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

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)
https://arxiv.org/abs/2405.10361
Astrobiology, Astrochemistry,