The effect of isotopic substitution on the excitation of CCS isotopologues in molecular clouds

Abstract

ABSTRACT CCS(3Σ−) is one of the few molecules which presents many observable isotopologues, with the reported detection of 13CCS, C13CS, CC34S, and CC33S. The accurate determination of CCS isotopologues abundances enables the study of the isotopic fraction in media where they are detected, and gives insights into their formation pathways. The availability of collisional rate coefficients, that are prerequisite for accurate determination of their abundances in astrophysical media under non-local thermodynamic equilibrium (LTE) conditions, allows drawing reliable conclusions on their abundances. In this study, fine and hyperfine-structure resolved excitation rate coefficients induced by collisions with helium are produced with a quantum approach for all detected CCS isotopologues for the 5–50 K temperature range. Radiative transfer models have been performed with the new data to discuss their impact on the excitation conditions of these species. The effect of the isotopic substitution appears to be weak between fine-structure rate coefficients. The observed lines of CCS, 13CCS, C13CS, and CC34S are found to be out of LTE conditions, and therefore, the proper determination of the abundance of CCS isotopologues in molecular clouds requires the use of the data produced in this work.