Chemistry Across Dust and Gas Gaps in Protoplanetary Disks: Modelling the Co-spatial Molecular Rings in the HD 100546 Disk

By Keith Cowing
Status Report
May 20, 2024
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Chemistry Across Dust and Gas Gaps in Protoplanetary Disks: Modelling the Co-spatial Molecular Rings in the HD 100546 Disk
Structure of the fiducial DALI model. From left to right and top to bottom: gas, small dust, and large dust density, gas and dust temperature, and UV field. Only the regions with a gas number density above 105 cm−3 are shown. Outside 250 au, the midplane gas density is too low for the gas and dust temperature to be coupled. — astro-ph.GA

High-resolution observations show that typically both the dust and the gas in nearby extended protoplanetary disks are structured, possibly related to radial and azimuthal variations in the disk density and/or chemistry.

The aim of this work is to identify the expected location and intensity of rings seen in molecular line emission of HCN, CN, C2H, NO, [CI], and HCO+ in gapped disks while exploring a range of physical conditions across the gap. In particular, we model HD 100546 disk where molecular rings are co-spatial with the dust rings at ∼20 and ∼200 au, in contrast to most other gapped disks. The fiducial model of a gapped disk with a 15 au gas cavity, a 20 au dust cavity, and a shallow (a factor of ≲10) gas and deep dust gap at 40-175 au provides a good fit to the continuum, CO isotopologues, HCN, and HCO+ in the HD 100546 disk.

However, the predictions for [CI], CN, C2H and NO do not match the intensity nor the morphology of the observations. An exploration of the parameter space shows that in general the molecular emission rings are only co-spatial with the dust rings if the gas gap between the dust rings is depleted by at least four orders of magnitude in gas or if the C/O ratio of the gas is varying as a function of radius. For shallower gaps the decrease in the UV field roughly balances the effect of a higher gas density for UV tracers such as CN, C2H, and NO.

Therefore, these radicals are not good tracers of the gas gap depth. The C/O ratio primarily effects the intensity of the lines without changing the morphology much. The co-spatial rings observed in the HD 100546 disk could be indicative of a radially varying C/O ratio in the HD 100546 disk with a C/O above 1 in a narrow region across the dust rings, together with a shallow gas gap that is depleted by a factor of ∼10 in gas, and a reduced background UV field.

M. Leemker, A. S. Booth, E. F. van Dishoeck, L. Wölfer, B. Dent

Comments: Accepted for publication in A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2405.10361 [astro-ph.EP] (or arXiv:2405.10361v1 [astro-ph.EP] for this version)
Submission history
From: Margot Leemker
[v1] Thu, 16 May 2024 18:00:01 UTC (28,668 KB)
Astrobiology, Astrochemistry,

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