New SiS destruction and formation routes via neutral-neutral reactions and their fundamental role in interstellar clouds at low and high metallicity values

Abstract

Among the silicon bearing species discovered in the interstellar medium, SiS and SiO stand out as key tracers due to their distinct chemistry and abundances in interstellar and circumstellar environments. Our objective is to enhance the network of Si- and S-bearing chemical reactions for a gas-grain model in molecular clouds, encompassing both low and high metallicities. We have calculated the energies and rate coefficients for 6 neutral atom-diatom reactions involved in the SiCS triatomic system, with a special focus on the C+SiS and S+SiC collisions. We employ the coupled cluster method with single and double substitutions and a perturbative treatment of triple substitutions (CCSD(T)) refined at the explicitly correlated CCSD(T)-F12 level. With these computational results in conjunction with data from the literature, we construct an extended network of neutral-neutral chemical reactions. We performed time-dependent models employing the Nautilus gas-grain code, setting the gas temperature to 10 K and the density to 2x10${}^4$ cm${}^{-3}$. The temperature dependence for the reactions involving SiS were modelled using $k(T)=\alpha {\left(T/300\right)}^{\beta }\exp (-\gamma /T)$. The high-metallicity models significantly boost the SiS production. Higher initial abundances of C, S, and Si, roughly $\sim$2, 190, and 210 times higher, respectively, contribute to this. Around 10${}^3$ yr, destruction mechanisms become relevant. The proposed production reaction S + SiC $\to$ C + SiS, mitigates these effects. By expanding the gas reaction network using a high metallicity model, we derived estimates for the abundances of interstellar molecules. The inclusion of neutral-neutral mechanisms, particularly via Si+HS and S+SiC channels, played a pivotal role in determining SiS abundance. These mechanisms carry a significance on a par with the well-known and fast ion-neutral reactions.

11 pages, 5 figures, accepted for publication in Astronomy and Astrophysics

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