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Communication Dans Un Congrès Année : 2023

Fine and hyperfine excitation of CCS isotopologues: isotopic effect on astrophysical modeling

Francois Lique

Résumé

Molecular spectra often serve as the sole source of information about astrophysical media. To interpret these spectra, collisional rate coefficients are essential. The CCS molecule has been detected in several molecular clouds, protostellar envelopes, and the circumstellar envelope of IRC+10216. Despite its abundance in these media, no reliable rate coefficients exist for this molecule yet. This is due to its complex fine structure, which complicates the computation of data. Additionally, various isotopologues of CCS, such as 13CCS, C13CS, CC34S, and recently CC33S, have been detected in molecular clouds. Having collisional data for all these isotopologues will aid in discussing isotopic fractionation in such media and understanding the formation mechanism of CCS through the analysis of the 13CCS and C13CS abundance ratio. Nevertheless, 13C and 33S isotopologues have non-zero molecular spin, resulting in the splitting of fine structure levels into 2 hyperfine levels for 13C-based isotopologues and 4 hyperfine levels for the CC33S isotopologue. In this study, we present the first CCS-He potential energy surface computed with CCSD(T)/aVQZ and mid-bond functions level of theory, as well as the first accurate CCS-He rate coefficients. These were compared to previously computed values, showing a global agreement within a factor of 2-10, and up to a factor of 100. The derived abundances in TMC-1 differed by 20% and were 2.5 times different from the CCS abundances derived from the ISA (https://isa.astrochem-tools.org/) database. Furthermore, we computed fine and hyperfine rate coefficients for 13CCS, C13CS, CC34S, and CC33S isotopologues. The isotopic effect on rate coefficients was found to be relatively weak, with differences in line intensity of 13C-based isotopologues primarily induced by a factor of 2.5 difference in abundances (not 4.2 as suggested by Sakai et al., 2007), rather than by collisional effects. Therefore, the formation of CCS cannot be induced by a symmetric molecule. In this study, accurate rate coefficients were computed for 5 CCS isotopologues, allowing us to revise the CCS abundance in TMC-1, derive 13CCS, C13CS, CC34S, and CC33S abundances, and provide insight into the CCS formation pathway.
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Dates et versions

hal-04147450 , version 1 (30-06-2023)

Identifiants

  • HAL Id : hal-04147450 , version 1

Citer

Amélie Godard Palluet, Francois Lique. Fine and hyperfine excitation of CCS isotopologues: isotopic effect on astrophysical modeling. Physical and Chemical processes of astrophysical interest: Towards the detection of new species, Alexandre Faure; François Lique; Jérome Loreau, Jun 2023, Saint-Florent, France. ⟨hal-04147450⟩
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