2006ApJ...646..258H

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 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 NH₃and 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 C³⁴S 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 in 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).