Volume 569, September 2014
|Number of page(s)||10|
|Published online||02 October 2014|
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
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
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.
Key words: astrochemistry / molecular processes / ISM: abundances / ultraviolet: ISM / methods: laboratory: solid state / dust, extinction
© ESO, 2014
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