Silicon and hydrogen chemistry under laboratory conditions mimicking the atmosphere of evolved stars

Abstract

Silicon is present in interstellar dust grains, meteorites and asteroids, and to date thirteen silicon-bearing molecules have been detected in the gas-phase towards late-type stars or molecular clouds, including silane and silane derivatives. In this work, we have experimentally studied the interaction between atomic silicon and hydrogen under physical conditions mimicking those at the atmosphere of evolved stars. We have found that the chemistry of Si, H and H${}_2$ efficiently produces silane (SiH${}_4$), disilane (Si${}_2$H${}_6$) and amorphous hydrogenated silicon (a-Si:H) grains. Silane has been definitely detected towards the carbon-rich star IRC\,+10216, while disilane has not been detected in space yet. Thus, based on our results, we propose that gas-phase reactions of atomic Si with H and H${}_2$ are a plausible source of silane in C-rich AGBs, although its contribution to the total SiH${}_4$ abundance may be low in comparison with the suggested formation route by catalytic reactions on the surface of dust grains. In addition, the produced a-Si:H dust analogs decompose into SiH${}_4$ and Si${}_2$H${}_6$ at temperatures above 500 K, suggesting an additional mechanism of formation of these species in envelopes around evolved stars. We have also found that the exposure of these dust analogs to water vapor leads to the incorporation of oxygen into Si-O-Si and Si-OH groups at the expense of SiH moieties, which implies that, if this type of grains are present in the interstellar medium, they will be probably processed into silicates through the interaction with water ices covering the surface of dust grains.

References in corpus (6)

• New molecules in IRC+10216: confirmation of C${}_5$S and tentative identification of MgCCH, NCCP, and SiH${}_3$CN
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• The chemistry of cosmic dust analogues from C, C${}_2$, and C${}_2$H${}_2$ in C-rich circumstellar envelopes
• Silicate Composition of the Interstellar Medium
• INFRA-ICE: an ultra-high vacuum experimental station for laboratory astrochemistry

Cited by in corpus (4)

• Impact of metals on (star)dust chemistry: a laboratory astrophysics approach
• Photocleavage of aliphatic C--C bonds in the interstellar medium
• New SiS destruction and formation routes via neutral-neutral reactions and their fundamental role in interstellar clouds at low and high metallicity values
• The Significant Role of Hydrogen in the Formation of Silicon Carbide in Evolved Stars