Infall in massive clumps harboring bright infrared sources
  Thirty massive clumps associated with bright infrared sources were observed
to detect the infall signatures and characterize infall properties in the
envelope of the massive clumps by APEX telescope in CO(4-3) and C$^{17}$O(3-2)
lines. Eighteen objects have "blue profile" in CO(4-3) line with virial
parameters less than 2, suggesting that global collapse is taking place in
these massive clumps. The CO(4-3) lines were fitted by the two-layer model in
order to obtain infall velocities and mass infall rates. Derived mass infall
rates are from 10$^{-3}$ to 10$^{-1}$ M$_{\odot}$yr$^{-1}$. A positive
relationship between clump mass and infall rate appears to indicate that
gravity plays a dominant role in the collapsing process. Higher luminosity
clump has larger mass infall rate, implying that the clump with higher mass
infall rate has higher star formation rate.
