The Effect of Carbon Grain Destruction on the Chemical Structure of Protoplanetary Disks

Chen-En Wei1, Hideko Nomura1, Jeong-Eun Lee2, Wing-Huen Ip3, Catherine Walsh4, and T. J. Millar5,6
Astrophysical Journal 870, 129 Link to Article [DOI: 10.3847/1538-4357/aaf390 ]
1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
2School of Space Research, Kyung Hee University, Seocheon-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do, 446-701, Republic of Korea
3Graduate Institute of Astronomy, National Central University, No. 300, Zhongda Road, Zhongli Dist., Taoyuan City 32001, Taiwan
4School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
5Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast, BT7 1NN, UK
6Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands

The bulk composition of Earth is dramatically carbon-poor compared to that of the interstellar medium, and this phenomenon extends to the asteroid belt. To interpret this carbon deficit problem, the carbonaceous component in grains must have been converted into the gas phase in the inner regions of protoplanetary disks (PPDs) prior to planetary formation. We examine the effect of carbon grain destruction on the chemical structure of disks by calculating the molecular abundances and distributions using a comprehensive chemical reaction network. When carbon grains are destroyed and the elemental abundance of the gas becomes carbon-rich, the abundances of carbon-bearing molecules, such as HCN and carbon-chain molecules, increase dramatically near the midplane, while oxygen-bearing molecules, such as ${{\rm{H}}}_{2}{\rm{O}}$ and ${\mathrm{CO}}_{2}$, are depleted. We compare the results of these model calculations with the solid carbon-to-silicon fraction in the solar system. Although we find a carbon depletion gradient, there are some quantitative discrepancies: the model shows a higher value at the position of the asteroid belt and a lower value at the location of Earth. In addition, using the obtained molecular abundance distributions, coupled with line radiative transfer calculations, we make predictions for ALMA to potentially observe the effect of carbon grain destruction in nearby PPDs. The results indicate that HCN, ${{\rm{H}}}^{13}\mathrm{CN}$, and c-${{\rm{C}}}_{3}{{\rm{H}}}_{2}$ may be good tracers.


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