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Register a new userOn the Origin of Banded Structure in Dusty Protoplanetary Discs: HL Tau and TW Hya
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Authors
Aaron C. Boley
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Recent observations of HL Tau revealed remarkably detailed structure within the systems circumstellar disc. A range of hypotheses have been proposed to explain the morphology, including, e.g., planet-disc interactions, condensation fronts, and secular gravitational instabilities. While embedded planets seem to be able to explain some of the major structure in the disc through interactions with gas and dust, the substructure, such as low-contrast rings and bands, are not so easily reproduced. Here, we show that dynamical interactions between three planets (only two of which are modelled) and an initial population of large planetesimals can potentially explain both the major and minor banded features within the system. In this context, the small grains, which are coupled to the gas and reveal the disc morphology, are produced by the collisional evolution of the newly-formed planetesimals, which are ubiquitous in the system and are decoupled from the gas.
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We report the detection of spiral substructure in both the gas velocity and temperature structure of the disk around TW~Hya, suggestive of planet-disk interactions with an unseen planet. Perturbations from Keplerian rotation tracing out a spiral pattern are observed in the SE of the disk, while significant azimuthal perturbations in the gas temperature are seen in the outer disk, outside 90~au, extending the full azimuth of the disk. The deviation in velocity is either $Delta v_{phi} , / , v_{rm kep} sim 0.1$ or $Delta v_{z} , / , v_{rm kep} sim 0.01$ depending on whether the perturbation is in the rotational or vertical direction, while radial perturbations can be ruled out. Deviations in the gas temperature are $pm 4$ K about the azimuthally averaged profile, equivalent to deviations of $Delta T_{rm gas} , / , T_{rm gas} sim 0.05$. Assuming all three structures can be described by an Archimedean spiral, measurements of the pitch angles of both velocity and temperature spirals show a radially decreasing trend for all three, ranging from 9$^{circ}$ at 70 au, dropping to 3$^{circ}$ at 200 au. Such low pitch-angled spirals are not readily explained through the wake of an embedded planet in the location of previously reported at 94 au, but rather require a launching mechanism which results in much more tightly wound spirals. Molecular emission tracing distinct heights in the disk is required to accurately distinguish between spiral launching mechanisms.
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.
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.
Disc-driven planet migration is integral to the formation of planetary systems. In standard, gas-dominated protoplanetary discs, low-mass planets or planetary cores undergo rapid inwards migration and are lost to the central star. However, several recent studies indicate that the solid component in protoplanetary discs can have a significant dynamical effect on disc-planet interaction, especially when the solid-to-gas mass ratio approaches unity or larger and the dust-on-gas drag forces become significant. As there are several ways to raise the solid abundance in protoplanetary discs, for example through disc winds and dust-trapping in pressure bumps, it is important to understand how planets migrate through a dusty environment. To this end, we study planet migration in dust-rich discs via a systematic set of high-resolution, two-dimensional numerical simulations. We show that the inwards migration of low-mass planets can be slowed down by dusty dynamical corotation torques. We also identify a new regime of stochastic migration applicable to discs with dust-to-gas mass ratios $gtrsim 0.3$ and particle Stokes numbers $gtrsim 0.03$. In these cases, disc-planet interaction leads to the continuous development of small-scale, intense dust vortices that scatter the planet, which can potentially halt or even reverse the inwards planet migration. We briefly discuss the observational implications of our results and highlight directions for future work.
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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