2019/10/03 by Romane Le Gal, Madison T. Brady, Karin I. Öberg +3 · 1 citation
Physics and Astronomy · #Abundance (ecology) #Astrochemistry #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Branching fraction #Cyanoacetylene #Ionization #Molecule #Nitrile #Stellar, planetary, and galactic studies #Ultraviolet #astro-ph.EP #astro-ph.GA #astro-ph.SR
paper · pdf · doi:10.3847/1538-4357/ab4ad9
15 pages, 7 figures, 2 tables, accepted for publication in ApJ
arxiv created 2019/10/03 · openalex created_date 2019/10/10 · openalex publication_date 2019/11/22 · arxiv updated 2020/01/08 · openalex updated_date 2026/08/05
Abstract Complex nitriles, such as HC 3 N, and CH 3 CN, are observed in a wide variety of astrophysical environments, including at relatively high abundances in photon-dominated regions (PDRs) and the ultraviolet exposed atmospheres of planet-forming disks. The latter have been inferred to be oxygen-poor, suggesting that these observations may be explained by organic chemistry in C-rich environments. In this study we first explore if the PDR complex nitrile observations can be explained by gas-phase PDR chemistry alone if the elemental C/O ratio is elevated. In the case of the Horsehead PDR, we find that gas-phase chemistry with C/O ≳ 0.9 can indeed explain the observed nitrile abundances, increasing predicted abundances by several orders of magnitude compared to standard C/O assumptions. We also find that the nitrile abundances are sensitive to the cosmic-ray ionization treatment, and provide constraints on the branching ratios between CH 3 CN and CH 3 NC productions. In a fiducial disk model, an elevated C/O ratio increases the CH 3 CN and HC 3 N productions by more than an order of magnitude, bringing abundance predictions within an order of magnitude to what has been inferred from observations. The C/O ratio appears to be a key variable in predicting and interpreting complex organic molecule abundances in PDRs across a range of scales.