On the nature of faint mid-infrared sources in M 33

Aims. We investigate the nature of 24 μm sources in M 33 that have weak or no associated Hα emission. Both bright evolved stars and embedded star-forming regions are visible as compact infrared sources in the 8 and 24 μm Spitzer maps of M 33 and contribute to the more diffuse and faint emission in these bands. Can we distinguish the two populations? Methods. We carry out deep CO J = 2–1 and J = 1–0 line searches at the location of 18 compact mid-IR sources and two optically selected ones to unveil an ongoing star formation process throughout the disk of M 33. In the absence of high-resolution CO maps we use different assumptions to estimate cloud masses from pointed observations. We also analyze if the spectral energy distribution and mid-IR colors can be used to discriminate between evolved stars and star-forming regions. Results. Molecular emission is detected at the location of 17 sources at the level of 0.3 K km s-1 or higher in at least one of the CO rotational lines. Even though the number of giant molecular clouds drops beyond 4 kpc in M 33, our deep observations reveal that clouds of smaller mass are common out to 6.8 kpc. Estimated cloud masses range between 104 and 105 M⊙, assuming likely values of the CO-to-H2 conversion factor and virial equilibrium. Sources that 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 μm sources without an Hα counterpart are genuine embedded star-forming regions. Sources with faint but compact Hα 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