Survey Observations to Study Chemical Evolution from High-mass Starless Cores to High-mass Protostellar Objects. II. HC3N and N2H+

We have carried out survey observations of molecular emission lines from HC3N, N2H+, CCS, and cyclic-C3H2 in the 81-94 GHz band toward 17 high-mass starless cores (HMSCs) and 28 high-mass prorostellar objects (HMPOs) with the Nobeyama 45 m radio telescope. We have detected N2H+ in all of the target sources except one and HC3N in 14 HMSCs and in 26 HMPOs. We investigate the N(N2H+)/N(HC3N) column density ratio as a chemical evolutionary indicator of massive cores. Using the Kolmogorov Smirnov (K-S) test and Welch's t test, we confirm that the N(N2H+)/N(HC3N) ratio decreases from HMSCs to HMPOs. This tendency in high-mass star-forming regions is opposite to that in low-mass star-forming regions. Furthermore, we find that the detection rates of carbon-chain species (HC3N, HC5N, and CCS) in HMPOs are different from those in low-mass protostars. The detection rates of cyanopolyynes (HC3N and HC5N) are higher and that of CCS is lower in high-mass protostars, compared to low-mass protostars. We discuss a possible interpretation for these differences.