Local constraints on alcohol--thioalcohol analogs in G35.2N

Abstract

The chemistry of sulfur-bearing complex organic molecules in dense star-forming environments remains uncertain, partly because the dominant sulfur reservoirs in dense gas and ices are poorly identified. Alcohol--thioalcohol pairs allow direct comparisons of structurally related O- and S-bearing molecules. We present ALMA Band 6 observations of CH$_3$OH/CH$_3$SH and C$_2$H$_5$OH/C$_2$H$_5$SH toward three spectral-extraction positions (MM3-pos, MM4-pos, and MM5-pos) in the MM3--MM5 region of G35.2N. Local thermodynamic equilibrium modeling yields column densities and abundance ratios. CH$_3$OH column densities were inferred from $^{13}$CH$_3$OH assuming $^{12}$C/$^{13}$C = 50 to mitigate optical-depth effects. CH$_3$OH, CH$_3$SH, and C$_2$H$_5$OH are robustly constrained at all three positions; C$_2$H$_5$SH is robustly constrained at MM3-pos and MM5-pos but remains tentative at MM4-pos. The corresponding adopted column-density ranges are $(1.8-2.7)\times10^{18}$, $(1.7-2.6)\times10^{16}$, $(5.7-8.5)\times10^{16}$, and $(2.4-4.6)\times10^{15}$ cm$^{-2}$. The CH$_3$OH/C$_2$H$_5$OH and CH$_3$OH/CH$_3$SH ratios are consistent across the positions within uncertainties, with nominal values of 31--32 and 100--110, respectively. Comparisons with chemically rich sources and warm-up chemical models show that CH$_3$OH/C$_2$H$_5$OH lies within the range measured elsewhere, whereas CH$_3$OH/CH$_3$SH exhibits greater source-to-source variation. The ethyl-level O/S comparison remains less certain because many literature C$_2$H$_5$SH measurements provide only lower limits. CH$_3$OH/CH$_3$SH is thus the best-constrained O/S alcohol--thioalcohol ratio in these data and provides an empirical probe of source-dependent sulfur-bearing organic chemistry. More sensitive C$_2$H$_5$SH observations are needed to test C$_2$H$_5$OH/C$_2$H$_5$SH.