Chemistry in the Inner Regions of Protoplanetary Disks
First Author:
Monika Kress
Email: mkress AT science.sjsu.edu
San Jose State University
SJSU, Dept. of Physics & Astronomy
San Jose, CA 95192-0106
Coauthors:
Tielens, Alexander G.G.M., NASA Ames Research Center
Frenklach, Michael, University of California, Berkeley
Abstract
Interstellar material is highly processed when subjected to the physical conditions that prevail in the inner regions of protoplanetary disks, the potential birthplace of habitable planets. Polycyclic aromatic hydrocarbons (PAHs) are abundant in the interstellar medium, and they have also been observed in the disks around young stars, with evidence for destruction as well as some modification in the latter. Recently, compounds such as C2H2, HCN and CO2 have been observed in high abundances within the central 3 AU in some disks as well. Using chemical models for sooting flames, we have investigated the thermally-driven reactions involving PAHs to understand what becomes of these compounds as they are incorporated into the region where habitable planets may eventually form. We find that PAHs are destroyed over disk timescales at temperatures ~900 K and higher. In the process, high abundances of C2H2 persist due to the kinetic inhibition of reactions that drive the carbon into CO and CH4. We also find that relatively high abundances of CH4 and CO2 are to be expected at chemical equilibrium at typical disk temperatures, pressures and C/O ratios. We suggest that PAH destruction in inner warm zones of protoplanetary disks is important and is the origin of abundant C2H2, CH4 and CO2.
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