survey is in synergy
with existing ALMA disk observations in the region (e.g. long19 ), so the
programme will directly link the processes occurring in the star-disk
interaction region and the overall disk structure, providing a unique, rich,
and robust observational reference for disk evolutionary models.

##  5 Studies of YSOs with CUBES

The high performance of CUBES in terms of spectral resolving power and
sensitivity will mostly broaden our knowledge on accretion and outflows in
YSOs, but will also open-up new science as it will be able to observe targets
that cannot be reached with current facilities. In this section we describe
some of the YSO topics that can be addressed with CUBES, and with the proposed
link to UVES. Except for line profile analysis (line shapes and peak position,
widths and kinematics), these studies require a good absolute flux calibration
($\sim$15% or better, see Sect. 2.1) throughout the widest spectral range. If
this requirement cannot be satisfied for simultaneous CUBES + UVES
observations, contemporaneous photometry will be needed, from the NUV to the
reddest possible wavelength.

###  5.1 Accretion

High throughput observations at R$\geq$20,000 will enable more detailed study
of the accretion process than currently possible, via precise modelling of the
Balmer jump, and by studying the higher-order Balmer lines. The high Balmer
and Ca ii H (3968 Å) & K (3933 Å) lines provide diagnostics of the accretion
funnel flows and heated chromosphere in the post-accretion shock region (e.g.,
alencar12 ).

As shown in Sect. 3.2, observations of objects down to V$=$22 mag with CUBES
will be possible with the low-res. mode. By scaling the X-Shooter spectrum of
Sz 88 A (V$=$13.2 mag) to V$=$22 mag, we derived line fluxes a factor
$\sim$3$\times$10-4 lower than those discussed in Sec 2.1.4. Thus, considering
the Gaia DR2 distance of 158 pc for Sz 88 A, and assuming $A_{\rm V}$$=$0 mag,
it would be possible in principle to observe strong accretors similar to Sz 88
A up to a distance of $\sim$9 kpc with CUBES, with line fluxes in the range of
7.5$\times$10-16 to 2.2$\times$10-17 erg s-1 cm2, respectively for the Balmer
lines from H$\alpha$ to H 15, and down to 4.2$\times$10-19 erg s-1 cm2 for H
25. Of course, the ultimate reach of such observations will be limited by
extinction, particularly given the average empirical rule of one magnitude of
extinction per kpc (implying $A_{\rm V}$$\approx$9 mag at 9 kpc) combined with
greater extinction towards the Galactic plane. Nonetheless, our tests
highlight that CUBES will allow us to study YSOs at $\sim$kpc scale distances
for the first time.

As seen in the previous sub-sections the high-resolution mode offers the
possibility of observations of relatively reddened ($A_{\rm V}$$=$3 mag) YSOs
with V$=$20 mag (see Sect. 3.1), which translates to objects similar to Sz 88
A at a distance of up to $\sim$3.5 kpc.

In the case of very low-mass objects ($M_{\star}$$<$0.1$M_{\odot}$), such as
2MASS J16095628$-$3859518 (or Lup818s in alcala14  and alcala19 ; SpT$=$M6;
V$\approx$18 mag; $\log$$L_{\rm acc}$$=-4.31$; $\log$$\dot{M}_{\rm
acc}$$=-10.96$) in Lupus, the limiting V$=$22 mag in the low-resolution mode
implies a dilution in flux by a factor $\sim$2.5$\times$10-2, theoretically
meaning a limiting $\log$($L_{\rm acc}$)$\approx-6$ and $\log$($\dot{M}_{\rm
acc}$) $\lesssim-11$, with $L_{\rm acc}$ and $\dot{M}_{\rm acc}$ in units of
$L_{\odot}$ and $M_{\odot}$ yr-1, respectively.

####  5.1.1 High Balmer lines and physical conditions of accreting gas

Our previous studies with X-Shooter (alcala14 , alcala17 , antoniucci17 )
showed an important similarity of the physical conditions in the accreting gas
for objects with very different $\dot{M}_{\rm acc}$, which was suggested by a
number of properties. One was the comparable behaviour of the Balmer
decrements down to H 15 for different objects, while another was the
similarity of the $L_{\rm acc}$–$L_{\rm line}$ relations which, to zeroth
order, are linear, with slopes varying between 0.99 and 1.18 for the hydrogen
lines, and 0.90 to 1.16 for the He I lines. This means that, over a range of
five orders of magnitude in $L_{\rm acc}$, a similar fraction of the accretion
energy is emitted in each line, independently of their excitation potential
and optical depth. This is shown in Fig. 9 where the fractional line
luminosities ($L_{\rm line}$/$L_{\rm acc}$) for a few example lines are
plotted as a function of $L_{\rm acc}$ in log scale. We used the $L_{\rm
acc}$–$L_{\rm line}$ relationships by alcala14  to produce this plot. The
relations are very stable across the examined stellar parameters, suggesting
that the physical conditions of the accreting gas are very similar in all
objects. Some evidence of a different behaviour of the Balmer decrements at
high Balmer lines, possibly indicating different physical conditions of the
accreting gas, were discussed in Sect. 2.1.4.

Figure 9: Fractional $L_{\rm line}$/$L_{\rm acc}$ line luminosity as a
function of $L_{\rm acc}$ in log scale for example diagnostic lines as
labelled. The relationships between line luminosity and accretion luminosity
by alcala14  were used to produce the plot.

CUBES observations of relatively bright YSOs (with V$<$19 mag) in the high-
res. mode will allow us to investigate the decrements in a more detailed
fashion, as well as extending the $L_{\rm acc}$–$L_{\rm line}$ calibrations to
the high-order Balmer lines, hence further investigate the physical conditions
of the accreting gas at high quantum numbers. The ODYSSEUS-PENELLOPE and GHOsT
projects (see Sect. 4) will provide bright and very well characterised targets
(in terms of physical and accretion properties) that will be perfect targets
for CUBES observations. The extended $L_{\rm acc}$–$L_{\rm line}$ calibrations
will be critical for estimates of $L_{\rm acc}$ in much fainter objects, where
fits to the Balmer jump will not be possible because of the low S/N of the
continuum, but where the emission lines are expected to be well detected.

CUBES alone would be in principle sufficient for these studies, but the
simultaneous UVES link would provide more diagnostics in the red for a more
complete analysis. The resolution of CUBES will also be helpful for the
analysis of the line profiles, providing further information on the physical
and kinematical properties of the accreting gas (e.g. antoniucci17 ).

####  5.1.2 Accretion at advanced stages

Transition disks (TDs) are protoplanetary disks that show evidence of inner
holes and gaps, as observed in millimeter interferometric observations
(andrews11 , vandermarel18 , andrews18 ) and in the dip of the mid-IR spectral
energy distribution (e.g. merin08 ). The leading explanation for the presence
of inner holes and gaps in the dust distribution of TDs is photo-evaporation
(e.g. ercolanopascucci17 , vandermarel18 ).

Low-mass pre-main-sequence stars with transitional disks accreting at very low
rates are likely in the final stages of inner disk evolution, and probably
have already formed proto-planets (owen_clarke12 ). Hence, identifying and
investigating such slow accretors may help us to understand planet formation.
However, measurements of low $\dot{M}_{\rm acc}$ are challenging. In general,
weak accretion is not easily detectable in the region of the Balmer jump. Some
previous studies attempted $\dot{M}_{\rm acc}$ measurements in TDs using other
tracers, such as trends between the lines width and $\dot{M}_{\rm acc}$ (e.g.
merin10 , cieza10 ). However, such estimates depend heavily on rather
uncertain scaling relations (see discussion in alcala14 ). Using the $L_{\rm
acc}$–$L_{\rm line}$ relationships is in principle possible. However, at very
low accretion levels the chromospheric contribution to the emission budget may
be comparable to $L_{\rm acc}$ and must therefore be disentangled.

Another important aspect regarding YSOs in advanced stages is represented by
the i possible degeneracy of stellar and accretion parameters in objects with
spectral types earlier than about K3. In these relatively early-type objects
the low contrast between the continuum excess emission and the photospheric +
chromospheric emission, may prevent a reliable assessment of accretion. For
example, Fig. 10 shows the case of the star MY Lup; the solution on the
accretion parameters based on low-res. spectra alone may be ambiguous, with up
to an order of magnitude difference in $L_{\rm acc}$ (see e.g. alcala19 ).
CUBES will allow us to measure low accretion rates, by more precisely
disentangling the contribution from chromospheric emission, which is difficult
to remove in general, but practically impossible if S/N is low in the NUV. The
simultaneous link with UVES will improve the fitting of continuum excess
emission.

Figure 10: Lower panel: extinction corrected low-resolution spectra of MY Lup
(in black) and the non-accreting template LkCa 19 (in red). The slab model
derived in alcala17  is also shown (in blue). The cyan spectrum shows the sum
of the non-accreting template plus the slab emission. Upper panel: as per the
lower panel, but now with the slab model multiplied by ten. Note the tiny
Balmer jump shown in the inset. Figure adapted from alcala19 .

####  5.1.3 Low end of $\dot{M}_{\rm acc}$ in large surveys

A crucial prediction of the YSO viscous accretion theory is the existence of
correlations between the disk mass ($M_{\rm disk}$) the stellar mass
($M_{\star}$) and the mass accretion rate ($\dot{M}_{\rm acc}$) (hartmann16 ).
Such correlations have been confirmed only recently for the disk populations
of the 1-3 Myr old Lupus and Chamaeleon I star-forming regions (see Sect. 4.1
and manara16  and references therein). The observed relations show, however, a
much larger scatter in $\dot{M}_{\rm acc}$ ($>$0.9 dex) than viscous evolution
would predict for 1-3 Myr old populations. The viscous disk theory predicts a
smaller spread of $\dot{M}_{\rm acc}$ ($<$0.4 dex) at older ages. We thus need
to measure the spread of $\dot{M}_{\rm acc}$ at given $M_{\rm disk}$ for
carefully selected samples of relatively old YSOs, where accretion activity is
expected to be weak. Recent work with X-Shooter showed the existence of very
high accretion rates in the ($>$5 Myr) Upper Scorpius association. CUBES will
contribute by measuring the weak Balmer jump and weak excess emission, as well
as disentangling the contribution of the chromospheric emission.

Another important point which deserves further investigation is that there is
growing evidence of a change in the slope of the $M_{\star}$–$\dot{M}_{\rm
acc}$ relationship for CTTS with ages of 2-3 Myr at $M_{\star}$$<$0.2
$M_{\odot}$ (manara17  and alcala17 , and see Fig. 11). Such a break could be
related to a faster disk evolution at low-masses (e.g. VorovyobBasu09 ). To
verify this, the slope of the $\dot{M}_{\rm acc}$–$M_{\star}$ relationship
needs to be sampled at much lower $M_{\star}$ and $\dot{M}_{\rm acc}$ values
than done so far. In Fig. 11 the theoretical limits that CUBES may be able to
reach are shown with the instrument logo. Note that such limits are much lower
than the values for $\sigma$-Ori 500 (2MASS J05382543$-$0242412), the V$=$ 20
mag young brown dwarf discussed in Sect. 2.1.4.

Figure 11: Mass accretion rate ($\dot{M}_{\rm acc}$) as a function of stellar
mass ($M_{\star}$) for YSOs in the Lupus clouds (black symbols). Transitional
disks are shown with crossed squares. The position of the young brown dwarf
2MASS J05382543$-$0242412 (or $\sigma$-Ori 500) is shown with the blue dot.
Average errors in $\log$($\dot{M}_{\rm acc}$) and $\log$($M_{\star}$) are
shown in the upper left. Dashed magenta lines represent the fits to the data
as in Eqs. (4) and (5) from alcala17 . The CUBES logo shows the region of the
diagram that the instrument may be able to reach. Figure adapted from alcala19
.

CUBES not only will complement the surveys of 1-2 Myr clusters with accurate
$\dot{M}_{\rm acc}$ determinations, but its sensitivity will allow us to
investigate the accretion process in young brown dwarfs, possibly down to
$M_{\star}$$\sim$30$M_{Jup}$ and $\dot{M}_{\rm acc}$ $\lesssim 10^{-11}$
$M_{\odot}$/yr. By studying the $\dot{M}_{\rm acc}$–$M_{\star}$ relationship
at such limits it will be possible to investigate whether low-mass objects
have indeed a faster disk evolution than their more massive counterparts, as
proposed by some disk accretion models (VorovyobBasu09 ).

Finally, another important issue regards the validity of the $L_{\rm
acc}$–$L_{\rm line}$ relationships at the very low-mass regime. Recently
aoyama21  have produced a theoretical $L_{\rm acc}$– L(H$\alpha$) relationship
drawn from modelling of a shock at the surface of a planetary mass object.
These authors show that the new theoretical relationship yields a much higher
$L_{\rm acc}$ for a given L(H$\alpha$) than what the relationships drawn from
YSO spectra would predict. Hence, the validation of the relationships at the
lowest possible mass is of key importance.

The high-sensitivity makes of CUBES a self-standing instrument for the studies
of accretion in very-low mass YSOs, although the simultaneous link with UVES
would be helpful for the YSO classification.

####  5.1.4 $\dot{M}_{\rm acc}$ in low metallicity environments

A very important aspect that has received little attention so far, mainly
because of the limited sensitivity of the existing intermediate/high
resolution instruments, is the spectroscopic study of YSOs in distant (a few
kpc) star-forming regions. At such distances, the low metallicity may have an
important impact on accretion.

Photometric studies in low-metallicity regions (e.g. demarchi17 , biazzo19
and references therein) suggest that metal-poor stars accrete at higher rates
than solar-metallicity stars in nearby Galactic star-forming regions. As a
consequence, most stars forming in a low-metallicity environment should
experience disk dispersal at an earlier stage (in less than about 1 Myr) than
those forming in solar metallicity environments (where disk dispersal may last
up to 5-6 Myr). Thus, metallicity may play a crucial role on circumstellar
disk evolution, hence on the time scale available for planet formation.

Typical solar-mass YSOs at kpc distances are faint (V$>$18 mag, see cusano11 )
so require the significant increase in sensitivity that CUBES will provide
compared to current instruments. CUBES observations will allow us to
investigate the Balmer jump, hence $L_{\rm acc}$ and $\dot{M}_{\rm acc}$ at
low Z ($<$0.2 Z⊙) and answer questions such as: are the $L_{\rm
acc}$–$L_{\star}$ relationships the same at Solar metallicity? Are the Balmer
decrements similar to those at Solar metallicity? What are the effects of
local and external UV radiation fields (i.e. external photo-evaporation)?

CUBES will enable studies of accretion at low metallicities, as well as the
effects of local UV fields on accretion and winds – weak fields should not
influence the disks, whereas strong fields may modify the ionisation rate in
the disk leading to larger mass-loss rates (guarcello21 ).

CUBES can be a self-standing instrument for this science case, but YSO
crowding may represent an issue for the farthermost SFRs.

###  5.2 Winds and outflows

As mentioned in Sect. 2.3, the outflow activity in YSOs is a phenomenon
strictly related to accretion at different mass regimes. The study of such
outflows enables us to elucidate the relationship between mass ejection and
accretion, and test the universality of the star formation mechanism as a
function of central YSO mass.

In particular, forbidden lines from the UV to the NIR spectral domain can be
used to measure the physical properties (density, temperature and ionisation
fraction) of the different outflow manifestations using diagnostic tools
specifically designed for the purpose. Analysis combining lines at different
wavelengths originating from various excitation regimes is important, as
applied to CTTS jets observed with X-Shooter (bacciotti11 , giannini15a ) and
more recently with the GIARPS instrument (giannini19 ) at much higher
resolution. The CUBES + UVES combination will provide sufficient sensitivity
and resolution to apply such diagnostic tools to studies of fainter and more
distant YSOs jets than possible so far.

To this aim, the CUBES spectral range covers several key lines for the study
of jets/winds in YSOs. We already mentioned the importance of the [O ii] 3726Å
line, in combination with the [O i] 6300 Å line, to estimate the ionization
degree of the jets/winds plasma. Other important lines providing complementary
information or tracing excitation regimes not probed by optical/IR lines are
also present in the CUBES + UVES range. For example, the [S ii] doublet at
4069/4076 Å is a useful diagnostic of dense winds in protoplanetary disks
(ballabio20  and references therein). Also, the little-studied [Ne iii] 3869 Å
line signals the presence of high-energy photons, thus probing either high
velocity ($>$ 100 km/s) shocks or X-ray heating liu14 ; liu21 . The two
scenarios can potentially be discriminated through CUBES + UVES velocity-
resolved observations.

Finally, the NUV spectral range includes several bright [Fe ii] and [Fe iii]
lines produced from high energy levels, which are therefore good tracers of
the temperature of the ionised gas. The observation of [Fe iii], combined with
[Fe ii], allows the simultaneous determination of temperature, density and
degree of ionisation, as well as a precise estimate of the Fe abundance in the
gas. The relevant non-LTE models required to predict the line fuxes already
exist (e.g., giannini15 , giannini15a ).

This analysis approach can be applied to a large sample of sources having
different stellar and accretion properties. In addition, the CUBES spectral
resolution will enable us to distinguish the different kinematical wind
components and separately study the properties of the jets and of the slow
winds, as well as investigating whether the relative importance of these two
outflow components changes with age, when the contribution of photo-
evaporation is expected to be dominant.

Following determination of the physical parameters, the mass ejection rate,
$\dot{M}_{\rm out}$, can be derived from the luminosity of spectrally-resolved
forbidden lines. This will allow us to measure the $\dot{M}_{\rm
out}$/$\dot{M}_{\rm acc}$ parameter, which critically depends on the jet
launching mechanism, and in turn to infer possible variations in the mass loss
efficency due to modifications of the inner disk magnetic and physical
properties.

While longer-wavelength data are required for this case, it is not a strong
requirement that they are simultaneous in time. In this example, the benefit
of the UVES fibrelink is primarily more efficient operations than separate
observations.

##  6 Summary and conclusions

The CUBES science case on accretion and outflows in YSOs has been presented.
The performance of CUBES, mainly in terms of its increased sensitivity in the
NUV (3000–4050 Å), will allow us to broaden investigations of the evolution of
circumstellar disks, by studying the accretion, jets/winds and photo-
evaporation processes down to the low-mass, brown dwarf regime and in more
distant YSOs than done so far. In particular CUBES will enable:

  * •

more precise flux measurements of continuum and emission lines in the NUV.
Contemporaneous photometric observations in the same spectral range will be
needed in case the CUBES flux calibration will not be precise at the 15% level
or better.

  * •

detection of weak emission lines (and their line morphology), in particular
for the Ca ii H&K and high-H lines, which are diagnostics of the accretion
funnel flows.

  * •

investigation of the physical conditions of the in-flowing gas via precise
Balmer decrement determinations, including the high-order Balmer lines.

  * •

refinement of the $L_{\rm acc}$–$L_{\rm line}$ relationships by including
Balmer lines with a high quantum number.

  * •

better definition of the Balmer jump, as well as detection of weak emission
lines, allowing for studies of low-accretion YSOs, including objects in the
latest stages of evolution.

  * •

studies of accretion and outflows in very low-mass browns dwarfs, and to study
YSOs at much larger distances than done so far, enabling investigation of the
effects of metallicity and external UV-radiation on accretion and winds.

In conclusion, CUBES observations of YSOs will lead to a step-change in the
study of accretion and outflows in solar-type young stars hence, on
protoplanetary disk evolution, as the spectrograph will enable investigation
of targets that are several magnitudes fainter than those reachable with
current instrumentation.

###### Acknowledgements.

We very much thank the two anonymous referees for their comments and
suggestions. Financial support from the project PRIN INAF 2019
”Spectroscopically tracing the disk dispersal evolution (STRADE)” is warmly
acknowledged. This research made use of the SIMBAD database, operated at the
CDS (Strasbourg, France).

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