In the atmosphere of carbon-rich (C-rich) asymptotic giant branch stars (AGBs), silicon chemistry plays an important role as is revealed by the detection of silane (SiH4) and organosilicon compounds such as methyl silane (CH3SiH3) and sylil cyanide (SiH3CN). To deepen the understanding on the formation of silicon bearing molecules in the circumstellar envelopes of AGBs, we have studied the interaction of atomic silicon and molecular hydrogen (H2) and we have found that, in addition to the formation SiH4, disilane (Si2H6) is efficiently produced, fostering the search for Si2H6 around C-rich stars. More interestingly, we have also observed the formation of amorphous hydrogenated silicon (a-Si:H) nanoparticles which decompose into SiH4 and Si2H6 at temperatures above 500 K, suggesting an additional mechanism for the formation of these species in envelopes around evolved stars. This novel mechanism can contribute to explain the high discrepancy between the observed SiH4 abundance and that obtained by gas-phase kinetical modelling, which predicts and abundance of SiH4 six orders of magnitude lower than that derived by astronomical observations.
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Silicon and Hydrogen Chemistry under Laboratory Conditions Mimicking the Atmosphere of Evolved Stars, Mario Accolla, Gonzalo Santoro, Pablo Merino, Lidia Martínez, Guillermo Tajuelo-Castilla, Luis Vázquez, Jesús M. Sobrado, Marcelino Agúndez, Miguel Jiménez-Redondo, Víctor J. Herrero, Isabel Tanarro, José Cernicharo and José Ángel Martín-Gago, Astrophys. J., 906 (2021) 44.