Chemical properties of two dense cores in a Planck Galactic Cold Clump
  G168.72-15.48
  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, \ce{c-C3H}, \ce{H2CO}, \ce{HC5N}, \ce{HC7N}, \ce{SO},
\ce{CCH}, \ce{N2H+}, \ce{CH3OH}, and \ce{CH3CCH}, toward two dense cores in
PGCC G168.72-15.48 using the Tianma Radio Telescope and Purple Mountain
Observatory Telescope. We detected \ce{c-C3H}, \ce{H2CO}, \ce{HC5N}, \ce{N2H+},
\ce{CCH}, and \ce{CH3OH} in both G168-H1 and G168-H2 cores, whereas \ce{HC7N}
and \ce{CH3CCH} were detected only in G168-H1 and SO was detected only in
G168-H2. Mapping observations reveal that the \ce{CCH}, \ce{N2H+}, \ce{CH3OH},
and \ce{CH3CCH} emissions are well coupled with the dust emission in G168-H1.
Additionally, \ce{N2H+} exhibits an exceptionally weak emission in the denser
and more evolved G168-H2 core, which may be attributed to the \ce{N2H+}
depletion. We suggest that the \ce{N2H+} depletion in G168-H2 is dominated by
\ce{N2} depletion, rather than the destruction by CO. The local thermodynamic
equilibrium calculations indicate that the carbon-chain molecules of \ce{CCH},
\ce{HC5N}, \ce{HC7N}, and \ce{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.
