Role of C/O in Nitrile Astrochemistry in PDRs and Planet-forming Disks
Abstract
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.
Additional Information
© 2019 The American Astronomical Society. Received 2019 July 30; revised 2019 October 1; accepted 2019 October 2; published 2019 November 22. The authors would like to thank the anonymous referee for constructive suggestions and comments. R.L.G. also thanksk Tom J. Millar for useful discussion. This work was supported by an award from the Simons Foundation (SCOL # 321183, KO). M.T.B. acknowledges support from the Noland Internship at Caltech. Software: Pandas (McKinney 2010), Matplotlib (Hunter 2007), NumPy (van der Walt et al. 2011), SciPy (Virtanen et al. 2019), Meudon PDR Code (Le Bourlot et al. 1993; Le Petit et al. 2006; Le Bourlot et al. 2012), Nautilus-v1.1 (Hersant et al. 2009; Ruaud et al. 2016; Wakelam et al. 2016).Attached Files
Published - Le_Gal_2019_ApJ_886_86.pdf
Accepted Version - 1910.01554.pdf
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Additional details
- Eprint ID
- 100048
- Resolver ID
- CaltechAUTHORS:20191125-153630631
- 321183
- Simons Foundation
- Caltech
- Created
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2019-11-25Created from EPrint's datestamp field
- Updated
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2021-11-16Created from EPrint's last_modified field