Line Observations of a Carbon - Chain - Rich Core L 492

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, C3S, HC3N, HC5N, and HC7N 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 NH3 and N2H + 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 CS 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). Subject headings: ISM:abundances — ISM:individual(L492) — ISM:Molecules — radio lines: ISM