Atom Addition Formation of Thionylimide (HNSO) on Interstellar Dust Grains: Chemical routes requiring oxygen and nitrogen atom surface diffusion

TL;DR

This study combines quantum chemical calculations and astrochemical models to elucidate the formation pathways of HNSO on interstellar dust grains, highlighting the roles of atomic oxygen and nitrogen diffusion.

Contribution

It introduces a detailed chemical route for HNSO formation involving surface reactions with O and N atoms, emphasizing the importance of diffusive chemistry in interstellar sulfur compounds.

Findings

HNSO is efficiently formed via surface reactions involving O and N atoms.

Solid HNSO can reach abundances comparable to icy OCS in interstellar ices.

Diffusive chemistry models improve agreement with astronomical observations.

Abstract

We investigate the formation of the recently detected HNSO molecule using quantum chemical calculations on ices and astrochemical models in tandem. Our results indicate that HNSO is efficiently produced on grain surfaces through reactions involving atomic oxygen and nitrogen atoms with the radicals NS and SO, forming NSO as a key intermediate. Subsequent hydrogenation of NSO leads to HNSO, with a clear preference for the lowest energy cis conformer, while the trans form is metastable and may be short-lived under typical interstellar conditions. The models predict that solid HNSO can reach abundances comparable to icy OCS, placing it among the major sulfur-bearing species in interstellar ices. Gas-phase abundances, in contrast, remain lower than those of OCS. The implementation of a multibinding scheme in our models clarifies the role of diffusive chemistry in the production of HNSO at early times, improving agreement with observations. These findings suggest that reactions involving diffusing O and N atoms on icy grains contribute significantly to sulfur chemistry and beyond in dense clouds and motivate further searches for molecules containing simultaneously H, N, O and S in other astronomical environments.