J. L. Chen, J. S. Zhang, J. X. Ge, Y. X. Wang, H. Z. Yu, Y. P. Zou, Y. T. Yan, X. Y. Wang, D. Y. Wei

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.