Formation and Destruction of SiS in Space

Abstract

The presence of SiS in space seems to be restricted to a few selected types of astronomical environments. It is long known to be present in circumstellar envelopes around evolved stars and it has also been detected in a handful of star-forming regions with evidence of outflows, like Sgr B2, Orion KL and more recently L1157-B1. The kinetics of reactions involving SiS is very poorly known and here we revisit the chemistry of SiS in space by studying some potentially important reactions of formation and destruction of this molecule. We calculated potential energy surfaces of the SiOS system and computed rate coefficients in the temperature range 50-2500 K for the reaction of destruction of SiS, in collisions with atomic O, and of its formation, through the reaction between Si and SO. We find that both reactions are rapid, with rate coefficients of a few times 10 cm s, almost independent of temperature. In the reaction between Si and SO, SiO production is 5-7 times more efficient than SiS formation. The reaction of SiS with O atoms can play an important role in destroying SiS in envelopes around evolved stars. We built a simple chemical model of a postshock gas to study the chemistry of SiS in protostellar outflows and we found that SiS forms with a lower abundance and later than SiO, that SiS is efficiently destroyed through reaction with O, and that the main SiS-forming reactions are Si + SO and Si + SO.

Structures and Properties of Known and Postulated Interstellar Cations

“…since the work of Podio et al (2017) and found to be plausible (Rosi et al 2018;Zanchet et al 2018;Rosi et al 2019).…”

IRC + 10°216 mass loss properties through the study ofλ3 mm emission

“…This assumption has been made on the basis of reproducing the interferometer observations of the region where photochemistry occurs (Agúndez et al 2017, and references therein). Particularly for the three species analysed in this work, we updated two important reactions in the chemistry of SiS according to Zanchet et al (2018), which are the destruction of SiS with atomic oxygen and the reaction between Si and SO. For the photo-dissociation rates of the studied species we took the values from Pattillo et al (2018) for CS and from Heays et al (2017) for SiO.…”

Nonadiabatic reaction dynamics to silicon monosulfide (SiS): A key molecular building block to sulfur-rich interstellar grains

“…Astrochemical models postulate the formation of silicon monosulfide is linked to reactions of interstellar anions with the silicon monosulfide cation (SiS + ), proton transfer from protonated silicon monosulfide (HSiS + ), and dissociative recombination of protonated silicon monosulfide with an electron ( 35 ). Alternatively, radiative associations between atomic silicon and sulfur ( 36 ) along with bimolecular collisions of atomic silicon with the thioxydroxyl radical (SH) ( 37 ) and with sulfur monoxide (SO) ( 38 , 39 ) have been assumed to initiate the formation of the first silicon-sulfur bond. Considering that none of the aforementioned postulated pathways to silicon monosulfide have been explored in laboratory studies, it is not unexpected that astrochemical models underestimate the astronomical observed fractional abundances of silicon monosulfide by up to two orders of magnitude ( 30 ).…”