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

¹ Dept. de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
² Department of Applied Chemistry, Science Building II, National Yang Ming Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 300098, Taiwan
³ Université de Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France
⁴ Observatorio de Yebes (IGN), Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain
⁵ Observatorio Astronómico Nacional (OAN, IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain

^★★ Corresponding authors: r.fuentetaja@csic.es, jose.cernicharo@csic.es

Received: 5 February 2025
Accepted: 9 July 2025

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 the starless core TMC-1 using the QUIJOTE¹ line survey. The observations were obtained with the Yebes 40 m radio telescope, and the sensitivity of the data varied between 0.08 and 0.2 mK in the 31–50 GHz range. Observations with the IRAM 30 m radio telescope of the most abundant isotopologues of these species are also presented and used to estimate volume densities and to constrain the excitation conditions of these molecules. Among these species, 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 starless object. These data were complemented with sensitive 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. The results allowed us to report the most complete analysis of the column densities of the C_nS family and to compare the abundance ratios of all detected isotopologues. Adopting a kinetic temperature for TMC-1 of 9 K, we found that n(H₂)=0.9–1.5×10⁴ 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 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 sulphur 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 in the 31–50 GHz domain (HCS, HSC, NCS, H₂CCS, HCSCN, HCCCS⁺, C₄S, and C₅S). The expected intensities for their ³⁴S and ¹³C 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.