On the shape of the mass-function of dense clumps in the Hi-GAL fields.
  I. SED determination and global properties of the mass-functions
  Stars form in dense, dusty clumps of molecular clouds, but little is known
about their origin and evolution. In particular, the relationship between the
mass distribution of these clumps (also known as the "clump mass function", or
CMF) and the stellar initial mass function (IMF), is still poorly understood.
In order to discern the "true" shape of the CMF and to better understand how
the CMF may evolve toward the IMF, large samples of bona-fide pre- and
proto-stellar clumps are required. The sensitive observations of the Herschel
Space Observatory (HSO) are now allowing us to look at large clump populations
in various clouds with different physical conditions. We analyse two fields in
the Galactic plane mapped by HSO during its science demonstration phase, as
part of the more complete and unbiased Herschel infrared GALactic Plane Survey
(Hi-GAL). These fields undergo a source-extraction and flux-estimation
pipeline, which allows us to obtain a sample with thousands of clumps. Starless
and proto-stellar clumps are separated using both color and positional criteria
to find those coincident with MIPS 24 micron sources. We describe the
probability density functions of the power-law and lognormal models that are
used to fit the CMFs, and we then find their best-fit parameters. For the
lognormal model we apply several statistical techniques to the data and compare
their results. The CMFs of the two SDP fields show very similar shapes, but
very different mass scales. This similarity is confirmed by the values of the
best-fit parameters of either the power-law or lognormal model. The power-law
model leads to almost identical CMF slopes, whereas the lognormal model shows
that the CMFs have similar widths. The similar CMF shape but different mass
scale represents an evidence that the overall process of star formation in the
two regions is very different.
