On the nature of faint mid-infrared sources in M33
  We investigate the nature of 24micron sources in M33 which have weak or no
associated Halpha emission. Both bright evolved stars and embedded star forming
regions are visible as compact infrared sources in the 8 and 24micron maps of
M33 and contribute to the more diffuse and faint emission in these bands. Can
we distinguish the two populations? We carry out deep CO J=2-1 and J=1-0 line
searches at the location of compact mid-IR sources to unveil an ongoing star
formation process. We use different assumptions to estimate cloud masses from
pointed observations and analyze if SED and mid-IR colours can be used to
discriminate between evolved stars and star forming regions. Molecular emission
has been detected at the location of several sources at the level of 0.3 K km/s
or higher in at least one of the CO rotational lines. Even though there are no
giant molecular clouds beyond 4kpc in M33, our deep observations have revealed
that clouds of smaller mass are very common. Sources which are known to be
evolved variable stars show weaker or undetectable CO lines. Evolved stars
occupy a well defined region of the IRAC color-color diagrams. Star forming
regions are scattered throughout a larger area even though the bulk of the
distribution has different IRAC colors than evolved variable stars. We estimate
that about half of the 24 micron sources without an Halpha counterpart are
genuine embedded star forming regions. Sources with faint but compact Halpha
emission have an incomplete Initial Mass Function (IMF) at the high-mass end
and are compatible with a population of young clusters with a stochastically
sampled, universal IMF.
