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We present the results of an integrated laboratory and modeling investigation into the impact of stellar X-rays on cosmic dust. Carbonaceous grains were prepared in a cooled (<200 K) supersonic expansion from aromatic molecular precursors, and were later irradiated with 970 eV X-rays. Silicate (enstatite) grains were prepared via laser ablation, thermally annealed, and later irradiated with 500 eV X-rays. Infrared spectra of the 3.4 ÎĽ m band of the carbon sample prepared with benzene revealed 84% ± 5% band area loss for an X-ray dose of 5.2 Ă—1023 eV.cmâˆ'2. Infrared spectra of the 8â€"12 ÎĽ m Siâ€"O band of the silicate sample revealed band area loss up to 63% ± 5% for doses of 2.3 Ă— 1023 eV.cmâˆ'2. A hybrid Monte Carlo particle trajectory approach was used to model the impact of X-rays and ensuing photoelectrons, Auger and collisionally ionized electrons through the bulk. As a result of X-ray ionization and ensuing Coulomb explosions on surface molecules, the calculated mass loss is 60% for the carbonaceous sample and 46% for the silicate sample, within a factor of 2 of the IR band loss, supporting an X-ray induced mass-loss mechanism. We apply the laboratory X-ray destruction rates to estimate the lifetimes of dust grains in protoplanetary disks surrounding 1 M ⊙ and 0.1 M ⊙ G and M stars. In both cases, X-ray destruction timescales are short (a few million years) at the disk surface, but are found to be much longer than typical disk lifetimes (≳10 Myr) over the disk bulk. [ABSTRACT FROM AUTHOR]

In the gas phase, most of the ionized or neutral molecules detected in the interstellar and circumstellar media contain at least one carbon atom. Carbon chemistry plays thus a dominant role in the understanding of the structure and evolution of the interstellar medium (ISM). One particular zone of interest to observe small carbonaceous radicals and molecules, are the sharp molecular clouds edges exposed to energetic photons. These photon-dominated regions are rich in these hydrocarbons (like CCH, c-C3H2, C4H), and provide tests for the chemistry models in the diffuse to molecular transition. The pure gas phase models generally fail in reproducing the abundance of many of the observed species, and several authors suggest such abundances may arise from the products of the VUV photodissociation of carbonaceous grains or PAHs. Hydrogenated amorphous carbons (a-C:H or HAC), abundantly observed in the ISM, could also be at the origin of many of these small carbonaceous radicals. Experimentally, this work investigates the production and release of hydrocarbons from the VUV photolysis of a-C:H interstellar analogues under ultra-high vacuum. The experimental results are applied to a Photon Dominated Region model to constrain the impact of this release on the observed gas phase species. [ABSTRACT FROM AUTHOR]

We investigate the formation of molecular hydrogen on ISM dust grains at T < 10 K. We use laboratory methods to detect newly formed D2 that is rovibrationally excited at v” = 4, J” = 2. We discover that recombination is catalyzed by pre-existing molecules on the dust surface and present this an important process in the coldest regions of the ISM. [ABSTRACT FROM PUBLISHER]