A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry

¹ Departments of Chemistry and AstronomyUniversity of Virginia, McCormick Road, Charlottesville, VA 22904, USA
e-mail: romane.legal@virginia.edu
² Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
³ NASA Goddard Space Flight Center (GSFC), Greenbelt, MD 20771, USA
⁴ NASA Ames Research Center, Moffett Field, CA 94035, USA

Received: 12 April 2017
Accepted: 29 May 2017

Abstract

A wide variety of molecules have recently been detected in the Horsehead nebula photodissociation region (PDR) suggesting that: (i) gas-phase and grain chemistries should both contribute to the formation of organic molecules; and (ii) far-ultraviolet (FUV) photodesorption may explain the release into the gas phase of grain surface species. In order to tackle these specific problems and more generally in order to better constrain the chemical structure of these types of environments we present a study of the Horsehead nebula gas-grain chemistry. To do so we used the 1D astrochemical gas-grain code Nautilus with an appropriate physical structure computed with the Meudon PDR code and compared our modeled outcomes with published observations and with previously modeled results when available. The use of a large set of chemical reactions coupled with the time-dependent code Nautilus allows us to reproduce most of the observations well, including those of the first detections in a PDR of the organic molecules HCOOH, CH₂CO, CH₃CHO and CH₃CCH, which are mostly associated with hot cores. We also provide some abundance predictions for other molecules of interest. Understanding the chemistry behind the detection of these organic molecules is crucial to better constrain the environments these molecules can probe.