Infall Motions in Massive Star-Forming Regions: Results from Years 1 & 2
  of the MALT90 Survey
  Massive star-forming regions with observed infall motions are good sites for
studying the birth of massive stars. In this paper, 405 compact sources have
been extracted from the APEX Telescope Large Area Survey of the Galaxy
(ATLASGAL) compact sources that also have been observed in the Millimetre
Astronomy Legacy Team 90 GHz (MALT90) survey during Years 1 and 2. These
observations are complemented with Spitzer GLIMPSE/MIPSGAL mid-IR survey data
to help classify the elected star-forming clumps into three evolutionary
stages: pre-stellar, proto-stellar and UCHII regions. The results suggest that
0.05 g cm$^{-2}$ is a reliable empirical lower bound for the clump surface
densities required for massive-star formation to occur. The optically thick
HCO$^{+}$(1-0) and HNC(1-0) lines, as well as the optically thin
N$_{2}$H$^{+}$(1-0) line were used to search for infall motions toward these
sources. By analyzing the asymmetries of the optically thick HCO$^{+}$(1-0) and
HNC(1-0) lines and the mapping observations of HCO$^{+}$(1-0), a total of 131
reliable infall candidates have been identified. The HCO$^{+}$(1-0) line shows
the highest occurrence of obvious asymmetric features, suggesting that it may
be a better infall motion tracer than other lines such as HNC(1-0). The
detection rates of infall candidates toward pre-stellar, proto-stellar and
UCHII clumps are 0.3452, 0.3861 and 0.2152, respectively. The relatively high
detection rate of infall candidates toward UCHII clumps indicates that many
UCHII regions are still accreting matter. The peak column densities and masses
of the infall candidates, in general, display a increasing trend with
progressing evolutionary stages. However, the rough estimates of the mass
infall rate show no obvious variation with evolutionary stage.
