In Search for Infall Motion in molecular clumps II: HCO+ (1-0) and HCN
  (1-0) Observations toward a Sub-sample of Infall Candidates
  Gravitational accretion accumulates the original mass, and this process is
crucial for us to understand the initial phases of star formation. Using the
specific infall profiles in optically thick and thin lines, we searched the
clumps with infall motion from the Milky Way Imaging Scroll Painting (MWISP) CO
data in previous work. In this study, we selected 133 sources of them as a
sub-sample for further research and identification. The excitation temperatures
of these sources are between 7.0 and 38.5 K, while the H_2 column densities are
between 10^21 and 10^23 cm^-2. We have observed optically thick lines HCO+
(1-0) and HCN (1-0) using the DLH 13.7-m telescope, and found 56 sources of
them with blue profile and no red profile in these two lines, which are likely
to have infall motions, with the detection rate of 42\%. It suggests that using
CO data to restrict sample can effectively improve the infall detection rate.
Among these confirmed infall sources, there are 43 associated with Class 0/I
young stellar objects (YSOs), and 13 are not. These 13 sources are probably
associated with the sources in earlier evolutionary stage. By comparison, the
confirmed sources which are associated with Class 0/I YSOs have higher
excitation temperatures and column densities, while the other sources are
colder and have lower column densities. Most infall velocities of the sources
we confirmed are between 10^-1 to 10^0 km s^-1, which is consistent with
previous studies.
