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

Abstract

S-bearing molecules are powerful tools for determining the physical conditions inside a massive star-forming region. The abundances of S-bearing molecules, including H$_ $S, H$_ $CS, and HCS$^ $, are highly dependent on physical and chemical changes, which means that they are good tracers of the evolutionary stage of massive star formation. We present observational results of H$_ $S 1$_ $, H$_ $34S 1$_ $, H$_ $CS 5$_ $, HCS$^ $ 4-3, SiO 4-3, HC$_ $N 19-18, and C18O 1-0 toward a sample of 51 late-stage massive star-forming regions, and study the relationships between H$_ $S, H$_ $CS, HCS$^ $, and SiO in hot cores. We discuss the chemical connections of these S-bearing molecules based on the relations between the relative abundances in our sources. H$_ $34S 1$_ $, as the isotopic line of H$_ $S 1$_ $, was used to correct the optical depths of H$_ $S 1$_ $. Beam-averaged column densities of all molecules were calculated, as were the abundances of H$_ $S, H$_ $CS, and HCS$^ $ relative to H$_ $, which were derived from C18O. Results from a chemical model that included gas, dust grain surface, and icy mantle phases, were compared with the observed abundances of H$_2$S, H$_2$CS, and HCS$^+$ molecules. H$_ $S 1$_ $, H$_ $34S 1$_ $, H$_ $CS 5$_ $, HCS$^ $ 4-3, and HC$_ $N 19-18 were detected in 50 of the 51 sources, SiO 4-3 was detected in 46 sources, and C18O 1-0 was detected in all sources. The Pearson correlation coefficients between H$_ $CS and HCS$^+$ normalized by H$_ $ and H$_ $S are 0.94 and 0.87, respectively, and a tight linear relationship with a slope of 1.00 and 1.09 is found; this relationship is 0.77 and 0.98 between H$_ $S and H$_ $CS and 0.76 and 0.97 between H$_ $S and HCS$^ $. The full widths at half maxima of H$_ $34S 1$_ $, H$_ $CS 5$_ $, HCS$^ $ 4-3, and HC$_ $N 19-18 in each source are similar to each other, which indicates that they may trace similar regions. By comparing the observed abundance with model results, we see that there is one possible time (2-3times 105 yr) a which each source in the model matches the measured abundances of H$_ $S, H$_ $CS, and HCS$^ $. The abundances of HCS$^ $, H$_ $CS, and H$_ $S increase with the SiO abundance in these sources, which implies that shock chemistry may be playing a large role. The close abundance relation of H$_2$S, H$_2$CS, and HCS$^+$ and the similar line widths in observational results indicate that these three molecules could be chemically linked, with HCS$^+$ and H$_2$CS 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 H$_ $S, H$_ $CS, and HCS$^ $ in hot cores with shock chemistry.