Dense gas in molecular cores associated with Planck Galactic cold clumps
  We present the first survey of dense gas towards Planck Galactic Cold Clumps
(PGCCs). Observations in the J=1-0 transitions of HCO+ and HCN towards 621
molecular cores associated with PGCCs were performed using the Purple Mountain
Observatory 13.7-m telescope. Among them, 250 sources have detection, including
230 cores detected in HCO+ and 158 in HCN. Spectra of the J=1-0 transitions
from CO, 13CO, and C18O at the centers of the 250 cores were extracted from
previous mapping observations to construct a multi-line data set. The
significantly low detection rate of asymmetric double-peaked profiles, together
with the well consistence among central velocities of CO, HCO+, and HCN
spectra, suggests that the CO-selected Planck cores are more quiescent compared
to classical star-forming regions. The small difference between line widths of
C18O and HCN indicates that the inner regions of CO-selected Planck cores are
not more turbulent than the exterior. The velocity-integrated intensities and
abundances of HCO+ are positively correlated with those of HCN, suggesting
these two species are well coupled and chemically connected. The detected
abundances of both HCO+ and HCN are significantly lower than values in other
low- to high-mass star-forming regions. The low abundances may be due to beam
dilution. On the basis of the inspection of the parameters given in the PGCC
catalog, we suggest that there may be about 1 000 PGCC objects having
sufficient reservoir of dense gas to form stars.
