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Register a new userVolatile carbon locking and release in protoplanetary disks. A study of TW Hya and HD 100546
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The composition of planetary solids and gases is largely rooted in the processing of volatile elements in protoplanetary disks. To shed light on the key processes, we carry out a comparative analysis of the gas-phase carbon abundance in two systems with a similar age and disk mass, but different central stars: HD 100546 and TW Hya. We combine our recent detections of C$^{0}$ in these disks with observations of other carbon reservoirs (CO, C$^{+}$, C$_{2}$H) and gas mass and warm gas tracers (HD, O$^{0}$), as well as spatially resolved ALMA observations and the spectral energy distribution. The disks are modelled with the DALI 2D physical-chemical code. Stellar abundances for HD 100546 are derived from archival spectra. Upper limits on HD emission from HD 100546 place an upper limit on the total disk mass of $leq0.1,M_{odot}$. The gas-phase carbon abundance in the atmosphere of this warm Herbig disk is at most moderately depleted compared to the interstellar medium, with [C]/[H]$_{rm gas}=(0.1-1.5)times 10^{-4}$. HD 100546 itself is a $lambda,$Bo{o}tis star, with solar abundances of C and O but a strong depletion of rock-forming elements. In the gas of the T Tauri disk TW Hya, both C and O are strongly underabundant, with [C]/[H]$_{rm gas}=(0.2-5.0)times 10^{-6}$ and C/O${>}1$. We discuss evidence that the gas-phase C and O abundances are high in the warm inner regions of both disks. Our analytical model, including vertical mixing and a grain size distribution, reproduces the observed [C]/[H]$_{rm gas}$ in the outer disk of TW Hya and allows to make predictions for other systems.
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The nearby Herbig Be star HD100546 is known to be a laboratory for the study of protoplanets and their relation with the circumstellar disk that is carved by at least 2 gaps. We observed the HD100546 environment with high contrast imaging exploiting several different observing modes of SPHERE, including datasets with/without coronagraphs, dual band imaging, integral field spectroscopy and polarimetry. The picture emerging from these different data sets is complex. Flux-conservative algorithms images clearly show the disk up to 200au. More aggressive algorithms reveal several rings and warped arms overlapping the main disk. The bright parts of this ring lie at considerable height over the disk mid-plane at about 30au. Our images demonstrate that the brightest wings close to the star in the near side of the disk are a unique structure, corresponding to the outer edge of the intermediate disk at ~40au. Modeling of the scattered light from the disk with a geometrical algorithm reveals that a moderately thin structure can well reproduce the light distribution in the flux-conservative images. We suggest that the gap between 44 and 113 au span between the 1:2 and 3:2 resonance orbits of a massive body located at ~70au that might coincide with the candidate planet HD100546b detected with previous thermal IR observations. In this picture, the two wings can be the near side of a ring formed by disk material brought out of the disk at the 1:2 resonance with the same massive object. While we find no clear evidence confirming detection of the planet candidate HD100546c in our data, we find a diffuse emission close to the expected position of HD100546b. This source can be described as an extremely reddened substellar object surrounded by a dust cloud or its circumplanetary disk. Its astrometry is broadly consistent with a circular orbital motion on the disk plane.
We report the first detection of a gap and a ring in 336 GHz dust continuum emission from the protoplanetary disk around TW Hya, using the Atacama Large Millimeter/Submillimeter Array (ALMA). The gap and ring are located at around 25 and 41 au from the central star, respectively, and are associated with the CO snow line at ~30 au. The gap has a radial width of less than 15 au and a mass deficit of more than 23%, taking into account that the observations are limited to an angular resolution of ~15 au. In addition, the 13CO and C18O J = 3 - 2 lines show a decrement in CO line emission throughout the disk, down to ~10 au, indicating a freeze-out of gas-phase CO onto grain surfaces and possible subsequent surface reactions to form larger molecules. The observed gap could be caused by gravitational interaction between the disk gas and a planet with a mass less than super-Neptune (2M_{Neptune}), or could be the result of the destruction of large dust aggregates due to the sintering of CO ice.
We present observations of the young stars TW Hya and HD 100546 made with the Australia Telescope Compact Array at 89 GHz with $sim2$ resolution and $sim3$ mJy continuum sensitivity. Compact thermal dust continuum emission is detected from disks surrounding both stars. HD 100546 also shows hints of extended emission, presumably a residual protostellar envelope, which is also visible in scattered light at optical wavelengths. For TW Hya, HCO$^+$ J=1--0 line emission from the circumstellar disk is detected and spatially resolved. The observed size and intensity are in good agreement with model calculations based on an irradiated disk with substantial depletions derived previously from single dish observations of higher-J HCO$^+$ transitions.
The volatile composition of a planet is determined by the inventory of gas and ice in the parent disk. The volatile chemistry in the disk is expected to evolve over time, though this evolution is poorly constrained observationally. We present ALMA observations of C18O, C2H, and the isotopologues H13CN, HC15N, and DCN towards five Class 0/I disk candidates. Combined with a sample of fourteen Class II disks presented in Bergner et al. (2019b), this data set offers a view of volatile chemical evolution over the disk lifetime. Our estimates of C18O abundances are consistent with a rapid depletion of CO in the first ~0.5-1 Myr of the disk lifetime. We do not see evidence that C2H and HCN formation are enhanced by CO depletion, possibly because the gas is already quite under-abundant in CO. Further CO depletion may actually hinder their production by limiting the gas-phase carbon supply. The embedded sources show several chemical differences compared to the Class II stage, which seem to arise from shielding of radiation by the envelope (impacting C2H formation and HC15N fractionation) and sublimation of ices from infalling material (impacting HCN and C18O abundances). Such chemical differences between Class 0/I and Class II sources may affect the volatile composition of planet-forming material at different stages in the disk lifetime.
Dark rings with bright rims are the indirect signposts of planets embedded in protoplanetary discs. In a recent first, an azimuthally elongated AU-scale blob, possibly a planet, was resolved with ALMA in TW Hya. The blob is at the edge of a cliff-like rollover in the dust disc rather than inside a dark ring. Here we build time-dependent models of TW Hya disc. We find that the classical paradigm cannot account for the morphology of the disc and the blob. We propose that ALMA-discovered blob hides a Neptune mass planet losing gas and dust. We show that radial drift of mm-sized dust particles naturally explains why the blob is located on the edge of the dust disc. Dust particles leaving the planet perform a characteristic U-turn relative to it, producing an azimuthally elongated blob-like emission feature. This scenario also explains why a 10 Myr old disc is so bright in dust continuum. Two scenarios for the dust-losing planet are presented. In the first, a dusty pre-runaway gas envelope of about 40 Earth mass Core Accretion planet is disrupted, e.g., as a result of a catastrophic encounter. In the second, a massive dusty pre-collapse gas giant planet formed by Gravitational Instability is disrupted by the energy released in its massive core. Future modelling may discriminate between these scenarios and allow us to study planet formation in an entirely new way -- by analysing the flows of dust and gas recently belonging to planets, informing us about the structure of pre-disruption planetary envelopes.
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