Initial phases of massive star formation in high infrared extinction
  clouds. II. Infall and onset of star formation
  The onset of massive star formation is not well understood because of
observational and theoretical difficulties. To find the dense and cold clumps
where massive star formation can take place, we compiled a sample of high
infrared extinction clouds, which were observed previously by us in the 1.2 mm
continuum emission and ammonia. We try to understand the star-formation stages
of the clumps in these high extinction clouds by studying the infall and
outflow properties, the presence of a young stellar object (YSO), and the level
of the CO depletion through a molecular line survey with the IRAM 30m and APEX
12m telescopes. Moreover, we want to know if the cloud morphology, quantified
through the column density contrast between the clump and the clouds, has an
impact on the star formation occurring inside it. We find that the HCO+(1-0)
line is the most sensitive for detecting infalling motions. SiO, an outflow
tracer, was mostly detected toward sources with infall, indicating that infall
is accompanied by collimated outflows. The presence of YSOs within a clump
depends mostly on its column density; no signs of YSOs were found below 4E22
cm-2. Star formation is on the verge of beginning in clouds that have a low
column density contrast; infall is not yet present in the majority of the
clumps. The first signs of ongoing star formation are broadly observed in
clouds where the column density contrast between the clump and the cloud is
higher than two; most clumps show infall and outflow. Finally, the most evolved
clumps are in clouds that have a column density contrast higher than three;
almost all clumps have a YSO, and in many clumps, the infall has already
halted. Hence, the cloud morphology, based on the column density contrast
between the cloud and the clumps, seems to have a direct connection with the
evolutionary stage of the objects forming inside.
