The Role of C/O in Nitrile Astrochemistry in PDRs and Planet-forming Disks

Complex nitriles, such as HC3N, and CH3CN, 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, sugges...

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Bibliographic Details
Published in:The Astrophysical journal 2019-12, Vol.886 (2), p.86
Main Authors: Le Gal, Romane, Brady, Madison T., Öberg, Karin I., Roueff, Evelyne, Petit, Franck Le
Format: Article
Language:English
Subjects:
Abundance
Accretion disks
Astrochemistry
Astrophysics
Chemistry
Computational methods
Cosmic ray ionization
Cosmic rays
Cyanides
Interstellar molecules
Ionization
Nitriles
Organic chemistry
Oxygen
Photodissociation regions
Physics
Planet formation
Planets
Protoplanetary disks
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Summary:Complex nitriles, such as HC3N, and CH3CN, 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 CH3CN and CH3NC productions. In a fiducial disk model, an elevated C/O ratio increases the CH3CN and HC3N 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.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ab4ad9