arXiv subjects

L. Ilsedore Cleeves

Publications and source records attributed to L. Ilsedore Cleeves.

At least 73 records · Page 4Linked to original sources

arXiv2021

Simulating Observations of Ices in Protoplanetary Disks

Ices are an important constituent of protoplanetary disks. New observational facilities, notably the James Webb Space Telescope (JWST), will greatly enhance our view of disk ices by measuring their infrared spectral features. We present a suite of models to complement these upcoming observations. Our models use a kinetics-based gas-grain chemical evolution code to simulate the distribution of ices in a disk, followed by a radiative transfer code using a subset of key ice species to simulate the observations. We present models reflecting both molecular inheritance and chemical reset initial conditions. We find that near-to-mid-IR absorption features of H$_2$O, CO$_2$, and CH$_3$OH are readily observable in disk-integrated spectra of highly inclined disks while CO, NH$_3$, and CH$_4$ ice do not show prominent features. CH$_3$OH ice has low abundance and is not observable in the reset model, making this species an excellent diagnostic of initial chemical conditions. CO$_2$ ice features exhibit the greatest change over disk lifetime, decreasing and increasing for the inheritance and reset models, respectively. Spatially resolved spectra of edge-on disks, possible with JWST's integral field unit observing modes, are ideal for constraining the vertical distribution of ices and may be able to isolate features from ices closer to the midplane (e.g., CO) given sufficient sensitivity. Spatially-resolved spectra of face-on disks can trace scattered-light features from H$_2$O, CO$_2$, and CH$_3$OH, plus CO and CH$_4$ from the outermost regions. We additionally simulate far-IR H$_2$O ice emission features and find they are strongest for disks viewed face-on.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) II: CLEAN Strategies for Synthesizing Images of Molecular Line Emission in Protoplanetary Disks

The Molecules with ALMA at Planet-forming Scales large program (MAPS LP) surveyed the chemical structures of five protoplanetary disks across more than 40 different spectral lines at high angular resolution (0.15" and 0.30" beams for Bands 6 and 3, respectively) and sensitivity (spanning 0.3 - 1.3 mJy/beam and 0.4 - 1.9 mJy/beam for Bands 6 and 3, respectively). In this article, we describe our multi-stage workflow -- built around the CASA tclean image deconvolution procedure -- that we used to generate the core data product of the MAPS LP: the position-position-velocity image cubes for each spectral line. Owing to the expansive nature of the survey, we encountered a range of imaging challenges; some are familiar to the sub-mm protoplanetary disk community, like the benefits of using an accurate CLEAN mask, and others less well-known, like the incorrect default flux scaling of the CLEAN residual map first described in Jorsater & van Moorsel 1995 (the "JvM effect"). We distill lessons learned into recommended workflows for synthesizing image cubes of molecular emission. In particular, we describe how to produce image cubes with accurate fluxes via the "JvM correction," a procedure that is generally applicable to any image synthesized via CLEAN deconvolution but is especially critical for low S/N emission. We further explain how we used visibility tapering to promote a common, fiducial beam size and contextualize the interpretation of signal to noise ratio when detecting molecular emission from protoplanetary disks. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) IV: Emission Surfaces and Vertical Distribution of Molecules

The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program provides a unique opportunity to study the vertical distribution of gas, chemistry, and temperature in the protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. By using the asymmetry of molecular line emission relative to the disk major axis, we infer the emission height ($z$) above the midplane as a function of radius ($r$). Using this method, we measure emitting surfaces for a suite of CO isotopologues, HCN, and C$_2$H. We find that $^{12}$CO emission traces the most elevated regions with $z/r > 0.3$, while emission from the less abundant $^{13}$CO and C$^{18}$O probes deeper into the disk at altitudes of $z/r \lesssim 0.2$. C$_2$H and HCN have lower opacities and SNRs, making surface fitting more difficult, and could only be reliably constrained in AS 209, HD 163296, and MWC 480, with $z/r \lesssim 0.1$, i.e., relatively close to the planet-forming midplanes. We determine peak brightness temperatures of the optically thick CO isotopologues and use these to trace 2D disk temperature structures. Several CO temperature profiles and emission surfaces show dips in temperature or vertical height, some of which are associated with gaps and rings in line and/or continuum emission. These substructures may be due to local changes in CO column density, gas surface density, or gas temperatures, and detailed thermo-chemical models are necessary to better constrain their origins and relate the chemical compositions of elevated disk layers with those of planet-forming material in disk midplanes. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) XIII: HCO$^+$ and disk ionization structure

We observed HCO$^+$ $J=1-0$ and H$^{13}$CO$^+$ $J=1-0$ emission towards the five protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480 as part of the MAPS project. HCO$^+$ is detected and mapped at 0.3\arcsec\,resolution in all five disks, while H$^{13}$CO$^+$ is detected (SNR$>6 σ$) towards GM Aur and HD 163296 and tentatively detected (SNR$>3 σ$) towards the other disks by a matched filter analysis. Inside a radius of $R\sim 100$ au, the HCO$^+$ column density is flat or shows a central dip. At outer radii ($\gtrsim 100$ au), the HCO$^+$ column density decreases outwards, while the column density ratio of HCO$^+$/CO is mostly in the range of $\sim 10^{-5}-10^{-4}$. We derived the HCO$^+$ abundance in the warm CO-rich layer, where HCO$^+$ is expected to be the dominant molecular ion. At $R\gtrsim 100$ au, the HCO$^+$ abundance is $\sim 3 \times 10^{-11} - 3\times 10^{-10}$, which is consistent with a template disk model with X-ray ionization. At the smaller radii, the abundance decreases inwards, which indicates that the ionization degree is lower in denser gas, especially inside the CO snow line, where the CO-rich layer is in the midplane. Comparison of template disk models with the column densities of HCO$^+$, N$_2$H$^+$, and N$_2$D$^+$ indicates that the midplane ionization rate is $\gtrsim 10^{-18}$ s$^{-1}$ for the disks around IM Lup, AS 209, and HD 163296. We also find hints of an increased HCO$^+$ abundance around the location of dust continuum gaps in AS 209, HD 163296, and MWC 480. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) XII: Inferring the C/O and S/H ratios in Protoplanetary Disks with Sulfur Molecules

Sulfur-bearing molecules play an important role in prebiotic chemistry and planet habitability. They are also proposed probes of chemical ages, elemental C/O ratio, and grain chemistry processing. Commonly detected in diverse astrophysical objects, including the Solar System, their distribution and chemistry remain, however, largely unknown in planet-forming disks. We present CS ($2-1$) observations at $\sim0."3$ resolution performed within the ALMA-MAPS Large Program toward the five disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. CS is detected in all five disks, displaying a variety of radial intensity profiles and spatial distributions across the sample, including intriguing apparent azimuthal asymmetries. Transitions of C$_2$S and SO were also serendipitously covered but only upper limits are found. For MWC 480, we present complementary ALMA observations at $\sim0."5$, of CS, $^{13}$CS, C$^{34}$S, H$_2$CS, OCS, and SO$_2$. We find a column density ratio N(H$_{2}$CS)/N(CS)$\sim2/3$, suggesting that a substantial part of the sulfur reservoir in disks is in organic form (i.e., C$_x$H$_y$S$_z$). Using astrochemical disk modeling tuned to MWC 480, we demonstrate that $N$(CS)/$N$(SO) is a promising probe for the elemental C/O ratio. The comparison with the observations provides a super-solar C/O. We also find a depleted gas-phase S/H ratio, suggesting either that part of the sulfur reservoir is locked in solid phase or that it remains in an unidentified gas-phase reservoir. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) I: Program Overview and Highlights

Planets form and obtain their compositions in dust and gas-rich disks around young stars, and the outcome of this process is intimately linked to the disk chemical properties. The distributions of molecules across disks regulate the elemental compositions of planets, including C/N/O/S ratios and metallicity (O/H and C/H), as well as access to water and prebiotically relevant organics. Emission from molecules also encodes information on disk ionization levels, temperature structures, kinematics, and gas surface densities, which are all key ingredients of disk evolution and planet formation models. The Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program was designed to expand our understanding of the chemistry of planet formation by exploring disk chemical structures down to 10 au scales. The MAPS program focuses on five disks - around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480 - in which dust substructures are detected and planet formation appears to be ongoing. We observed these disks in 4 spectral setups, which together cover ~50 lines from over 20 different species. This paper introduces the ApJS MAPS Special Issue by presenting an overview of the program motivation, disk sample, observational details, and calibration strategy. We also highlight key results, including discoveries of links between dust, gas, and chemical sub-structures, large reservoirs of nitriles and other organics in the inner disk regions, and elevated C/O ratios across most disks. We discuss how this collection of results is reshaping our view of the chemistry of planet formation.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS). XV. Tracing protoplanetary disk structure within 20 au

Constraining the distribution of gas and dust in the inner 20 au of protoplanetary disks is difficult. At the same time, this region is thought to be responsible for most planet formation, especially around the water ice line at 3-10 au. Under the assumption that the gas is in a Keplerian disk, we use the exquisite sensitivity of the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA large program to construct radial surface brightness profiles with a ~3 au effective resolution for the CO isotopologue J=2-1 lines using the line velocity profile. IM Lup reveals a central depression in 13CO and C18O that is ascribed to a pileup of ~500 $M_\oplus$ of dust in the inner 20 au, leading to a gas-to-dust ratio of around <10. This pileup is consistent with efficient drift of grains ($\gtrsim$ 100 $M_\oplus$ Myr$^{-1}$) and a local gas-to-dust ratio that suggests that the streaming instability could be active. The CO isotopologue emission in the GM Aur disk is consistent with a small (~15 au), strongly depleted gas cavity within the ~40 au dust cavity. The radial surface brightness profiles for both the AS 209 and HD 163296 disks show a local minimum and maximum in the C18O emission at the location of a known dust ring (~14 au) and gap (~10 au), respectively. This indicates that the dust ring has a low gas-to-dust ratio ($>$ 10) and that the dust gap is gas-rich enough to have optically thick C18O.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS). VII. Sub-stellar O/H and C/H and super-stellar C/O in planet feeding gas

The elemental composition of the gas and dust in a protoplanetary disk influences the compositions of the planets that form in it. We use the Molecules with ALMA at Planet-forming Scales (MAPS) data to constrain the elemental composition of the gas at the locations of potentially forming planets. The elemental abundances are inferred by comparing source-specific gas-grain thermochemical models, with variable C/O ratios and small-grain abundances, from the DALI code with CO and C2H column densities derived from the high-resolution observations of the disks of AS 209, HD 163296, and MWC 480. Elevated C/O ratios (~2.0), even within the CO ice line, are necessary to match the inferred C2H column densities, over most of the pebble disk. Combined with constraints on the CO abundances in these systems, this implies that both the O/H and C/H ratios in the gas are substellar by a factor of 4-10, with the O/H depleted by a factor of 20-50, resulting in the high C/O ratios. This necessitates that even within the CO ice line, most of the volatile carbon and oxygen is still trapped on grains in the midplane. Planets accreting gas in the gaps of the AS 209, HD 163296, and MWC 480 disks will thus acquire very little carbon and oxygen after reaching the pebble isolation mass. In the absence of atmosphere-enriching events, these planets would thus have a strongly substellar O/H and C/H and superstellar C/O atmospheric composition.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) XI: CN and HCN as Tracers of Photochemistry in Disks

UV photochemistry in the surface layers of protoplanetary disks dramatically alters their composition relative to previous stages of star formation. The abundance ratio CN/HCN has long been proposed to trace the UV field in various astrophysical objects, however to date the relationship between CN, HCN, and the UV field in disks remains ambiguous. As part of the ALMA Large Program MAPS (Molecules with ALMA at Planet-forming Scales), we present observations of CN N=1-0 transitions at 0.3'' resolution towards five disk systems. All disks show bright CN emission within $\sim$50-150 au, along with a diffuse emission shelf extending up to 600 au. In all sources we find that the CN/HCN column density ratio increases with disk radius from about unity to 100, likely tracing increased UV penetration that enhances selective HCN photodissociation in the outer disk. Additionally, multiple millimeter dust gaps and rings coincide with peaks and troughs, respectively, in the CN/HCN ratio, implying that some millimeter substructures are accompanied by changes to the UV penetration in more elevated disk layers. That the CN/HCN ratio is generally high (>1) points to a robust photochemistry shaping disk chemical compositions, and also means that CN is the dominant carrier of the prebiotically interesting nitrile group at most disk radii. We also find that the local column densities of CN and HCN are positively correlated despite emitting from vertically stratified disk regions, indicating that different disk layers are chemically linked. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) XIX. Spiral Arms, a Tail, and Diffuse Structures Traced by CO around the GM Aur Disk

The concentric gaps and rings commonly observed in protoplanetary disks in millimeter continuum emission have lent the impression that planet formation generally proceeds within orderly, isolated systems. While deep observations of spatially resolved molecular emission have been comparatively limited, they are increasingly suggesting that some disks interact with their surroundings while planet formation is underway. We present an analysis of complex features identified around GM Aur in $^{12}$CO $J=2-1$ images at a spatial resolution of $\sim40$ au. In addition to a Keplerian disk extending to a radius of $\sim550$ au, the CO emission traces flocculent spiral arms out to radii of $\sim$1200 au, a tail extending $\sim1800$ au southwest of GM Aur, and diffuse structures extending from the north side of the disk up to radii of $\sim1900$ au. The diffuse structures coincide with a "dust ribbon" previously identified in scattered light. The large-scale asymmetric gas features present a striking contrast with the mostly axisymmetric, multi-ringed millimeter continuum tracing the pebble disk. We hypothesize that GM Aur's complex gas structures result from late infall of remnant envelope or cloud material onto the disk. The morphological similarities to the SU Aur and AB Aur systems, which are also located in the L1517 cloud, provide additional support to a scenario in which interactions with the environment are playing a role in regulating the distribution and transport of material in all three of these Class II disk systems. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS). IX. Distribution and Properties of the Large Organic Molecules HC$_3$N, CH$_3$CN, and $c$-C$_3$H$_2$

The precursors to larger, biologically-relevant molecules are detected throughout interstellar space, but determining the presence and properties of these molecules during planet formation requires observations of protoplanetary disks at high angular resolution and sensitivity. Here we present 0.3" observations of HC$_3$N, CH$_3$CN, and $c$-C$_3$H$_2$ in five protoplanetary disks observed as part of the Molecules with ALMA at Planet-forming Scales (MAPS) Large Program. We robustly detect all molecules in four of the disks (GM Aur, AS 209, HD 163296 and MWC 480) with tentative detections of $c$-C$_3$H$_2$ and CH$_3$CN in IM Lup. We observe a range of morphologies -- central peaks, single or double rings -- with no clear correlation in morphology between molecule nor disk. Emission is generally compact and on scales comparable with the millimetre dust continuum. We perform both disk-integrated and radially-resolved rotational diagram analysis to derive column densities and rotational temperatures. The latter reveals 5-10 times more column density in the inner 50-100 au of the disks when compared with the disk-integrated analysis. We demonstrate that CH$_3$CN originates from lower relative heights in the disks when compared with HC$_3$N, in some cases directly tracing the disk midplane. Finally, we find good agreement between the ratio of small to large nitriles in the outer disks and comets. Our results indicate that the protoplanetary disks studied here are host to significant reservoirs of large organic molecules, and that this planet- and comet-building material can be chemically similar to that in our own Solar System. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement Series.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) XVI: Characterizing the impact of the molecular wind on the evolution of the HD 163296 system

During the main phase of evolution of a protoplanetary disk, accretion regulates the inner-disk properties, such as the temperature and mass distribution, and in turn, the physical conditions associated with planet formation. The driving mechanism behind accretion remains uncertain; however, one promising mechanism is the removal of a fraction of angular momentum via a magnetohydrodynamic (MHD) disk wind launched from the inner tens of astronomical units of the disk. This paper utilizes CO isotopologue emission to study the unique molecular outflow originating from the HD 163296 protoplanetary disk obtained with the Atacama Large Millimeter/submillimeter Array. HD~163296 is one of the most well-studied Class II disks and is proposed to host multiple gas-giant planets. We robustly detect the large-scale rotating outflow in the 12CO J=2-1 and the 13CO J=2-1 and J=1-0 transitions. We constrain the kinematics, the excitation temperature of the molecular gas, and the mass-loss rate. The high ratio of the rates of ejection to accretion (5 - 50), together with the rotation signatures of the flow, provides solid evidence for an MHD disk wind. We find that the angular momentum removal by the wind is sufficient to drive accretion through the inner region of the disk; therefore, accretion driven by turbulent viscosity is not required to explain HD~163296's accretion. The low temperature of the molecular wind and its overall kinematics suggest that the MHD disk wind could be perturbed and shocked by the previously observed high-velocity atomic jet. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) VI: Distribution of the small organics HCN, C2H, and H2CO

Small organic molecules, such as C2H, HCN, and H2CO, are tracers of the C, N, and O budget in protoplanetary disks. We present high angular resolution (10-50 au) observations of C2H, HCN, and H2CO lines in five protoplanetary disks from the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program. We derive column density and excitation temperature profiles for HCN and C2H, and find that the HCN emission arises in a temperate (20-30 K) layer in the disk, while C2H is present in relatively warmer (20-60 K) layers. In the case of HD 163296, we find a decrease in column density for HCN and C2H inside one of the dust gaps near 83 au, where a planet has been proposed to be located. We derive H2CO column density profiles assuming temperatures between 20 and 50 K, and find slightly higher column densities in the colder disks around T Tauri stars than around Herbig Ae stars. The H2CO column densities rise near the location of the CO snowline and/or millimeter dust edge, suggesting an efficient release of H2CO ices in the outer disk. Finally, we find that the inner 50 au of these disks are rich in organic species, with abundances relative to water that are similar to cometary values. Comets could therefore deliver water and key organics to future planets in these disks, similar to what might have happened here on Earth. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

Molecules with ALMA at Planet-forming Scales. XX. The Massive Disk Around GM Aurigae

Gas mass remains one of the most difficult protoplanetary disk properties to constrain. With much of the protoplanetary disk too cold for the main gas constituent, H2, to emit, alternative tracers such as dust, CO, or the H2 isotopolog HD are used. However, relying on disk mass measurements from any single tracer requires assumptions about the tracer's abundance relative to \hh\ and the disk temperature structure. Using new Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program as well as archival ALMA observations, we construct a disk physical/chemical model of the protoplanetary disk GM Aur. Our model is in good agreement with the spatially resolved CO isotopolog emission from eleven rotational transitions with spatial resolution ranging from 0.15'' to 0.46'' (24-73 au at 159 pc) and the spatially unresolved HD J=1-0 detection from Herschel. Our best-fit model favors a cold protoplanetary disk with a total gas mass of approximately 0.2 solar masses, a factor of 10 reduction in CO gas inside roughly 100 au and a factor of 100 reduction outside of 100 au. Despite its large mass, the disk appears to be on the whole gravitationally stable based on the derived Toomre Q parameter. However, the region between 70 and 100 au, corresponding to one of the millimeter dust rings, is close to being unstable based on the calculated Toomre Q of <1.7. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

An ALMA Survey of Chemistry in Disks around M4-M5 Stars

M-stars are the most common hosts of planetary systems in the Galaxy. Protoplanetary disks around M-stars thus offer a prime opportunity to study the chemistry of planet-forming environments. We present an ALMA survey of molecular line emission toward a sample of five protoplanetary disks around M4-M5 stars (FP Tau, J0432+1827, J1100-7619, J1545-3417, and Sz 69). These observations can resolve chemical structures down to tens of AU. Molecular lines of $^{12}$CO, $^{13}$CO, C$^{18}$O, C$_2$H, and HCN are detected toward all five disks. Lines of H$_2$CO and DCN are detected toward 2/5 and 1/5 disks, respectively. For disks with resolved C$^{18}$O, C$_2$H, HCN, and H$_2$CO emission, we observe substructures similar to those previously found in disks around solar-type stars (e.g., rings, holes, and plateaus). C$_2$H and HCN excitation conditions estimated interior to the pebble disk edge for the bright disk J1100-7619 are consistent with previous measurements around solar-type stars. The correlation previously found between C$_2$H and HCN fluxes for solar-type disks extends to our M4-M5 disk sample, but the typical C$_2$H/HCN ratio is higher for the M4-M5 disk sample. This latter finding is reminiscent of the hydrocarbon enhancements found by previous observational infrared surveys in the innermost ($<$10AU) regions of M-star disks, which is intriguing since our disk-averaged fluxes are heavily influenced by flux levels in the outermost disk, exterior to the pebble disk edge. Overall, most of the observable chemistry at 10-100AU appears similar for solar-type and M4-M5 disks, but hydrocarbons may be more abundant around the cooler stars.

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arXiv2021

Ice-Coated Pebble Drift as a Possible Explanation for Peculiar Cometary CO/H2O Ratios

To date, at least three comets -- 2I/Borisov, C/2016 R2 (PanSTARRS), and C/2009 P1 (Garradd) -- have been observed to have unusually high CO concentrations compared to water. We attempt to explain these observations by modeling the effect of drifting solid (ice and dust) material on the ice compositions in protoplanetary disks. We find that, independent of the exact disk model parameters, we always obtain a region of enhanced ice-phase CO/H2O that spreads out in radius over time. The inner edge of this feature coincides with the CO snowline. Almost every model achieves at least CO/H2O of unity, and one model reaches a CO/H2O ratio > 10. After running our simulations for 1 Myr, an average of 40% of the disk ice mass contains more CO than H2O ice. In light of this, a population of CO ice enhanced planetesimals are likely to generally form in the outer regions of disks, and we speculate that the aforementioned CO-rich comets may be more common, both in our own Solar System and in extrasolar systems, than previously expected.

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arXiv2021

Near-IR observations of the young star [BHB2007]-1: A sub-stellar companion opening the gap in the disk

The presence of planets or sub-stellar objects still embedded in their native protoplanetary disks is indirectly suggested by disk sub-structures like gaps, cavities, and spirals. However, these companions are rarely detected. We present VLT/NACO high-contrast images in $J$, $H$, $K_S$, and $L^{\prime}$ band of the young star [BHB2007]-1 probing the inclined disk in scattered light and revealing the probable presence of a companion. The point source is detected in the $L^{\prime}$ band in spatial correspondence with complementary VLA observations. This object is constrained to have a mass in the range of 37-47 M$_{Jup}$ and is located at 50 au from the central star, inside the 70 au-large disk cavity recently imaged by ALMA, that is absent from our NACO data (down to 20 au). This mass range is compatible with the upper end derived from the size of the ALMA cavity. The NIR disk brightness is highly asymmetric around the minor axis, with the southern side 5.5 times brighter than the northern side. The constant amount of asymmetry across all wavelengths suggests that it is due to a shadow cast by a misaligned inner disk. The massive companion that we detect could, in principle, explain the possible disk misalignment, as well as the different cavity sizes inferred by the NACO and ALMA observations. The confirmation and characterization of the companion is entrusted to future observations.

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arXiv2021

The TW Hya Rosetta Stone Project IV: A hydrocarbon rich disk atmosphere

Connecting the composition of planet-forming disks with that of gas giant exoplanet atmospheres, in particular through C/O ratios, is one of the key goals of disk chemistry. Small hydrocarbons like $\rm C_2H$ and $\rm C_3H_2$ have been identified as tracers of C/O, as they form abundantly under high C/O conditions. We present resolved $\rm C_3H_2$ observations from the TW Hya Rosetta Stone Project, a program designed to map the chemistry of common molecules at $15-20$ au resolution in the TW Hya disk. Augmented by archival data, these observations comprise the most extensive multi-line set for disks of both ortho and para spin isomers spanning a wide range of energies, $E_u=29-97$ K. We find the ortho-to-para ratio of $\rm C_3H_2$ is consistent with 3 throughout extent of the emission, and the total abundance of both $\rm C_3H_2$ isomers is $(7.5-10)\times10^{-11}$ per H atom, or $1-10$% of the previously published $\rm C_2H$ abundance in the same source. We find $\rm C_3H_2$ comes from a layer near the surface that extends no deeper than $z/r=0.25$. Our observations are consistent with substantial radial variation in gas-phase C/O in TW Hya, with a sharp increase outside $\sim30$ au. Even if we are not directly tracing the midplane, if planets accrete from the surface via, e.g., meridonial flows, then such a change should be imprinted on forming planets. Perhaps interestingly, the HR 8799 planetary system also shows an increasing gradient in its giant planets' atmospheric C/O ratios. While these stars are quite different, hydrocarbon rings in disks are common, and therefore our results are consistent with the young planets of HR 8799 still bearing the imprint of their parent disk's volatile chemistry.

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