Vacuum ultraviolet of hydrogenated amorphous carbons

II. Small hydrocarbons production in Photon Dominated Regions^⋆

¹ CNRS-INSU, Institut d’Astrophysique Spatiale, UMR 8617, 91405 Orsay, France
e-mail: emmanuel.dartois@u-psud.fr
² Université Paris Sud, Institut d’Astrophysique Spatiale, UMR 8617, Bâtiment 121, 91405 Orsay, France
³ CNRS-IN2P3, Institut de Physique Nucléaire d’Orsay, UMR 8608, 91406 Orsay, France
⁴ Université Paris Sud, Institut de Physique Nucléaire d’Orsay, UMR 8608, IN2P3-CNRS, Bâtiment 103, 91406 Orsay, France
⁵ Centro de Astrobiología, INTA-CSIC, Carretera de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁶ Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ CNRS-INP, Institut des Sciences Moléculaires d’Orsay, UMR 8214, 91405 Orsay, France
⁸ Université Paris-Sud, Institut des Sciences Moléculaires d’Orsay, UMR 8214, Bâtiment 210, 91405 Orsay, France

Received: 21 April 2015
Accepted: 16 October 2015

Abstract

Context. Hydrogenated amorphous carbons (a–C:H) are a major component of the carbonaceous solids present in the interstellar medium. The production and existence of these grains is connected in particular with the balance between their photolysis, radiolysis, and hydrogenation. During grain processing, H₂ and other small organic molecules, radicals, and fragments are released into the gas phase.

Aims. We perform photolytic experiments on laboratory produced interstellar a–C:H analogues to monitor and quantify the release of species and compare to relevant observations in the interstellar medium.

Methods. Hydrogenated amorphous carbon analogues at low temperature are exposed to ultraviolet (UV) photons, under ultra-high vacuum conditions. The species produced are monitored using mass spectrometry and post irradiation temperature-programmed desorption. Additional experiments are performed using deuterated analogues and the species produced are unambiguously separated from background contributions. We implement the laboratory measured yields for the released species in a time dependent model to investigate the effect of the UV photon irradiation of hydrogenated amorphous carbons in a photon dominated region, and estimate the associated time scale.

Results. The UV photolysis of hydrogenated amorphous carbons leads to the production of H₂ molecules and small hydrocarbons. The model shows that the photolytic evolution of a–C:Hs in photon dominated regions, such as the Horsehead Nebula, can raise the abundance of carbonaceous molecules by several orders of magnitude at intermediate visual extinctions, i.e., after the C⁺ maximum and before the dense cloud conditions prevail where models generally show a minimum abundance for such carbonaceous species. The injection time peak ranges from a thousand to ten thousand years in the models, considering only the destruction of such grains and no re-hydrogenation. This time scale is consistent with the estimated advection front of a photon dominated region, which replenishes it with freshly exposed material.