Studying
massive star forming clumps at the CSO 
Most stars in our Galaxy,
including all massive stars, form in Giant Molecular Cloud (GMC)
clumps.
These clumps are very massive (several thousands of solar mass), dense
(number density of about 106 cm-3),
and highly obscured by dust. They can only be explored by dust
continuum emission or molecular emission
lines of dense gas tracers like Hydrogen cyanide (HCN) and Carbon
monosulfide (CS). Astronomers led by
Dr. Jingwen Wu at Harvard-Smithsonian Center for Astrophysics have used
the CSO and
another radio
telescope at Five College Radio Astronomy Observatory (FCRAO) to map
more than fifty such massive
clumps in the Milky Way, with multiple HCN and CS
transitions. Figure 1 presents the
contour maps of
one of these massive clumps obtained at the CSO and FCRAO. They have
studied their properties
revealed
by the dense gas, including their mass, size, luminosity, volume
density, and dynamics. They also
found
these clumps are centrally condensed, very turbulent, and many may be
experiencing collapse.
Astronomers conclude that the most relevant
parameter for their star formation rate is the total mass of dense gas
the clumps contain. They found all dense gas tracers show a linear
correlation between their star formation rate
(indicated by their infrared luminosity) and the total amount of
dense gas, a relationship similar
to that found in galaxies (as shown in Figure 2). The physical reason
underlying this correlation may be that
star formation in GMC has a threshold in terms of the mass of dense
gas. These GMC clumps may be building
blocks of galaxies. The results were published by Wu et al.
2010 in the Astrophysical Journal Supplement Series.
Figure 1. Contour maps of one massive dense clump (G10.6-0.4) with
several HCN and CS lines.
Figure 2. Linear correlation exists between the infrared luminosity and
the line luminosity (for all
dense gas tracers) for Galactic massive clumps that brighter than 104.5
Lsun; Some correlations
extend to galaxy scales (the dots at the right side in the panels).
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