Simulating the impact of C/O ratio on chemical evolution of N-bearing species in the Hot molecular core

Abstract

Nitrogen-bearing species play a pivotal role in the chemical complexity of the interstellar medium, serving as key tracers of physical conditions and potential precursors to prebiotic molecules in star-forming environments. In this study, the chemical evolution of six nitrogen-bearing species (HC${}_3$N, HC${}_5$N, HNCO, NH${}_2$CN, C${}_2$H${}_5$CN, and NH${}_2$CHO) in high-mass star-forming regions have been investigated using a three-stage physical model, encompassing cloud collapse, warm-up, and post-warm-up stages. Simulations were conducted across varying carbon-to-oxygen (C/O) ratios (0.5, 0.75, 1, and 1.25) to analyze the time-dependent abundances of these species over a period of $2\times 10^6$ years. The results reveal distinct evolutionary patterns for each species and demonstrate their sensitivity to the C/O ratio.

For HC${}_3$N and HC${}_5$N, early-stage abundances increase during the cloud collapse stage, while higher C/O ratios favor their enhanced production during the warm-up stage. However, post-warm-up simulations overestimate their observed abundances. The production of HNCO aligns best with the observed values at a C/O ratio of 0.5, highlighting its dependence on lower C/O ratios. NH${}_2$CN and C${}_2$H${}_5$CN exhibit contrasting trends during the warm-up stage, with NH${}_2$CN decreasing and C${}_2$H${}_5$CN increasing as C/O ratios rise. Despite their significant abundance during the warm-up stage, post-warm-up simulations overestimate the observed values for these species.

Overall, while the model successfully captures key trends in nitrogen-bearing species evolution, discrepancies with observed abundances — especially in the post-warm-up stage — underline the need for improved chemical networks and refined physical parameters to better understand the complex astrochemistry of high-mass star-forming regions.