Dust and Gas in the disc of HL Tauri: Surface density, dust settling, and dust-to-gas ratio (1508.00584v2)

Abstract: The recent ALMA observations of the disc surrounding HL Tau reveal a very complex dust spatial distribution. We present a radiative transfer model accounting for the observed gaps and bright rings as well as radial changes of the emissivity index. We find that the dust density is depleted by at least a factor 10 in the main gaps compared to the surrounding rings. Ring masses range from 10-100 M${\oplus}$ in dust, and, we find that each of the deepest gaps is consistent with the removal of up to 40 M${\oplus}$ of dust. If this material has accumulated into rocky bodies, these would be close to the point of runaway gas accretion. Our model indicates that the outermost ring is depleted in millimetre grains compared to the central rings. This suggests faster grain growth in the central regions and/or radial migration of the larger grains. The morphology of the gaps observed by ALMA - well separated and showing a high degree of contrast with the bright rings over all azimuths - indicates that the millimetre dust disc is geometrically thin (scale height $\approx$ 1 au at 100 au) and that a large amount of settling of large grains has already occurred. Assuming a standard dust settling model, we find that the observations are consistent with a turbulent viscosity coefficient of a few $10^{-4}$. We estimate the gas/dust ratio in this thin layer to be of the order of 5 if the initial ratio is 100. The HCO$^+$ and CO emission is consistent with gas in Keplerian motion around a 1.7 $M_\odot$ star at radii from $\leq 10 - 120\,$au.

• The paper finds that dust density in disk gaps drops by at least tenfold compared to rings, suggesting key sites for planetesimal formation.
• It employs radiative transfer modeling to estimate dust masses between 10 and 100 Earth masses in rings and significant depletions of up to 40 Earth masses in gaps.
• The analysis determines a turbulent viscosity coefficient of around 10⁻⁴ and confirms gas in Keplerian motion around a 1.7 M⊙ star, shaping disc dynamics.

Analysis of "Dust and Gas in the Disk of HL Tauri"

In the paper "Dust and Gas in the Disk of HL Tauri: Surface Density, Dust Settling, and Dust-to-Gas Ratio," the authors present a comprehensive analysis of the protoplanetary disk surrounding HL Tau, utilizing recent Atacama Large Millimeter/submillimeter Array (ALMA) observations. This research focuses on the dust spatial distribution and the underlying physical processes driving dust dynamics in the early stages of planet formation.

Main Findings

The authors employ a radiative transfer model to interpret the high-resolution ALMA data, highlighting the presence of intricate dust structures, including gaps and rings. This paper reveals several key findings:

1. Dust Density and Distribution: The dust density in the gaps observed in the HL Tau disk is reduced by at least a factor of ten compared to the surrounding bright rings. The paper suggests that the observed substructures could imply significant dust accumulation or planetesimal formation at specific regions within the disk.
2. Ring and Gap Masses: The mass of dust present in the rings ranges from 10 to 100 Earth masses (M$_{\oplus}$), whereas the depletion in some of the deepest gaps is compatible with the removal of up to 40 M$_{\oplus}$ of dust, indicating potential sites of significant solid body formation.
3. Morphological Implications: The distinct contrast and separation of the gaps and rings across all azimuths suggest the HL Tau disk is geometrically thin, with millimeter grains undergoing substantial vertical settling.
4. Dust Settling and Turbulent Viscosity: To reproduce the observed dust settling, the researchers estimate a turbulent viscosity coefficient (α) on the order of $10^{-4}$. This value aligns with standard dust settling models and indicates a considerable level of grain settling toward the midplane.
5. Gas and Dust Dynamics: Analysis of HCO$^+$ and CO emissions corroborates that gas is in Keplerian motion around the central 1.7 M$_\odot$ star, with significant implications on gas disk dynamics and the mass estimation of the central star.

Implications and Future Directions

This paper provides insightful contributions to our understanding of early planet formation within protoplanetary disks. The detailed modeling of dust density and distribution offers a more in-depth view of the potential processes of planetesimal formation in young star systems.

The findings carry significant implications for theoretical models of dust growth and migration, as well as for the scaling of turbulent viscosity within protoplanetary disks. The suggestion of substantial grain growth in the disk's central regions and the radial migration of large grains offers avenues for future research to explore the efficiency of these mechanisms in different disk environments.

Moreover, the revelation that the dust disk is substantially thinner than initially believed challenges conventional perceptions of protoplanetary disk structure, advocating for refinements in models of vertical dust settling and radial migration.

Future studies should aim to extend these findings by considering the role of potential planetary bodies in shaping the disk's structure, exploring the disk's time evolution, and utilizing similar high-resolution observational techniques on other protoplanetary systems for comparative analysis. This work serves as a foundation for the ongoing exploration of planet formation processes and the dynamic interactions within protoplanetary disks.