A survey of CCS, HC3N, HC5N, and NH3 toward dark cloud cores and their production chemistry

Abstract

Survey observations of CCS (JN = 43-32, JN = 21-10), HC3N (J = 5-4), HC5N (J = 9-8, J = 17-16), and NH3 (J, K = 1, 1) were carried out toward 49 dark cloud cores to examine the existence of a systematic relation between the chemical evolution and the physical evolution of dark clouds. The JN = 33-22 and JN = 34-23 lines of CCS and the J = 4-3 line of C3S were also observed in several cores. The CCS radical was revealed to be abundant in cold and quiescent dark cloud cores, while it is much less abundant in starforming regions. From the intensity ratio between the JN = 43-32 and JN = 21-10 lines, the H2 densities of the CCS cores are estimated to be (0.4-5) × 104 cm-3 by using the large velocity gradient calculations. The column density of CCS shows a good positive correlation with those of HC3N and HC5N, indicating that the production chemistry of CCS is closely related to those of other carbon-chain molecules in dark clouds. On the other hand, the column density of CCS shows no correlation with that of NH3; NH3 tends to be abundant in star-forming regions. A possible chemical model for the production of Cn S (n = 1-3) is proposed. Pseudo-time-dependent calculations based on the proposed model show that the calculated abundance of CCS in the early stage of chemical evolution agrees mostly with the observed value in TMC-1. The relations among the observed column densities are qualitatively interpreted as an effect of chemical evolution of dark clouds on the basis of simulation of the molecular synthesis; carbon-chain molecules including CCS are abundant in the early stages of chemical evolution, whereas NH3 is abundant in the later stages. A change in the chemistry of a dark cloud core associated with the cloud contraction and formation of low-mass stars is discussed, and it is proposed that the ratio [CCS]/[NH3] is a possible indicator of cloud evolution and star formation.