Molecular Line Observations of a Carbon-Chain-Rich Core L492

Abstract

We report on molecular abundances and distributions in a starless dense core L492. We have found that the abundances of carbon-chain molecules such as CCS, C${}_3$S, HC${}_3$N, HC${}_5$N, and HC${}_7$N are comparable to those in chemically young dark cloud cores called "carbon-chain--producing regions", such as L1495B, L1521B, L1521E, and TMC-1. This is the first dark cloud core with extremely rich in carbon-chain-molecules that is found outside the Taurus region. In addition, the deuterium fractionation ratios of DNC/HNC and DCO${}^{+}$/HCO${}^{+}$ are also comparable to those in carbon-chain--producing regions, being significantly lower than those in the evolved prestellar cores such as L1498 and L1544. On the other hand, the abundances of NH${}_3$ and N${}_2$H${}^{+}$ are systematically higher than those in carbon-chain--producing regions. Our mapping observations reveal that the central hole of molecular distributions, which were reported for CCS and C${}^{34}$S in evolved prestellar cores is not significant in L492, indicating that the depletion factor of molecules is not very high. Furthermore, L492 is dynamically more evolved than carbon-chain--producing regions, and the protostellar collapse has started like L1498 and L1544. Therefore, it is likely that the chemical and dynamical evolutionary stage of L492 is intermediate between carbon-chain--producing regions (L1495B, L1521B, L1521E, and TMC-1) and evolved prestellar cores (L1498 and L1544).

26 pages, 8 figures, accepted for publication in The Astrophysical Journal (Preprint with high resolution figures will be available at http://www.nro.nao.ac.jp/library/report/list.html)

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