2025AJ....170...74C

Using the Institut de Radioastronomie Millmétrique 30 m telescope, we presented observations of N₂H⁺J = 1-0, CCS J_N = 8₇-7₆ and 7₇-6₆ lines toward a large sample of ultracompact H II regions (UC H IIs). Among our 88 UC H IIs, 87 and 33 sources were detected in the N₂H⁺J = 1-0 and CCS J_N = 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 J_N = 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 through high-mass protostellar cores to UC H IIs. 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.