TMC-1 analysis of C and S isotopologues

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. The results allowed us to 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 kinetic temperature for TMC-1 of 9K, we found that n(H_2_)=0.9-1.5x10^4^cm^-3^ can explain the observed decline in intensity with increasing rotational levels J for all observed molecules.

We derived the rotational constants for the C^13^C^34^S, CC^33^S, CCC^33^S, HC^33^S+, and HCC^34S^+ isotopologues from new laboratory data and complemented them with the frequencies of the observed lines. We find that all sulphur isotopologues are consistent with solar isotopic abundance ratios. Accurate ^12^C/^13^C abundances were derived and, as previously suggested, the ^13^C isotopologues of CCS and CCCS show strong abundance anomalies depending on the position of the substituted carbon.

Nevertheless, the ^12^C/^13^C abundance ratio is practically identical to the solar value for CS, HCS+, and H_2_CS. We also searched for the isotopologues of other S-bearing molecules in the 31-50 GHz domain (HCS, HSC, NCS, H_2_CCS, HCSCN, HCCCS+, C_4_S, and C5S). The expected intensities for their ^34^S and ^13^C isotopologues are too low to be detected with the present sensitivity of the QUIJOTE line survey, however.

The results presented in this work provide new insights into the molecular composition, isotopic abundances, and physical conditions of the cold starless core TMC-1