Analysis of the isotopologues of CS, CCS, CCCS, HCS+, HCCS+, and H2CS in TMC-1 with the QUIJOTE line survey

Abstract

We performed a detailed analysis of the isotopologues with 13C, 34S, 33S, and 36S of the sulphur-bearing molecules CS, CCS, CCCS,HCS+, HCCS+, and H2CS towards TMC-1 using the QUIJOTE1. The observations were obtained with the Yebes radio telescope. Observations with the IRAM 30m of the most abundant isotopologues of these species are also presented and used to estimate volume densities and to constrain the excitation conditions. We report the first detection in space of C13C34S, CC33S, CCC33S, HC33S+, and HCC34S+. C36S is also detected for the first time in a cold object. We also complemented with maps that provide the spatial distribution of most of these species. Using the available collisional rate coefficients for each species, we modeled the observed line intensities using the large velocity gradient method for the radiative transfer. We report the most complete analysis of the column densities of the CnS family and to compare the abundance ratios of all detected isotopologues. Adopting a T_k for TMC-1 of 9K, we found that n(H2)=0.9-1.5X10^4cm-3 can explain the observed decline in intensity with increasing J. We derived the rot. constants for the C13C34S, CC33S, CCC33S, HC33S+, and HCC34S+ isotopologues from new laboratory data and complemented them with the frequencies of the observed lines. We find that all S isotopologues are consistent with solar isotopic abundance ratios. Accurate 12C/13C abundances were derived and, as previously suggested, the 13C isotopologues of CCS and CCCS show strong abundance anomalies depending on the position of the substituted carbon. Nevertheless, the 12C/13C abundance ratio is practically identical to the solar value for CS, HCS+, and H2CS. We also searched for the isotopologues of other S-bearing molecules. The expected intensities for their 34S and 13C isotopologues are too low to be detected with the present sensitivity of the QUIJOTE, however.