2019ApJ...887..243T

To deepen our understanding of the chemical properties of the Planck Galactic Cold Clump (PGCC) G168.72-15.48, we performed observations of nine molecular species, namely, c-C₃H, H₂CO, HC₅N, HC₇N, SO, CCH, N₂H⁺, CH₃OH, and CH₃CCH, toward two dense cores in PGCC G168.72-15.48 using the Tianma Radio Telescope and Purple Mountain Observatory Telescope. We detected c-C₃H, H₂CO, HC₅N, N₂H⁺, CCH, and CH₃OH in both G168-H1 and G168-H2 cores, whereas HC₇N and CH₃CCH were detected only in G168-H1 and SO was detected only in G168-H2. Mapping observations reveal that the CCH, N₂H⁺, CH₃OH, and CH₃CCH emissions are well coupled with the dust emission in G168-H1. Additionally, N₂H⁺ exhibits an exceptionally weak emission in the denser and more evolved G168-H2 core, which may be attributed to the N₂H⁺ depletion. We suggest that the N₂H⁺ depletion in G168-H2 is dominated by N₂ depletion, rather than the destruction by CO. The local thermodynamic equilibrium calculations indicate that the carbon-chain molecules of CCH, HC₅N, HC₇N, and CH₃CCH are more abundant in the younger G168-H1 core. We found that starless core G168-H1 may have the properties of cold dark clouds based on its abundances of carbon-chain molecules. While, the prestellar core G168-H2 exhibits lower carbon-chain molecular abundances than the general cold dark clouds. With our gas-grain astrochemical model calculations, we attribute the observed chemical differences between G168-H1 and G168-H2 to their different gas densities and different evolutionary stages.