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 contribut
e 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.
