The evolution of sulphur-bearing molecules in high-mass star-forming cores

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
e-mail: francesco.fontani@inaf.it
² LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 92190 Meudon, France
³ Centre for Astrochemical Studies, Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Centro de Astrobiología (CSIC-INTA), Ctra Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain

Received: 26 July 2023
Accepted: 15 October 2023

Abstract

Context. To understand the chemistry of sulphur (S) in the interstellar medium, models need to be tested by observations of S-bearing molecules in different physical conditions.

Aims. We aim to derive the column densities and abundances of S-bearing molecules in high-mass dense cores in different evolutionary stages and with different physical properties.

Methods. We analysed observations obtained with the Institut de RadioAstronomie Millimétrique (IRAM) 30 m telescope towards 15 well-known cores classified in the three main evolutionary stages of the high-mass star formation process: high-mass starless cores, high-mass protostellar objects, and ultracompact HII regions.

Results. We detected rotational lines of SO, SO⁺, NS, C³⁴S, ¹³CS, SO₂, CCS, H₂S, HCS⁺, OCS, H₂CS, and CCCS. We also analysed the lines of the NO molecule for the first time to complement the analysis. From a local thermodynamic equilibrium approach, we derived the column densities of each species and excitation temperatures for those that are detected in multiple lines with different excitation. Based on a statistical analysis of the line widths and the excitation temperatures, we find that NS, C³⁴S, ¹³CS, CCS, and HCS⁺ trace cold, quiescent, and likely extended material; OCS, and SO₂ trace warmer, more turbulent, and likely denser and more compact material; SO and perhaps SO⁺ trace both quiescent and turbulent material, depending on the target. The nature of the emission of H₂S, H₂CS, and CCCS is less clear. The molecular abundances of SO, SO₂, and H₂S show the strongest positive correlations with the kinetic temperature, which is thought to be an indicator for evolution. Moreover, the sum of all molecular abundances shows an enhancement of gaseous S from the less evolved to the more evolved stages. These trends could be due to the increasing amount of S that is sputtered from dust grains owing to the increasing protostellar activity with evolution. The average abundances in each evolutionary group increase, especially in the oxygen-bearing molecules, perhaps due to the increasing abundance of atomic oxygen with evolution owing to photodissociation of water in the gas phase.

Conclusions. Our observational work represents a test-bed for theoretical studies aimed at modelling the chemistry of sulphur during the evolution of high-mass star-forming cores.