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 variable abundances in interstellar and circumstellar environments. Nevertheless, while the origins of SiO are well documented, the SiS chemistry remains relatively unexplored. 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. To achieve this, we calculated the energies and rate coefficients for six neutral atom-diatom reactions involved in the SiCS triatomic system, with a special focus on the C+SiS and S+SiC collisions. We employed 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 supplementary data from the literature, we construct an extended network of neutral-neutral chemical reactions involving Si- and S-bearing molecules. To assess the impact of these chemical reactions, we performed time-dependent models employing the Nautilus gas-grain code, setting the gas temperature to 10 K and the H$_2$ density to 2times 10$^4$ cm$^ $. The models considered two initial abundance scenarios, corresponding to low- and high-metallicity levels. Abundances were computed using both the default chemical network and the constrained network, enriched with newly calculated reactions. The temperature dependence for the reactions involving SiS were modelled to the $k(T)= T/300 beta gamma/T) $ expression, and the coefficients are provided for the first time. The high-metallicity models significantly boost the SiS production, resulting in abundances nearly four orders of magnitude higher compared to low-metallicity models. Higher initial abundances of C, S, and Si, roughly sim 2, 190, and 210 times higher, respectively, contribute to this. Around the age of 10$^3$ yr, destruction mechanisms become relevant, impacting the abundance of SiS. The proposed production reaction S + SiC longrightarrow C + SiS, mitigates these effects in later stages. By expanding the gas reaction network using a high-metallicity model, we derived estimates for the abundances of observed interstellar molecules, including SiO, SO, and SO$_2$. We demonstrate the significance of both SiC+S and C+SiS channels in the SiS chemistry. Notably, 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 level on a par with that of the well-known and fast ion-neutral reactions.

A Possible Additional Formation Pathway for the Interstellar Diatomic SiS

“…The last of these above contributions to the formation of SiS (Reaction ( 5)) shows that even minor contributors can have a role to play in the overall chemical modeling for any potential reaction mechanism. While other mechanisms have been suggested (Rosi et al 2018;Zanchet et al 2018;Campanha et al 2022), the above have risen to the top as the most viable mechanisms for current models that are in line with current observations of SiS (Mendoza et al 2024). However, other reactions could still be present as contributions to SiS abunance, and these have exciting correlations with currently hypothesized but as-of-yet undetected molecules.…”

“…An additional pathway to SiS through SH + SiH (Reaction ( 6)) is shown here to have many advantages for gas-phase chemistry: a large exothermicity, submerged barriers, and the formation of H 2 as a leaving group. While Reactions (1)-( 5) certainly cannot be discounted for their roles in the creation of SiS, this additional pathway may help to further explain SiS abundance in observed regions, augmenting recent work (Mendoza et al 2024). Reaction (6) (SiH + SH) is dependent upon a high enough population of SiH to be present in regions where SH is also found, but such coupling between SiS abundance and the presence of SiH would give evidence for the existence of the currently undetected SiH diatomic.…”

“…Silicon monosulfide (SiS) has been observed in a variety of astronomical environments since it was first observed in the circumstellar environment of the evolved carbon star IRC +10216 in 1975 (Morris et al 1975). Currently, SiS is believed to be created from the following set of reactions, with their percent contributions given for SiS formation over t = 1 × 10 3 yr assuming a molecular cloud with T = 10 K, n(H 2 ) = 2 × 10 4 cm −3 , and a cosmic-ray ionization rate of 1.3 × 10 −17 s −1 (Mendoza et al 2024 ( )…”