# MagAO-X and HST high-contrast imaging of the AS209 disk at H$\alpha$

Gabriele Cugno Department of Astronomy, University of Michigan, Ann Arbor, MI
48109, USA  Yifan Zhou Department of Astronomy, The University of Texas at
Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA  Thanawuth
Thanathibodee Institute for Astrophysical Research, Department of Astronomy,
Boston University, Boston, MA 02215, USA  Per Calissendorff  Department of
Astronomy, University of Michigan, Ann Arbor, MI 48109, USA  Michael R. Meyer
Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
Suzan Edwards  Five College Astronomy Department, Smith College, Northampton,
MA 01063, USA  Jaehan Bae Department of Astronomy, University of Florida,
Gainesville, FL 32611, USA  Myriam Benisty Laboratoire Lagrange, Université
Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, 06304 Nice, France  Univ.
Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France  Edwin Bergin  Department
of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA  Matthew De
Furio Department of Astronomy, University of Michigan, Ann Arbor, MI 48109,
USA  Stefano Facchini Università degli Studi di Milano, via Giovanni Celoria
16, 20133 Milano, Italy  Jared R. Males  Steward Observatory, University of
Arizona, Tucson, AZ 85179, USA  Laird M. Close  Steward Observatory,
University of Arizona, Tucson, AZ 85179, USA  Richard D. Teague  Department of
Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of
Technology, Cambridge, MA 02139, USA  Olivier Guyon  Steward Observatory,
University of Arizona, Tucson, AZ 85179, USA  James C. Wyant College of
Optical Sciences, University of Arizona, 1630 E University Blvd, Tucson, AZ
85719, USA  National Astronomical Observatory of Japan, Subaru Telescope,
National Institutes of Natural Sciences, Hilo, HI 96720, USA  Astrobiology
Center, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo,
JAPAN  Sebastiaan Y. Haffert NASA Hubble Fellow  Steward Observatory,
University of Arizona, Tucson, AZ 85179, USA  Alexander D. Hedglen  James C.
Wyant College of Optical Sciences, University of Arizona, 1630 E University
Blvd, Tucson, AZ 85719, USA  Maggie Kautz  James C. Wyant College of Optical
Sciences, University of Arizona, 1630 E University Blvd, Tucson, AZ 85719, USA
Andrés Izquierdo European Southern Observatory, Karl-Schwarzschild-Str. 2,
85748 Garching bei München, Germany  Leiden Observatory, Leiden University,
P.O. Box 9513, 2300 RA Leiden, The Netherlands  Joseph D. Long Steward
Observatory, University of Arizona, Tucson, AZ 85179, USA  Jennifer Lumbres
James C. Wyant College of Optical Sciences, University of Arizona, 1630 E
University Blvd, Tucson, AZ 85719, USA  Avalon L. McLeod  James C. Wyant
College of Optical Sciences, University of Arizona, 1630 E University Blvd,
Tucson, AZ 85719, USA  Logan A. Pearce Steward Observatory, University of
Arizona, Tucson, AZ 85179, USA  Lauren Schatz  Air Force Research Laboratory,
Directed Energy Directorate, Space Electro-Optics Division, Starfire Optical
Range, Kirtland Air Force Base, NM, 87117,USA  Kyle Van Gorkom  Steward
Observatory, University of Arizona, Tucson, AZ 85179, USA

###### Abstract

The detection of emission lines associated with accretion processes is a
direct method for studying how and where gas giant planets form, how young
planets interact with their natal protoplanetary disk and how volatile
delivery to their atmosphere takes place. H$\alpha$ ($\lambda=0.656~{}\mu$m)
is expected to be the strongest accretion line observable from the ground with
adaptive optics systems, and is therefore the target of specific high-contrast
imaging campaigns. We present MagAO-X and HST data obtained to search for
H$\alpha$ emission from the previously detected protoplanet candidate orbiting
AS209, identified through ALMA observations. No signal was detected at the
location of the candidate, and we provide limits on its accretion. Our data
would have detected an H$\alpha$ emission with $F_{\mathrm{H\alpha}}>2.5\pm
0.3\times 10^{-16}$ erg s-1 cm-2, a factor 6.5 lower than the HST flux
measured for PDS70 b (Zhou et al., 2021). The flux limit indicates that if the
protoplanet is currently accreting it is likely that local extinction from
circumstellar and circumplanetary material strongly attenuates its emission at
optical wavelengths. In addition, the data reveal the first image of the jet
north of the star as expected from previous detections of forbidden lines.
Finally, this work demonstrates that current ground-based observations with
extreme adaptive optics systems can be more sensitive than space-based
observations, paving the way to the hunt for small planets in reflected light
with extremely large telescopes.

planet-disk interactions — planets and satellites: detection — planets and
satellites: formation

††facilities: Magellan: Clay (MagAO-X); Hubble Space Telescope
(WFC3)††software: PynPoint (Stolker et al., 2019), applefy (Bonse et al.,
2023),

##  1 Introduction

In recent years observations by the Atacama Large Millimeter and Submillimeter
Array (ALMA) and high contrast imagers including SPHERE and GPI revealed that
circumstellar disks are highly structured, with gaps, rings and spirals being
among the most important observable features (e.g., Andrews et al., 2018;
Avenhaus et al., 2018; Tschudi & Schmid, 2021; Bae et al., 2022a; Benisty et
al., 2022). While other explanations have been proposed (e.g., snowlines or
turbulence, Zhang et al., 2015; Flock et al., 2015), it is widely accepted
that at least some of these structures are the result of the interaction with
forming planets. This hypothesis is supported by the direct detection of two
confirmed protoplanets in the cavity of the transition disk PDS70 (Keppler et
al., 2018; Haffert et al., 2019). However, despite significant observational
efforts, no other confirmed planet in the cavities of other disks has been
detected, neither in the infrared (Asensio-Torres et al., 2021; Cugno et al.,
2023) nor in emission lines associated to accretion (Cugno et al., 2019; Zurlo
et al., 2020; Xie et al., 2020; Huélamo et al., 2022; Follette et al., 2022).
Two candidates have been directly detected through H$\alpha$ emission (LkCa15
b and AB Aur b, Sallum et al., 2015; Currie et al., 2022); the first has never
been redetected, and near-infrared observations suggest it could be a
reprocessed scattered light feature (Currie et al., 2019), while the second
one shows H$\alpha$ emission consistent with scattered light from the disk
(Zhou et al., 2022) and requires further investigation.

An alternative method for inferring the existence of protoplanets and studying
their formation is to examine the effect they have on the disk structure
(Pinte et al., 2022). Indeed, young gas giant planets leave observational
traces on the velocity field of the surrounding gas due to their gravitational
force, manifesting as kinks (Pinte et al., 2018, 2020), doppler flips
(Casassus & Pérez, 2019), deviations from Keplerian velocity (Teague et al.,
2018, 2021), or molecular emission line broadening along the planet orbit
(Dong et al., 2019; Izquierdo et al., 2022).

AS209 (alternative name V 1121 Oph) is a young ($\approx 1-2$ Myr, Andrews et
al. 2009) K5 ($V=11.3$ mag) star with a mass of $1.2~{}M_{\odot}$ (Öberg et
al., 2021; Teague et al., 2021). Fernandez et al. (1995) measured the
H$\alpha$ flux of the star to be $8.4\times 10^{-12}$ erg s-1 cm-2, which
translates in a mass accretion rate of
$\dot{M}_{\mathrm{acc}}=10^{-8.2}~{}M_{\odot}$ yr-1 when using the line
luminosity vs. accretion luminosity relationship from Fang et al. (2009). This
value is in strong agreement with the mass accretion rate of
$\dot{M}_{\mathrm{acc}}=10^{-8.3}~{}M_{\odot}$ yr-1 obtained by Fang et al.
(2018) using multiple emission line luminosities other than H$\alpha$. It is
surrounded by a well-studied protoplanetary disk ($i=35\pm 0.1^{\circ}$, Huang
et al. 2018) that has been the subject of several major surveys such as DSHARP
(Andrews et al., 2018; Huang et al., 2018; Guzmán et al., 2018), DARTTS-S
(Avenhaus et al., 2018), and MAPS (Öberg et al., 2021). As part of the MAPS
program, Bae et al. (2022b) identified a candidate circumplanetary disk (CPD)
detected in 13CO within a gap at $\sim 200$ au previously identified in 12CO
and scattered light observations (Guzmán et al., 2018; Avenhaus et al., 2018).
Subsequent kinematic analysis of the molecular line data revealed disk winds
emerging from the gap around the candidate, showing a complex interplay
between forming planets and disk winds (Galloway-Sprietsma et al., 2023;
Izquierdo et al., 2023). A wind from the central star was also inferred
through high resolution spectroscopy of forbidden lines (Fang et al., 2018;
Banzatti et al., 2019).

Figure 1: MagAO-X AS209 residuals in the H$\alpha$ (left panel), in the
continuum (middle panel), and in the continuum-subtracted H$\alpha$ frames
(right panel). North points to the top and East to the left. The white star
indicates the position of the central star. The position of the CPD candidate
is marked with a white dashed circle. The jet can be identified as the
extended emission North of the star in both the left and the right panels. The
rectangular mask applied for the jet brightness measurement is shown in the
right panel.

In this paper we search for H$\alpha$ emission from the the CPD candidate
orbiting AS209 with the Magellan Adaptive Optics eXtreme (MagAO-X) instrument
(Males et al., 2022a) and with HST/WFC3/UVIS. MagAO-X is a new high-contrast
imaging instrument that pushes extreme adaptive optics to visible wavelengths
(Males et al., 2018; Close et al., 2018; Males et al., 2022a), including
H$\alpha$ (Close, 2020). MagAO-X is a successor to MagAO (Close et al., 2014a)
and has an increased number of controllable modes (1600 versus 300), AO loop
speed (2 kHz versus 1 kHz) and sensitivity (twice the throughput at
H$\alpha$). The data reveal a stellar jet at short separations, while no
localized H$\alpha$ emission at the location of the CPD candidate or anywhere
else within the disk is detected. This Paper is structured as follows: in
Sect. 2 we present the MagAO-X and HST observations and in Sect. 3 we detail
our data reductions. The main results are reported in Sect. 4 and discussed in
Sect. 5. The conclusions of our work can be found in Sect. 6.

##  2 Observations

###  2.1 MagAO-X

AS209 was observed with the MagAO-X instrument at the Las Campanas Observatory
on April 17 2022 and April 20 2022. The observing conditions were very
different between the two nights. On April 17, data were taken under very
stable conditions, with seeing $<0\farcs 5$ for most of the night,
airmass$<1.2$ and photometric sky. Conversely, due to extremely unstable
atmospheric conditions on April 20, the second set of data were not usable. As
a result, this manuscript focuses solely on the data obtained on April 17.

AS209 was observed in H$\alpha$ dual-band imaging mode, which involves
splitting the light into the H$\alpha$ ($\lambda_{c}=0.656~{}\mu$m,
$\Delta\lambda=9$ nm) and nearby continuum (Cnt, $\lambda_{c}=0.668~{}\mu$m,
$\Delta\lambda=9$ nm) filters after it has undergone the same optical path
through the instrument. This results in the very similar diffraction and
speckle pattern in both filters, allowing the continuum image to be used to
remove the stellar contribution from the H$\alpha$ image without affecting any
potential line flux emitted from a protoplanet. Typical protoplanets do not
contribute in the continuum image at $\sim 0.65~{}\mu$m due to their
relatively low temperatures ($\sim 1000-2000$ K) compared to stars. For more
details on this observational mode and the reasoning behind it we refer to
Close et al. (2014b) and Cugno et al. (2019).

We obtained a total of 209 frames in each filter with a Detector Integration
Time (DIT) of 60 s per exposure. Thanks to the high sky rotation rate, we
achieved a total field rotation of $\sim 106^{\circ}$, ensuring a high
throughput for the angular differential imaging (ADI; Marois et al., 2006)
post-processing algorithm.

Figure 2: HST AS209 residuals in the F656N filter (H$\alpha$). North points to
the top and East to the left. The white star indicates the position of the
central star, while the position of the CPD candidate is marked with a white
dashed circle. The jet North of the star is re-detected in agreement with
MagAO-X data. The rectangular mask applied for the jet brightness measurement
is shown.

###  2.2 HST/WFC3/UVIS

AS209 was observed with the HST/WFC3/UVIS instrument on 2023-05-03 for three
orbits in the F656N (H$\alpha$ narrowband, $\lambda_{c}=6561.5$ Å,
$\Delta\lambda=17.9$ Å) filter using the UVIS2/C512C subarray (field of view
$20\farcs 5\times 20\farcs 5$). To improve the spatial sampling, a four-points
dithering pattern was used, in which the telescope moves by 0.5 pixels between
exposures. In total we obtained 108 frames of 13 s each, for a total time on
target amounting to 23.4 min. In order to perform ADI, the roll angle in Orbit
2 differs by 35∘ from those in Orbits 1 and 3.

##  3 Data reduction

###  3.1 MagAO-X

The MagAO-X data were reduced using the high-contrast imaging pipeline
PynPoint (Amara & Quanz, 2012; Stolker et al., 2019). After dark and flat
calibration, frames are flipped along the x-axis to correct for a reflection
within the instrument and bad pixels are corrected by 4-sigma clipping. Images
are then aligned to each other using cross-correlation and then centered by
fitting a 2D Gaussian function to the mean image. Continuum and H$\alpha$
frames were found to display an azimuthal offset of about $0.3^{\circ}$ and we
corrected this by rotating the continuum images. This process resulted in
squared images of $3\farcs 7$ in size (MagAO-X pixel scale is $0\farcs 0059$).

We applied a frame selection based on the peak intensity of the PSF in the
H$\alpha$ filter, measured in an aperture of radius $0\farcs 0135$ (2.3
pixels) in order not to be biased by the position of the star on the pixel
grid. Since the instrument response to a point source is the same for every
source, when the stellar PSF has a lower peak due to low Strehl, the same
applies to faint protoplanet signals, making them more difficult to detect. We
removed images with peak fluxes that deviated by $>2\sigma$ from the maximum
value of the entire dataset. This step removed 45 frames out of 209 from the
dataset, most of them temporally located towards the end of the night, when
weather conditions started to deteriorate.

Two different PSF subtraction techniques were applied. First, we removed the
stellar PSF from individual filters (both H$\alpha$ and Continuum) using ADI
based on principal component analysis (PCA, Amara & Quanz, 2012). We found
that at least 10 principal components have to be subtracted in order to remove
the bright stellar noise. Second, the PSF was removed using a combination of
dual-filter differential imaging (Close et al., 2014b; Cugno et al., 2019)
followed by ADI. The continuum frames are first spatially downscaled to match
the PSF size (which is $\propto\lambda$) at the H$\alpha$
wavelength111Information on the MagAO-X H$\alpha$ filters can be found at
https://magao-x.org/docs/handbook/observers/filters.html and their flux is
upscaled by a factor 2.1 to match the total flux in the corresponding
H$\alpha$ image within an aperture of $r=1\farcs 0$. This is necessary to
compensate for the substantial line emission contribution due to stellar
accretion in the H$\alpha$ filter. We found that the continuum subtraction
accurately cancels most of the stellar noise, and only a few components have
to be removed to reveal faint sources. After the final residuals were median-
combined, we applied a Gaussian filter of the size of the PSF
(FWHM=0$\farcs$034) to reduce pixel-to-pixel variation and highlight
protoplanetary candidates.

###  3.2 HST

We initiated the data reduction process using the flc file obtained from the
MAST archive. To begin with, we visually inspected the data in order to
identify any hot pixels and cosmic rays present. These pixels were then
replaced by employing linear interpolations based on the neighboring pixels.
Subsequently, we reconstructed Nyquist sampled images by interlacing sets of
four dithered images in Fourier space (for details on the image reconstruction
we refer the reader to Lauer 1999; Zhou et al. 2021). This step resulted in 27
Nyquist sampled images with pixelscale $0\farcs 02$.

For the primary subtraction, we again employed PynPoint. To align the images,
we registered them according to the centroids of the PSFs, which were
determined through two-dimensional Gaussian fitting. The centroid-aligned
image cube was then fed into the PCA algorithm, where images from one roll
position are used to model and subtract the central PSF in the other roll
position. For HST data, only a few components were removed. Finally, after
derotating and median-combining the frames we applied again the highpass
filter (FWHM$=0\farcs 04$).

##  4 Results

The residual images (shown in Fig. 1 and Fig. 2) do not reveal a signal at the
location of the CPD candidate ($1\farcs 4$, Bae et al., 2022b), and in Sect.
4.1 we calculate detection limits at the candidate location and discuss the
non-detection. In addition, the residuals show extended emission North of the
star coming from a jet both in the MagAO-X and HST data, presented in Sect.
4.2.

Figure 3: H$\alpha$ $5\sigma$ contrast limits and H$\alpha$ luminosity limits
of AS209 as a function of separation. The grey dashed lines represent the
separation of the CPD candidate (Bae et al., 2022b). Shaded regions represent
the systematic uncertainty due to the speckle and residuals noise. Left:
contrast curves obtained with MagAO-X. The curve obtained with the H$\alpha$
differential technique is shown with an orange line, while the curve obtained
with ADI PSF-subtraction without the continuum removal is shown with a blue
line. Subtracting the scaled continuum improves the contrast performance by up
to 1 mag, especially at short separations ($\lesssim 0\farcs 3$). Middle:
contrast curves obtained with HST. Due to different stellar fluxes in the
filters used, the contrasts from the left and central panels are not directly
comparable. Right: H$\alpha$ luminosity upper limits as a function of
separation for the two instruments: the curves are directly comparable,
indicating that our MagAO-X data are more sensitive than our HST data at every
separation in the image.

###  4.1 The CPD candidate

The protoplanet candidate surrounded by a circumplanetary disk detected in
13CO by Bae et al. (2022b) was not detected neither with MagAO-X nor with HST
in H$\alpha$. Its expected position is shown in the residual images of Fig. 1
and Fig. 2.

To quantify the relevance of the non detection, we estimated detection limits
on the presence of H$\alpha$ emitting sources. These were obtained with the
applefy tool presented in Bonse et al. (2023) based on the metric proposed by
Mawet et al. (2014). The detection threshold was fixed to a false positive
fraction (FPF) of $2.87\times 10^{-7}$, equivalent to 5$\sigma$ for large
separations and Gaussian noise. Artificial protoplanetary signals were
inserted every $0\farcs 06$ at four different PAs. The planet signal and the
noise were measured in an aperture of $r=3.2$ pix and 2.0 pix for MagAO-X and
HST data (half FWHM), respectively. For every separation the noise for 360
different aperture placements is estimated and we report the median over all
results with the standard deviation representing the systematic uncertainty on
the contrast measurement (see Bonse et al. 2023 for more details). Because the
presence of the bright jet could bias the contrast curves, especially at short
separations, we rotated the residuals in the opposite direction before
combining them and estimating the noise (Pairet et al., 2019). This approach
maintains the stellar and speckle noise, but avoids the presence of physical
signals in the images used to sample the noise.

The obtained $5\sigma$ contrast curves from MagAO-X data are shown in the left
panel of Fig. 3, in orange for the H$\alpha-$Cnt and in blue for the H$\alpha$
stack with PCA-ADI PSF-subtraction. For the injection of planets we used the
PSF from the continuum filter, as about 25% of the data in the H$\alpha$
filter is saturated in the inner few pixels. In addition, stellar line
emission may suffer from variability on different timescales and therefore the
H$\alpha$ flux is more difficult to calibrate. We note that the different
filter throughputs might introduce a bias in the flux estimate of a few
percent, much smaller than the contrast uncertainties. At small separations,
H$\alpha$ differential imaging outperforms ADI, providing contrasts $\sim 1$
mag deeper. At larger separations, simple ADI reaches higher contrasts than
H$\alpha$ differential imaging. We attribute this behavior to the higher
detector and readout noise introduced in the data by the continuum subtraction
in the outer regions of the images, where the stellar emission does not
dominate.

The contrast limits of the left panel of Fig. 3 show that at the separation of
the CPD candidate we reach a 5$\sigma$ contrast of $11.5\pm 0.1$ mag. For the
absolute calibration, we adopted $F^{*}_{\mathrm{Cnt}}=1.01~{}(\pm 0.1)\times
10^{-11}$ erg s-1 cm-2 for the stellar flux in the continuum filter of the
MagAO-X data (Henden et al., 2015), where the 10% uncertainty was
conservatively chosen to include potential variability (the nominal
measurement uncertainty is 4%). Combining this value with the contrast limit
measured for the CPD candidate, we obtained a $5\sigma$ H$\alpha$ flux limit
for the protoplanet of $F^{\mathrm{H\alpha}}_{\mathrm{pl}}\lesssim 2.5\pm
0.3\times 10^{-16}$ erg s-1 cm-2, which once corrected for the stellar
distance ($d=121\pm 0.4$ pc, Gaia Collaboration 2022) corresponds to a
luminosity of $L_{\mathrm{H\alpha}}^{\mathrm{obs}}\lesssim 1.15\pm 0.15\times
10^{-7}~{}L_{\odot}$. The overall luminosity upper limit as a function of
separation from the MagAO-X data is shown in green in the right panel of Fig.
3. At each separation, we adopted the higher contrast between the two curves
reported in the left panel.

The contrast limits measured by HST are shown in the central panel of Fig. 3.
At the separation of the CPD candidate we reach a contrast of $9.9\pm 0.1$
mag. The contrast measured with respect to the stellar flux in the only HST
filter available is not directly comparable to the one obtained above for the
MagAO-X data for two reasons: (i) the HST filter is centred on the H$\alpha$
line, while for a more reliable flux calibration the MagAO-X contrast limits
were measured with respect to stellar flux obtained in the continuum filter,
and (ii) the HST filter has a width of $18$ Å, 5$\times$ smaller than the
MagAO-X filters ($\sim 90$ Å). From the HST images, we were able to directly
measure the stellar flux in the F656N filter: $3.22\pm 0.05\times 10^{-13}$
erg s-1 cm-2 Å-1.Applying the contrast at the separation of the CPD candidate,
we obtain a flux limit from the HST data of
$F^{\mathrm{H\alpha}}_{\mathrm{pl}}\lesssim 5.7\pm 0.6\times 10^{-16}$ erg s-1
cm-2, and once corrected for stellar distance a luminosity upper limit of
$L_{\mathrm{H\alpha}}^{\mathrm{obs}}\lesssim 2.6\pm 0.3\times
10^{-7}~{}L_{\odot}$. These two quantities are directly comparable to the
MagAO-X data, which provided lower limits and are able to exclude planets with
dimmer luminosities. Hence, for the interpretation of the non-detection we
will only focus on the upper limits provided by MagAO-X. The comparison of
sensitivities achieved by the two observatories is further discussed in Sect.
5.4.

Figure 4: Surface brightness profile of the jet from AS209 as measured from
MagAO-X (orange) and HST (violet) data. Errorbars represent $1\sigma$
uncertainties. The central mask with radius $0\farcs 15$ is shown with the
dashed grey line.

###  4.2 Jet

Figure 1 and Figure 2 reveal a jet north of the star imaged for the first time
in AS209. Its extension in the 2D projected image goes up to a separation of
$0\farcs 65$ in the MagAO-X data. The jet in the HST data is detected only up
to $0\farcs 5$, thus the values provided next only refer to MagAO-X data.
Assuming the jet is perpendicular to the disk plane, its physical extension is
estimated to be $\sim 169$ au. Its width in the residuals is $\sim 0\farcs 3$,
equivalent to $\sim 36$ au at AS209 distance. This value is in line with FWHM
of jets from other TTauri stars (Ray et al., 2007). No receding jet is
detected, possibly due to the obscuring effect of the circumstellar disk.

To estimate the jet profile, we applied a rectangular mask ( width $=0\farcs
3$ for both instruments) to focus only on the image region with the jet signal
(see dashed lines in the right panel of Fig. 1 and in Fig. 2). To avoid the
more aggressive subtraction induced by PCA, we used residuals produced with
classical ADI: given the large field rotation in the MagAO-X data and the roll
angle observing strategy used to obtain the HST data, we expect subtraction of
the extended jet signal to be minimal (throughput $\sim 100\%$) at large
enough separations. However, some level of self-subtraction is expected in the
innermost region of the images. In addition, those same regions suffer from
strong speckles residuals from the PSF subtraction. Hence, we masked the inner
$0\farcs 15$ in each dataset. The flux from the the jet was measured in annuli
in steps of 1 FWHM ($\sim 6.5$ pix and $\sim 4$ pix for MagAO-X and HST data
respectively), and normalized to the measurement area, obtaining the surface
brightness profile. To calibrate the flux, we estimated the PSF counts within
an aperture of $2\farcs 0$ and we compare it to the stellar flux as described
in Sect. 4.1 to find a count-to-flux conversion factor for each instrument.
The obtained conversion factors were applied to the brightness profiles, and
the calibrated profiles are reported in Fig. 4. The profiles estimated from
the two instruments are in agreement. For the errorbars, we considered two
types of uncertainties. The first one, corresponding to the noise induced by
stellar residuals, was estimated by considering the residuals outside the jet
mask at the same separations and assigning an uncertainty to the measurement
corresponding to the fraction of the noise counts over the jet counts
(normalized by area). The second one corresponds to photon noise. At each
separation we considered the largest source of uncertainty (residuals
$\lesssim 0\farcs 35$, photon noise $\gtrsim 0\farcs 35$). The profiles
obtained with the two instruments largely agree with each other.

The jet appears rather smooth and does not show significant knots as it was
observed for RY Tau (Garufi et al., 2019; Uyama et al., 2022) and HD163296
(Ellerbroek et al., 2014; Xie et al., 2021). However, it seems some intensity
variation is present between $0\farcs 15$ and $0\farcs 3$ in the MagAO-X data,
even though with the current data it is difficult to assess if the bumps in
the profile are physical or due to the post-processing.

##  5 Discussion

###  5.1 The protoplanet mass

AS209 has been observed with VLT/NaCo in the $L^{\prime}$-band as part of the
NaCo-ISPY survey (Launhardt et al., 2020). The data, presented in Cugno et al.
(2023), exclude the presence of a companion brighter than 17.2 mag, which
translates in an absolute magnitude of 11.8 mag at $L^{\prime}$. Even though
Cugno et al. (2023) showed the risk of using evolutionary models to transform
IR brightness measurements into mass estimates, we use the detection limits to
obtain a rough estimate of what near-IR high-contrast imaging can exclude,
assuming a clear view of the planet photosphere and no contribution of the
accretion luminosity at $L^{\prime}$. For hot-start scenarios like AMES-Cond,
NaCo limits exclude objects with $M_{\mathrm{p}}>1.5~{}M_{\mathrm{J}}$, a very
tight upper limit. However, for a colder scenario like the for the BEX-Warm
models presented in Marleau et al. (2019) the mass limit is
$10.6~{}M_{\mathrm{J}}$ (assuming an age of $\tau=2$ Myr), providing a much
more loose constraint. We note that a planet with such a high mass would have
opened a much deeper and wider gap in the gas distribution of the disk than
observed in 12CO (Kanagawa et al., 2015).

Bae et al. (2022b) used an empirical planet mass $-$ gap width relation and
calculated the mass of the planet embedded in the CPD candidate to be
$0.42-4.2~{}M_{\mathrm{J}}$, depending on the disk viscosity, assuming it is
responsible for carving out the gas gap at $\sim 200$ au. We note however that
more recent work suggested that even lower masses are possible from a
dynamical point of view. In fact, Galloway-Sprietsma et al. (2023) found disk
winds emerging from the gap the CPD candidate is embedded in, and suggested
that the kinematics could be dominated by the winds, not the planet. In this
case, the planet mass inferred from the gap width could be over-estimated.
These mass estimates agree with the VLT/NaCo detection limits.

###  5.2 The non-detection from the candidate

Different models can be used to transform measured H$\alpha$ luminosities to
mass accretion rates. Here, we compare two sets of models: the emission model
of shock-heated gas for planetary mass objects (Aoyama et al., 2018, 2020),
and the magnetospheric accretion model applied to planetary objects presented
in Thanathibodee et al. (2019). Based on their non-Local Thermal Equilibrium
(non-LTE) models, Aoyama et al. (2021) determined the
$L_{\mathrm{H\alpha}}-L_{\mathrm{acc}}$ relation for protoplanets. Applying
the relation to our measurement from Sect. 4.1, we obtain
$\log(L_{\mathrm{acc}}/L_{\odot})=-6.16\pm 0.05$. Following Gullbring et al.
(1998), the mass accretion rate $\dot{M}_{\mathrm{acc}}$ can then be estimated
with

$\dot{M}_{\mathrm{acc}}=\left(1-\frac{R_{\mathrm{p}}}{R_{in}}\right)^{-1}\frac{L_{\mathrm{acc}}R_{\mathrm{p}}}{GM_{\mathrm{p}}},$
(1)

where $R_{\mathrm{p}}$ is the planet radius, $R_{in}$ is the disk truncation
radius, assumed to be $\approx 5R_{\mathrm{p}}$, and $G$ is the gravitational
constant. The mass of the planet has been assumed to be between
$0.5~{}M_{\mathrm{J}}$ and $4~{}M_{\mathrm{J}}$ (see Sect. 5.1) and radii were
drawn uniformly between 1 and $3~{}R_{\mathrm{J}}$.

In order to consider magnetospheric accretion, we use the models from
Thanathibodee et al. (2019) to generate a large grid of scenarios producing
H$\alpha$ lines. For each calculation, we systematically varied the planets’
and model’s parameters encompassing a wide range of values whose boundaries
are given either by the AS209 system possible properties or by extreme
scenarios. Using masses between 0.5 and 4 $M_{\mathrm{J}}$, we adopted
evolutionary models from Mordasini et al. (2017) to estimate radii for each
mass consistent with 1-2 Myr ($R_{\mathrm{p}}$ in the
$1.9-3.1~{}R_{\mathrm{J}}$ range). For a given set of mass and radius, we
calculate a grid of 4050 models varying the magnetospheric parameters. The
mass accretion rate $\log(\dot{M}_{\mathrm{acc}}/M_{\mathrm{J}}$ yr${}^{-1})$
was taken between $-9$ (weak H$\alpha$ produced) to $-5$ (typical stellar
accretion rate), the maximum flow temperature from 7000 K to 8000 K (lower
temperature don’t produce H$\alpha$)222We note this parameter is very
uncertain even for the more studied case of accreting stars (Muzerolle et al.,
2001)., the magnetospheric truncation radius from $1.5~{}R_{\mathrm{p}}$
(smaller values would bring the disk too close to the planet) to
$6~{}R_{\mathrm{p}}$ (larger values require an unlikely strong magnetic
field), flow width from $0.5~{}R_{\mathrm{p}}$ to $2.5~{}R_{\mathrm{p}}$
(similar to stellar scenario), and the inclination from 20 to 50 degree
(consistent with the circumstellar disk’s inclination). We then calculate the
line luminosity from each model and select the model from which the luminosity
is within 1$\sigma$ of the upper limits.

Initially, we did not consider extinction ($A_{\mathrm{H\alpha}}=0$ mag).
Figure 5 shows the mass accretion rate estimate as a function of the planet
mass for the H$\alpha$ limit of the CPD candidate. The orange dashed line
represents the average mass accretion rate $\overline{\dot{M}_{\mathrm{acc}}}$
necessary to reach the planet mass on the $x$-axis in 2 Myr. As it is still
unclear if and how accretion rates onto protoplanets evolve with time and on
which timescales, we use $\overline{\dot{M}_{\mathrm{acc}}}$ as a proxy for a
constant mass accretion rate over the stellar lifetime (2 Myr). If
$\dot{M}_{\mathrm{acc}}>\overline{\dot{M}_{\mathrm{acc}}}$, then the planet is
either undergoing a period of vigorous (above average) accretion (meaning it
underwent lower accretion in the past) or the planet age is lower than 2 Myr.
If $\dot{M}_{\mathrm{acc}}<\overline{\dot{M}_{\mathrm{acc}}}$, then a large
fraction of the planet mass was accreted in the past and currently the mass
accretion rate is significantly lower than earlier phases. In Fig. 5 the blue
circles represent the distribution of possible
$M_{\mathrm{p}}-\dot{M}_{\mathrm{acc}}$ using the shock-heated gas models,
while the black diamonds report the results from the magnetospheric accretion
models.

The two models show different trends for increasing planet masses. The reason
is that in the magnetospheric accretion model, to the first order, the line
flux scales with the flow density $\rho\propto\dot{M}/\sqrt{M_{p}}$ (Hartmann
et al., 1994, , Eq. 9) and hence for fixed line fluxes $\rho$ does not vary
and $\dot{M}_{\mathrm{acc}}\propto\sqrt{M_{\mathrm{p}}}$. Conversely, in the
accretion shock models free fall velocity of the infalling material is higher
for a more massive planet, increasing the H$\alpha$ flux. As a consequence, to
produce the same H$\alpha$ emission lower $\dot{M}_{\mathrm{acc}}$ is required
at higher masses. The comparison betwen $\overline{\dot{M}_{\mathrm{acc}}}$
and the results for both sets of models suggests that the accretion rate is
$10-100$ times lower than $\overline{\dot{M}_{\mathrm{acc}}}$. Hence, assuming
no extinction, it seems the planet is accreting at a relatively low rate,
indicating that in the past the accretion rate was likely much more vigorous.

However, circumstellar and circumplanetary material can strongly affect line
emission at H$\alpha$ wavelengths, especially in the presence of dust.
Avenhaus et al. (2018) detected scattered light from the disk gap in AS209
indicative of the presence of small grains. Moreover, the non-zero disk
inclination ($i=35^{\circ}$, Huang et al. 2018) possibly increases its effect
on the detectable line emission. Unfortunately, the extinction properties of
the outer regions of a circumstellar disk have never been measured, not to
mention that we expect it to vary between sources and within individual
sources depending on the location with respect to substructures and disk
geometry.

From the non-detection of H$\beta$ and the re-detection of H$\alpha$,
Hashimoto et al. (2020) constrained the extinction towards PDS70 b to be
$A_{\mathrm{H\alpha}}\gtrsim 2$ mag, while Uyama et al. (2021) inferred
$A_{V}\approx 0.9$ and $2.0$ mag for PDS70 b and c respectively considering
the non-detection of Pa$\beta$. Following these estimates, we adopt here
$A_{\mathrm{H\alpha}}=2.0$ mag at the position of the CPD candidate, as it is
located, similar to the PDS70 planets, in a deep gap, where the amount of
small grains and pebble is expected to be relatively low (but not absent, see
Avenhaus et al. 2018).

Including the effect of the extinction from the circumstellar disk material in
the gap shifts the results from both models vertically along the $y$-axis of
Fig. 5. The new mass accretion rates are now closer to
$\overline{\dot{M}_{\mathrm{acc}}}$, but still $1-2$ orders of magnitude
lower. If this is the case, we conclude again that the protoplanet candidate
accreted material much more vigorously in the first phases of its life (Lubow
& Martin, 2012; Brittain et al., 2020). The consequence of this early
accretion scenario for direct imaging surveys of H$\alpha$ emission would be
drastic: at younger ages, when planets would accrete at larger rates, stars
are still embedded in their envelopes and the direct detection of line
emission would likely be hindered by the much larger extinction in the
optical.

We note however, that the PDS70 planets (even though located in a different
region of the disk) show signs of relatively strong accretion
($>10^{-7}~{}M_{\mathrm{J}}$ yr-1, Hashimoto et al. 2020; Zhou et al. 2021) at
much older ages ($\tau\sim 8$ Myr, Wang et al., 2021). If the protoplanet
candidate continues to accrete material at least at a similar rate, an
additional source of attenuation is needed to explain the non-detection,
likely from circumplanetary material333We note that Marleau et al. (2022)
showed that the accreting material only is not able to substantially increase
the extinction along the line of sight.. This scenario would be consistent
with the interpretation of the 13CO detection from Bae et al. (2022b), in
which a circumplanetary disk surrounds the forming planet. We note that, under
the assumption of a planetary atmosphere with physical and chemical properties
similar to those of more mature planets, Cugno et al. (2021) estimated the
extinction from the circumplanetary environment necessary to suppress
molecular features from VLT/SINFONI data of PDS70 b to be
$A_{\mathrm{V}}\approx 10-15$ mag ($A_{\mathrm{H\alpha}}\approx 8-12$ mag).
For the same object, Wang et al. (2021) estimated the extinction from fitting
the protoplanet’s SED with atmospheric models to be up to $A_{V}\sim 10$ mag.
A similar value for the extinction towards the protoplanet candidate would
allow the $L_{\mathrm{H\alpha}}$ upper limits to be consistent with
$\overline{\dot{M}_{\mathrm{acc}}}$. The much larger extinction from the CPD
material is expected to strongly suppress the H$\alpha$ signal produced by
accretion processes. If this is the case, accretion tracers at longer
wavelengths (e.g., Br$\gamma$ at $2.16~{}\mu$m, Br$\alpha$ at $4.05~{}\mu$m)
could help reducing the effect of the extinction and confirming the presence
of an accreting CPD.

A comparison of the CPD candidate around AS209 with the PDS70 planets reveals
significant environmental differences that could explain the non-detection of
the former and the multiple detections of the latter. The more advanced
developmental stage of the PDS70 system potentially indicates a different
evolutionary phase, wherein the planets may be less embedded compared to much
younger protoplanetary disks like AS209, as with larger masses and longer
times they are more capable of clearing gaps (Szulágyi & Ercolano, 2020;
Sanchis et al., 2020). Moreover, the presence of two planets inhabiting the
same gap likely ensures an even higher level of gap clearing, thereby
mitigating the effect of extinction on the emitted H$\alpha$ flux. In order to
thoroughly characterize the evolution of forming planets, a greater number of
confirmed protoplanet direct detections is necessary, but aforementioned
dissimilarities suggest that distinct features are observable during the
various phases of their formation.

Figure 5: Mass accretion rate upper limit of the protoplanet candidate as a
function of the planet mass $M_{\mathrm{p}}$ under different scenarios. The
orange dashed line represents the average mass accretion necessary to build a
planet of a given mass in 2 Myr. The blue and grey circles represent the mass
accretion rate estimated using the shock-heated accretion models from Aoyama
et al. (2021) using the observed H$\alpha$ luminosity
($A_{\mathrm{H\alpha}}=0,2$ respectively). Black and green diamonds indicate
the mass accretion rate when applying the magnetospheric accretion model for
$A_{\mathrm{H\alpha}}=0$ mag and 2 mag respectively.

###  5.3 A jet in a planet-forming disk

Images of microjets (extensions $\sim 100$ au) in Class II sources are
relatively rare and are mostly confined to forbidden lines (Ray et al., 2007;
Pascucci et al., 2022). However, microjets from TTauri stars with high
accretion rates are inferred to be common via spatially unresolved spectra of
strong forbidden lines such as [O I] ($\lambda$6300) and [S II]
($\lambda$6731), where a ‘high velocity component’ is attributed to collimated
outflowing gas with centroids around $-100$ km s-1, with the red side of the
line occulted by the disk (Hartigan et al., 1995). In contrast, detection of a
microjet at H$\alpha$ in a TTauri star is usually prohibitive due to the high-
contrast required to distinguish it from the broad emission arising from
magnetospheric accretion onto the star. As an example, in AS209 the equivalent
width of H$\alpha$ is 113 Å and the line wings extend to $\pm$ 300 km s-1
(Alencar & Basri, 2000). Instruments such as MagAO-X promise to be game
changing for imaging microjets at H$\alpha$ in accreting Class II sources
that, in tandem with forbidden lines, will enable physical parameters, such as
density, temperature and mass ejection rates to be determined, as is currently
done for resolved jets from Class I and 0 sources (e.g. Nisini et al., 2005).

Of two published profiles of [O I] 6300 in AS209, one from KECK’s HIRES
spectrograph (Fang et al., 2018) and the other from Magellan’s MIKE
spectrograph (Banzatti et al., 2019), only the second shows a high velocity
component associated with jet emission, while both show a low velocity
component associated with a slow, extended disk wind. Whether this is due to
jet variability or to different slit alignments is unclear. The accretion rate
for AS209 has been estimated from calibrated relationships between emission
lines and accretion luminosity established from Balmer Jump emission (Alcalá
et al., 2017) to be
$\log(\dot{M}_{\mathrm{acc}}/(M_{\odot}~{}\mathrm{yr}^{-1}))=-8.3$ (Fang et
al., 2018), sufficiently high for jet emission to be expected (Nisini et al.,
2018).

The receding jet was not detected, likely due to absorption from the
protoplanetary disk material. Although receding jets are sometimes seen, (for
example the Ae/Be star HD163296, see Xie et al., 2021), this is a relatively
rare occurrence in TTauri stars.

Future work with instruments like the Visible Integral-field Spectrograph
eXtreme (VIS-X, Haffert et al., 2021) will be able to spectrally and spatially
resolve the jet at H$\alpha$, allowing for the investigation of its velocity
components. In addition, the calculation of ratios with other emission lines
in the jet will enable the estimation of the level of ionization and thus jet
mass loss rate. Finally, an exciting prospect is the comparison between the
stellar and the planetary mass accretion rates, once emission lines, possibly
at longer wavelengths, will be detected.

###  5.4 The future of ground-based high-contrast imaging in the optical

The detection limits reported in Fig. 3 show that in this case the data
obtained with MagAO-X are more sensitive than those obtained from space with
HST at every separation. Even though at least partially this is due to the
very stable weather conditions during the observations and the longer amount
of time spent on target (3.5 hr versus 23.4 min)444 Note that including
overheads, the amount f time used for MagAO-X and HST observations is
comparable, showcasing the efficiency of ground-based imaging., this work
demonstrates the success of the recent efforts from the community to improve
AO correction at short wavelengths. An additional point to consider is that
AS209 is a relatively faint star ($V=11.3$ mag) and the AO-correction will be
even better for brighter targets like nearby stars. This is an exciting
premise for the future development of ground-based high-contrast imagers like
ELT/EPICS (Kasper et al., 2010) and GMagAO-X (Males et al., 2022b) that in the
next decade will search for small terrestrial planets in reflected light.

##  6 Summary and Conclusions

We have observed AS209 with the MagAO-X instrument on the 6.5m Magellan Clay
telescope at Las Campanas Observatory in dual imaging mode and with the
HST/WFC3/UVIS camera in the F656N filter. We detected a collimated jet in
H$\alpha$, which can be spectrally resolved with future observations to study
the different velocity components. In addition, no signal from the CPD
candidate proposed in Bae et al. (2022b) has been detected, suggesting that
either the instantaneous mass accretion rate is very low, or that local
extinction from the circumstellar and circumplanetary environments is strongly
attenuating line emission in the optical.

The interpretation of the protoplanet candidate orbiting AS209 and its mass
accretion rate is highly dependent on the flux absorption. Our current
inability to accurately characterize the extinction from these elements
prevents us from further constraining the accretion onto the protoplanet
candidate. Future observations should try to detect other hydrogen
recombination lines in the near-IR, where extinction plays a minor role. The
comparison of line luminosities will further constrain the extinction from the
line emitting region.

GC thanks the Swiss National Science Foundation for financial support under
grant number P500PT_206785. Y.Z. acknowledges support from the Heising-Simons
Foundation 51 Pegasi b Fellowship. This project has received funding from the
European Research Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (PROTOPLANETS, grant agreement No.
101002188). SF is funded by the European Union under the European Union’s
Horizon Europe Research & Innovation Programme 101076613 (UNVEIL). SYH was
supported by NASA through the NASA Hubble Fellowship grant #HST-
HF2-51436.001-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. LC and the MaxProtoPlanetS
survey are supported by NASA eXoplanet Research Program (XRP) grant
80NSSC21K0397. LAP acknowledges research support from the NSF Graduate
Research Fellowship under Grant No. DGE-1746060. The results reported herein
include data gathered with the Magellan Clay 6.5-meter telescope located at
Las Campanas Observatory, Chile. MagAO-X was developed under NSF MRI Award
#1625441. The observations and data analysis works were supported by program
HST-GO-17283. Some of the data presented in this paper were obtained from the
Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science
Institute. The specific observations analyzed can be accessed via
https://doi.org/10.17909/nf39-4h66 (catalog
https://doi.org/10.17909/nf39-4h66). Supports for Program numbers HST-GO-17283
were provided by NASA through a grant from the Space Telescope Science
Institute, which is operated by the Association of Universities for Research
in Astronomy, Incorporated, under NASA contract NAS5-26555.

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