A Possible Chemical Clock in High-mass Star-forming Regions: N(HC3N)/N(N2H+)?

We conducted observations of multiple HC₃N (J = 10-9, 12-11, and 16-15) lines and the N₂H⁺ (J = 1-0) line toward a large sample of 61 ultracompact (UC) H II regions, through the Institut de Radioastronomie Millmétrique 30 m and the Arizona Radio Observatory 12 m telescopes. The N₂H⁺ J = 1-0 line is detected in 60 sources and HC₃N is detected in 59 sources, including 40 sources with three lines, 9 sources with two lines, and 10 sources with one line. Using the rotational diagram, the rotational temperature and column density of HC₃N were estimated toward sources with at least two HC₃N lines. For 10 sources with only one HC₃N line, their parameters were estimated, taking one average value of T _rot. For N₂H⁺, we estimated the optical depth of the N₂H⁺ J = 1-0 line, based on the line intensity ratio of its hyperfine structure lines. Then the excitation temperature and column density were calculated. When combining our results in UC H II regions and previous observation results on high-mass starless cores, the N(HC₃N)/N(N₂H⁺) ratio clearly increases from the region stage. This means that the abundance ratio changes with the evolution of high-mass star-forming regions (HMSFRs). Moreover, positive correlations between the ratio and other evolutionary indicators (dust temperature, bolometric luminosity, and luminosity-to-mass ratio) are found. Thus we propose the ratio of N(HC₃N)/N(N₂H⁺) as a reliable chemical clock of HMSFRs.