Simulations for the evolution of the chemical clock HC3N/N2H+ in high-mass star-forming regions

Abstract: From observations, column density ratios or integrated intensity ratios between some species exhibit monotonic increase or decrease along with the evolution of high-mass star-forming regions (HMSFRs). Such ratios are defined as chemical clocks, which can be used to constrain the evolutionary stage. We performed chemical simulations to reproduce the observed column density ratio of HC3N/N2H+ and the abundances of these two species across various evolutionary stages in HMSFRs. Simultaneously, we identified the chemical processes responsible for the observed time-dependent trends in these stages. Our simulations utilized the astrochemical code Nautilus and the existing 1D models of HMSFRs that cover four evolutionary stages, accompanied by variations in density and temperature throughout the entire evolution. When averaging over large spatial scales, the best model produced successfully matches the observed column density ratio of HC3N/N2H+ and the abundances of the species involved at specific times for each evolutionary stage; that is, the late high-mass starless core stage, the early high-mass protostellar object stage, and the early ultracompact HII stage. HC3N is mainly affected by the warm carbon-chain chemistry (WCCC) and its own thermal desorption, while N2H+ is primarily influenced by the thermal desorption of N2, CO, CH4, NH3, and H2O followed by dissociative recombination and ion-molecule reactions. The results obtained from the best-fitting model timescales broadly agree with statistical estimates. Based on our best-fit model, we further examined other 350 ratios involving 27 species, and 178 ratios exhibit an increasing or decreasing evolutionary trend around the best-fit timescales of HC3N/N2H+. Among them, 157 ratios are observable and could be considered as candidate chemical clocks.