The Chemical Clock of High-mass Star-forming Regions: N2H+/CCS

TL;DR

This study uses molecular line observations to establish the N(N2H+)/N(CCS) ratio as a chemical clock for tracking the evolutionary stages of high-mass star-forming regions, supported by chemical modeling.

Contribution

It introduces the N(N2H+)/N(CCS) ratio as a new chemical clock for high-mass star-forming regions, validated by observations and chemical modeling.

Findings

N(N2H+)/N(CCS) ratio increases with evolutionary stage.

The ratio effectively traces the progression from starless cores to UC HII regions.

Chemical models support the ratio's increase with age.

Abstract

Using the IRAM 30 m telescope, we presented observations of N2H+ J = 1-0, CCS JN = 87-76 and 77-66 lines toward a large sample of ultracompact HII regions (UC HIIs). Among our 88 UC HIIs, 87 and 33 sources were detected in the N2H+ J = 1-0 and CCS JN = 87-76 lines, respectively. For the CCS 77-66 transition, we detected emission in 10 out of 82 targeted sources, all of which also exhibited emission in the CCS JN = 87-76 line. Physical parameters are derived for our detections, including the optical depth and excitation temperature of N2H+, the rotational temperature of CCS and the column density. Combining our results and previous observation results in different stages of high-mass star-forming regions (HMSFRs), we found that the column density ratio N(N2H+)/N(CCS) increases from high-mass starless cores (HMSCs) through high-mass protostellar cores (HMPOs) to UC HIIs. This implies that N(N2H+)/N(CCS) can trace the evolution process of HMSFRs. It was supported by our gas-grain chemical model, which shows that N(N2H+)/N(CCS) increases with the evolution age of HMSFRs. The temperature, density and chemical age were also constrained from our best-fit model at each stage. Thus, we propose N(N2H+)/N(CCS) as a reliable chemical clock of HMSFRs.