Observational studies of S-bearing molecules in massive star-forming regions

¹ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, PR China
² Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi, Xinjiang 830011, PR China
³ Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou 311121, PR China
⁴ Shanghai Astronomical Observatory, Chinese Academy of Sciences No. 80 Nandan Road Shanghai 200030, PR China
⁵ School of Chemistry and Chemical Engineering, Chongqing University, Daxuecheng South Road. 55, Chongqing 401331, PR China
⁶ Department of Physics, Anhui Normal University, Wuhu, Anhui 241002, PR China

^★ Corresponding authors; junzhiwang@gxu.edu.cn

Received: 3 February 2024
Accepted: 14 October 2024

Abstract

Context. S-bearing molecules are powerful tools for determining the physical conditions inside a massive star-forming region. The abundances of S-bearing molecules, including H₂S, H₂CS, and HCS⁺, are highly dependent on physical and chemical changes, which means that they are good tracers of the evolutionary stage of massive star formation.

Aims. We present observational results of H₂S 1₁₀-1₀₁, H₂³⁴S 1₁₀-1₀₁, H₂CS 5₁₄-4₁₄, HCS⁺ 4-3, SiO 4-3, HC₃N 19-18, and C¹⁸O 1-0 toward a sample of 51 late-stage massive star-forming regions, and study the relationships between H₂S, H₂ CS, HCS⁺, and SiO in hot cores. We discuss the chemical connections of these S-bearing molecules based on the relations between the relative abundances in our sources.

Methods. H₂³⁴S 1₁₀-1₀₁, as the isotopic line of H₂S 1₁₀-1₀₁, was used to correct the optical depths ofH₂S 1₁₀-1₀₁. Beam-averaged column densities of all molecules were calculated, as were the abundances of H₂S, H₂CS, and HCS⁺ relative to H₂, which were derived from C¹⁸ O. Results from a chemical model that included gas, dust grain surface, and icy mantle phases, were compared with the observed abundances of H₂S, H₂CS, and HCS⁺ molecules.

Results. H₂S 1₁₀-1₀₁, H₂³⁴S 1₁₀-1₀₁, H₂CS 5₁₄-4₁₄, HCS⁺ 4-3, andHC₃N 19-18 were detected in 50 of the 51 sources, SiO 4-3 was detected in 46 sources, and C¹⁸O 1-0 was detected in all sources. The Pearson correlation coefficients between H₂CS and HCS⁺ normalized by H₂ and H₂S are 0.94 and 0.87, respectively, and a tight linear relationship with a slope of 1.00 and 1.09 is found; this relationship is 0.77 and 0.98 between H₂S and H₂CS and 0.76 and 0.97 between H₂S and HCS⁺. The full widths at half maxima of H₂³⁴S 1₁₀-1₀₁, H₂CS 5₁₄-4₁₄, HCS⁺ 4-3, and HC₃N 19-18 in each source are similar to each other, which indicates that they may trace similar regions. By comparing the observed abundance with model results, we see that there is one possible time (2−3 × 10⁵ yr) a which each source in the model matches the measured abundances of H₂S, H₂CS, and HCS⁺. The abundances of HCS⁺, H₂CS, and H₂S increase with the SiO abundance in these sources, which implies that shock chemistry may be playing a large role.

Conclusions. The close abundance relation of H₂S, H₂CS, and HCS⁺ and the similar line widths in observational results indicate that these three molecules could be chemically linked, with HCS⁺ and H₂CS the most correlated. The comparison of the observational results with chemical models shows that the abundances can be reproduced for almost all the sources at a specific time. The observational results, including the abundances in these sources need to be considered in further modeling of H₂S, H₂CS, and HCS⁺ in hot cores with shock chemistry.