The Si+SO$_2$ collision and an extended network of neutral-neutral reactions between silicon and sulphur bearing species

TL;DR

This study investigates the Si+SO$_2$ reaction and extends the network of neutral-neutral reactions involving silicon and sulfur species, analyzing their impact on molecular abundances in astrophysical shock environments.

Contribution

It provides new computational insights into the Si+SO$_2$ reaction pathways and develops an extended reaction network for silicon- and sulfur-bearing molecules in astrochemical models.

Findings

Si($^3$P) and SO$_2$ react barrierlessly to produce SO and SiO.

The reaction does not contribute to SiS abundance.

SiS can reach abundances of ~10$^{-8}$ in shock conditions from 1 million-year-old clouds.

Abstract

The Si+SO$_2$ reaction is investigated to verify its impact on the abundances of molecules with astrochemical interest, such as SiS, SiO, SO and others. According to our results Si($^3$P) and SO$_2$ react barrierlessly yielding only the monoxides SO and SiO as products. No favourable pathway has been found leading to other products, and this reaction should not contribute to SiS abundance. Furthermore, it is predicted that SiS is stable in collisions with O$_2$, and that S($^3$P)+SiO$_2$ and O($^3$P)+OSiS will also produce SO+SiO. Using these results and gathering further experimental and computational data from the literature, we provide an extended network of neutral-neutral reactions involving Si- and S-bearing molecules. The effects of these reactions were examined in a protostellar shock model, using the Nautilus gas-grain code. This consisted in simulating the physicochemical conditions of a shocked gas evolving from $i.$ primeval cold core, $ii.$ the shock region itself, $iii.$ and finally the gas bulk conditions after the passage of the shock. Emphasising on the cloud ages and including systematically these chemical reactions, we found that [SiS/H$_2$] can be of the order of $\sim$ 10$^{-8}$ in shocks that evolves from clouds of $t=1\times 10^6$ yr, whose values are mostly affected by the SiS+O $\longrightarrow$SiO+S reaction. Perspectives on further models along with observations are discussed in the context of sources harbouring molecular outflows.