The ‘soot line’: Destruction of presolar polycyclic aromatic hydrocarbons in the terrestrial planet-forming region of disks Monika E. Kress a, * , Alexander G.G.M. Tielens b , Michael Frenklach c a Department of Physics & Astronomy, San Jose State University, San Jose, CA 95192, United States b Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden, The Netherlands c Department of Mechanical Engineering, University of California, Berkeley, CA 94720, United States Received 12 August 2009; received in revised form 5 February 2010; accepted 7 February 2010 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 some modification in the latter. Using a chemical model developed for sooting flames, we have investigated the chemical evolution of PAHs in warm (1000–2000 K) and oxygen-rich (C/O < 1) conditions appropriate for the region where habitable planets may eventually form. Our study focuses on (1) delineating the conditions under which PAHs will react and (2) identifying the key reaction pathways and reaction products characterizing this chemical evolution. We find that reactions with H, OH and O are the main pathways for destroying PAHs over disk timescale at tem- peratures greater than about 1000 K. In the process, high abundances of C 2 H 2 persist over long timescales due to the kinetic inhibition of reactions that eventually drive the carbon into CO, CO 2 and CH 4 . The thermal destruction of PAHs may thus be the cause of the abun- dant C 2 H 2 that has been observed in disks. We propose that protoplanetary disks have a ‘soot line’, within which PAHs are irreversibly destroyed via thermally-driven reactions. The soot line will play an important role, analogous to that of the ‘snow line’, in the bulk car- bon content of meteorites and habitable planets. Ó 2010 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: PAH; Protoplanetary disk; Chemical model; Presolar organics; Solar nebula 1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are extremely stable, in oxidizing or reducing conditions, at high temper- atures, and in the presence of intense, high-energy UV radi- ation, allowing them to persist in a very wide range of astrophysical and planetary environments. In particular, PAHs have been observed in the interstellar medium, and, at much lower abundances, in the disks around young stars (e.g. Bouwman et al., 2008; Geers et al., 2007; Boersma et al., 2008). Bright emission features at 3.3, 6.2, 7.7, 11.2, and 12.7 lm dominate the mid-IR spectra of regions of star and planet formation. These emission features are gener- ally attributed to infrared fluorescence of PAH molecules excited by ultraviolet radiation. Observations show that large (50 C-atom) PAH molecules are an abundant – con- taining some 5–10% of the elemental C – and ubiquitous component of the interstellar medium of galaxies (see Tielens, 2008 for a recent review). These molecules are thought to form at high temperatures in the ejecta of car- bon stars in a chemical process akin to incomplete combus- tion in sooting flames (Frenklach and Feigelson, 1989; 0273-1177/$36.00 Ó 2010 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2010.02.004 * Corresponding author. Tel.: +1 408 924 5255. E-mail addresses: mkress@science.sjsu.edu (M.E. Kress), tielens@strw. leidenuniv.nl (A.G.G.M. Tielens), myf@me.berkeley.edu (M. Frenklach). www.elsevier.com/locate/asr Available online at www.sciencedirect.com Advances in Space Research 46 (2010) 44–49