Sulfur-bearing molecules in a sample of active star-forming cores

Abstract

Astrochemical processes involving sulfur are not yet well understood because cosmic sulfur reservoirs and the production pathways of sulfur-bearing species remain elusive. Addressing this, requires high-resolution interferometric observations capable of probing individual molecular cores. Following our previous study focused on early molecular cores, we are motivated to expand this investigation toward a sample of evolved cores to understand their chemical transition. We analyzed data from ALMA toward 16 molecular cores in massive star-forming regions associated with methanol masers, targeting the same six sulfur-bearing species studied in a previous work toward early molecular cores: SO, SO2, H2CS, SO+, NS, and 34SO. Column densities and abundances were derived assuming LTE, and temperatures were estimated from methanol transitions. Comparisons were made between the results obtained for the evolved cores and those previously obtained for the early ones. We find that the abundances of the sulfur-bearing molecules are higher in the evolved cores than in the early ones, confirming a general time-dependent enrichment of sulfur in the gas phase. While abundances increase within the 100-220 K range, their correlation with temperature weakens, suggesting that gas kinematics become increasingly more important in the sulfur chemistry. This evolutionary transition involves a chemical reorganization where SO2 becomes dominant. We confirm the validity of SO2/SO as a chemical clock, though chemical modeling reveals a discrepancy in sources with more pronounced kinematic processes, namely a steeper increase in the SO2/SO ratio. The line-width analysis of the molecular species indicates that core evolution and kinematics lead to a well-mixed gas, erasing the spatial stratification where different species trace distinct layers in the cores at earlier stages.