Interpreting high spatial resolution line observations of planet-forming disks with gaps and rings: the case of HD 163296

Abstract

Context. Spatially resolved continuum observations of planet-forming disks show prominent ring and gap structures in their dust distribution. However, the picture from gas observations is much less clear and constraints on the radial gas density structure (i.e. gas gaps) remain rare and uncertain. Aims. We want to investigate the importance of thermo-chemical processes for the interpretation of high-spatial-resolution gas observations of planet-forming disks and their impact on the derived gas properties. Methods. We applied the radiation thermo-chemical disk code PRODIMO (PROtoplanetary DIsk MOdel) to model the dust and gas disk of HD 163296 self-consistently, using the DSHARP (Disk Substructure at High Angular Resolution) gas and dust observations. With this model we investigated the impact of dust gaps and gas gaps on the observables and the derived gas properties, considering chemistry, and heating and cooling processes. Results. We find distinct peaks in the radial line intensity profiles of the CO line data of HD 163296 at the location of the dust gaps. Our model indicates that those peaks are not only a consequence of a gas temperature increase within the gaps but are mainly caused by the absorption of line emission from the back side of the disk by the dust rings. For two of the three prominent dust gaps in HD 163296, we find that thermo-chemical effects are negligible for deriving density gradients via measurements of the rotation velocity. However, for the gap with the highest dust depletion, the temperature gradient can be dominant and needs to be considered to derive accurate gas density profiles. Conclusions. Self-consistent gas and dust thermo-chemical modelling in combination with high-quality observations of multiple molecules are necessary to accurately derive gas gap depths and shapes. This is crucial to determine the origin of gaps and rings in planet-forming disks and to improve the mass estimates of forming planets if they are the cause of the gap.

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

“…In HD 163296, the derived CO surfaces, and in particular, the measured depths of vertical substructures, in HD 163296 most closely match the models of Rab et al (2020) that include deep gas gaps, i.e., similar depletion as for the dust, at the locations of the observed millimeter continuum gaps. Similarly, the gas gap models (versus that of CO depletion) from Calahan et al (2021) are better able to reproduce the Z81 dip in the C 18 O emission surface.…”

“…Localized vertical substructures are observed in many of the emission surfaces derived from the MAPS data. The properties of these substructures, namely their radial locations, widths, and depths, provide important constraints that are necessary for detailed thermochemical modelling (e.g., Rab et al 2020;Calahan et al 2021). In the following subsections, we identify and catalogue all substructures present in the derived emitting layers and compare them with the gas temperature profiles, and with substructures observed in the millimeter continuum and CO line emission.…”

Vortex-like kinematic signal, spirals, and beam smearing effect in the HD 142527 disk

“…Additionally, part of the 13 CO and C 18 O emission from the back side of the disk can be absorbed by dust particles located in the midplane (Isella et al 2018;Rab et al 2020). This process can theoretically delete up to half of the emission if the line is optically thin and the dust highly optically thick.…”

Second-generation dust in planetary systems: The case of HD 163296

“…In particular, Teague et al (2018) obtained fractional deviations of a few percent in the region occupied by the second and third planet, between ≈ 80 and ≈ 200 au, in good accord with the results presented in the Figure (note that we refer to the unperturbed velocity 𝑢 * 𝜙 rather than to the Keplerian velocity 𝑣 Kep ). Rab et al (2020) also performed a detail analysis of the rotational velocity deviations of the ring region and arrived at very similar conclusions (see total fractional variations in their Figure 7). Pinte et al (2018) reported deviations at ≈ 260 au amounting to ≈ 15% of the local Keplerian velocity, which they modelled as induced by a 2 𝑀 J planet orbiting at that distance.…”

“…The right panel of Figure 2 shows the ratio 𝑢 𝜙 /𝑢 * 𝜙 , averaged in azimuth around the star. Deviations of the gas rotation profile from a Keplerian pattern have been reported for HD 163296 by several groups (e.g., Teague et al 2018;Pinte et al 2018;Rab et al 2020), based on observations of CO spectral features. In particular, Teague et al (2018) obtained fractional deviations of a few percent in the region occupied by the second and third planet, between ≈ 80 and ≈ 200 au, in good accord with the results presented in the Figure (note that we refer to the unperturbed velocity 𝑢 * 𝜙 rather than to the Keplerian velocity 𝑣 Kep ).…”