Volume 644, December 2020
|Number of page(s)||12|
|Section||Planets and planetary systems|
|Published online||24 November 2020|
Using HCO+ isotopologues as tracers of gas depletion in protoplanetary disk gaps
Max Planck Institute for Astronomy,
2 Department of Chemistry, Ludwig Maximilian University, Butenandtstr. 5-13, 81377 Munich, Germany
3 Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya str. 48, Moscow 119017, Russia
Accepted: 8 September 2020
Context. The widespread rings and gaps seen in the dust continuum in protoplanetary disks are sometimes accompanied by similar substructures seen in molecular line emission. One example is the outer gap at ~100 au in AS 209, which shows that the H13CO+ and C18O emission intensities decrease along with the continuum in the gap, while the DCO+ emission increases inside the gap.
Aims. We aim to study the behavior of DCO+/H13CO+ and DCO+/HCO+ ratios in protoplanetary disk gaps assuming the two scenarios: (A) the gas depletion follows the dust depletion and (B) only the dust is depleted.
Methods. We first modeled the physical disk structure using the thermo-chemical model ANDES. This 1+1D steady-state disk model calculates the thermal balance of gas and dust and includes the far ultraviolet, X-rays, cosmic rays, and other ionization sources together with the reduced chemical network for molecular coolants. Afterward, this physical structure was adopted for calculations of molecular abundances with the extended gas-grain chemical network with deuterium fractionation. Ideal synthetic spectra and 0th-moment maps were produced with the LIne Modeling Engine.
Results. We are able to qualitatively reproduce the increase in the DCO+ intensity and the decrease in the H13CO+ and C18O intensities inside the disk gap, which is qualitatively similar to what is observed in the outer AS 209 gap. The corresponding disk model (A) assumes that both the gas and dust are depleted in the gap. The model (B) with the gas-rich gap, where only the dust is depleted, produces emission that is too bright in all HCO+ isotopologues and C18O.
Conclusions. The DCO+/H13CO+ line ratio can be used to probe gas depletion in dust continuum gaps outside of the CO snow line. The DCO+/C18O line ratio shows a similar, albeit weaker, effect; however, these species can be observed simultaneously with a single (sub)mm interferometer setup.
Key words: astrochemistry / methods: numerical / protoplanetary disks / stars: pre-main sequence / ISM: molecules / submillimeter: planetary systems
© G. V. Smirnov-Pinchukov et al. 2020
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Open Access funding provided by Max Planck Society.
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