The relation between DCN/DNC ratios and gas temperature in late-stage massive star-forming regions using Yebes observations

Deuterated molecules are important probes of the physical and chemical processes that regulate molecular cloud evolution and star formation. Their abundances are affected by the surrounding environment, such as temperature, density, and evolutionary stage. DCN and DNC are relatively abundant deuterated isomers that provide an opportunity to investigate temperature-dependent deuterium chemistry. Although the abundance ratio between their hydrogenated counterparts, HCN and HNC, has often been thought to increase in warmer gas, the temperature dependence of the abundance ratio DCN / DNC remains less well constrained. To investigate how DCN / DNC depends on the gas kinetic temperature, we observed the J=1-0 transitions of DCN, DNC, H 13 CN, HN^13C, HC^15N, and H^15NC toward 101 late-stage massive star-forming regions using the Yebes 40m telescope. Gas kinetic temperatures were estimated from CH_3CCH 5-4, whose rotation temperature is closely related to the kinetic temperature of dense gas. These temperatures were adopted in the column density calculations and in the analysis of temperature-dependent abundance ratios. We find a moderate positive correlation between the abundance ratio DCN / DNC and kinetic temperature, which is statistically significant and indicates that DCN becomes progressively more abundant relative to DNC in warmer gas. A weak negative correlation with temperature is observed for D_ frac ( HNC )$, whereas no statistically significant correlation is found for D_ frac ( HCN ). The HCN / HNC ratios show substantial scatter and no statistically significant correlation with temperature, possibly due in part to optical-depth effects in the ^13C isotopologues and the limited sensitivity of the $^ 15 N isotopologue observations. These results suggest that DCN / DNC may be governed not only by HCN/HNC isomeric chemistry but also by temperature-dependent deuterium fractionation pathways and the chemical evolution of dense gas.