Vacuum ultraviolet photolysis of hydrogenated amorphous carbons

I. Interstellar H₂ and CH₄ formation rates

¹ CNRS-INSU, Institut d’Astrophysique Spatiale, UMR 8617, 91405 Orsay, France
e-mail: ialata@ias.u-psud.fr
² Université Paris Sud, Institut d’Astrophysique Spatiale, UMR 8617, Bâtiment 121, 91405 Orsay, France
³ Centro de Astrobiología, INTA-CSIC, Carretera de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain

Received: 22 November 2013
Accepted: 9 July 2014

Abstract

Context. The interstellar hydrogenated amorphous carbons (HAC or a-C:H) observed in the diffuse medium are expected to disappear in a few million years, according to the destruction time scale from laboratory measurements. The existence of a-C:H results from the equilibrium between photodesorption, radiolysis, hydrogenation and resilience of the carbonaceous network. During this processing, many species are therefore injected into the gas phase, in particular H₂, but also small organic molecules, radicals or fragments.

Aims. We perform experiments on interstellar a-C:H analogs to quantify the release of these species in the interstellar medium.

Methods. The vacuum ultraviolet (VUV) photolysis of interstellar hydrogenated amorphous carbon analogs was performed at low (10 K) to ambient temperature, coupled to mass-spectrometry detection and temperature-programed desorption. Using deuterium isotopic substitution, the species produced were unambiguously separated from background contributions.

Results. The VUV photolysis of hydrogenated amorphous carbons leads to the efficient production of H₂ molecules, but also to small hydrocarbons.

Conclusions. These species are formed predominantly in the bulk of the a-C:H analog carbonaceous network, in addition to the surface formation. Compared with species made by the recombination of H atoms and physisorbed on surfaces, they diffuse out at higher temperatures. In addition to the efficient production rate, it provides a significant formation route in environments where the short residence time scale for H atoms inhibits H₂ formation on the surface, such as PDRs. The photolytic bulk production of H₂ with carbonaceous hydrogenated amorphous carbon dust grains can provide a very large portion of the contribution to the H₂ molecule formation. These dust grains also release small hydrocarbons (such as CH₄) into the diffuse interstellar medium, which contribute to the formation of small carbonaceous radicals after being dissociated by the UV photons in the considered environment. This extends the interstellar media environments where H₂ and small hydrocarbons can be produced.