Volume 584, December 2015
|Number of page(s)||9|
|Section||Interstellar and circumstellar matter|
|Published online||04 December 2015|
Vacuum ultraviolet of hydrogenated amorphous carbons
II. Small hydrocarbons production in Photon Dominated Regions⋆
CNRS-INSU, Institut d’Astrophysique Spatiale, UMR
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
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.
Key words: astrochemistry / molecular processes / ISM: abundances / ultraviolet: ISM / methods: laboratory: solid state / photon-dominated region (PDR)
Appendices are available in electronic form at http://www.aanda.org
This work was supported by the ANR COSMISME project, grant ANR-2010-BLAN-0502 of the French Agence Nationale de la Recherche. Part of the equipment used in this work, as well as missions, have been financed by the ANR and French INSU-CNRS program “Physique et Chimie du Milieu Interstellaire” (PCMI).
© ESO, 2015
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.