Evolution of Chemistry in the envelope of HOt CorinoS (ECHOS). II. The puzzling chemistry of isomers as revealed by the HNCS/HSCN ratio

The observational detection of some metastable isomers in the interstellar medium with abundances comparable to those of the most stable isomer, or even when the stable isomer is not detected, highlights the importance of non-equilibrium chemistry. This challenges our understanding of the interstell...

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Published in:Astronomy and astrophysics (Berlin) 2024-11
Main Authors: Esplugues, G., Rodríguez-Baras, M., Navarro-Almaida, D., Fuente, A., Fernández-Ruiz, P., Spezzano, S., Drozdovskaya, M.N., Sánchez-Monge, Á., Caselli, P., Rivière-Marichalar, P., L., Beitia-Antero
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Language:English
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Summary:The observational detection of some metastable isomers in the interstellar medium with abundances comparable to those of the most stable isomer, or even when the stable isomer is not detected, highlights the importance of non-equilibrium chemistry. This challenges our understanding of the interstellar chemistry and shows the need to study isomeric forms of molecular species to constrain chemical processes occurring in the interstellar medium. Our goal is to study the chemistry of isomers through the sulphur isomer pair HNCS and HSCN, since HSCN has been observed in regions where its stable isomer has not been detected, and the observed HNCS/HSCN ratio seems to significantly vary from cold to warm regions. We used the Nautilus time-dependent gas-grain chemical code to model the formation and destruction paths of HNCS and HSCN in different astrochemical scenarios, as well as the time evolution of the HNCS/HSCN ratio. We also analysed the influence of the environmental conditions on their chemical abundances. We present an observational detection of the metastable isomer HSCN in the Class I object B1-a ($N$=(1.1pm 0.6)times 1012 cm$^ $), but not of the stable isomer HNCS, despite HNCS lying 3200 K lower in energy than HSCN. Our theoretical results show an HNCS/HSCN ratio sensitive to the gas temperature and the evolutionary time, with the highest values obtained at early stages ($t$lesssim 10$^4$ yr) and low ($T_ g $lesssim 20 K) temperatures. A more detailed analysis also shows that the main mechanism forming HNCS in young ($t$$
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361/202451902