Young stellar clusters and associations in M33

Aims. We analyse multi-wavelength observations of 32 young star clusters and associations in M33 with known oxygen abundance (8 < 12+ log(O/H) < 8.7). The data set includes ultraviolet (UV), optical, mid-infrared (MIR), CO (1-0) and 21-cm line (HI) observations. We derive the spectral energy distribution (SED) of these systems and the properties of their gaseous environment to investigate the process of star formation and the interplay with the interstellar medium (ISM). Methods. We determine age, bolometric luminosities, masses, and the extinction by comparing the multi-band integrated photometry to single-age stellar population models. The best-fit solutions have been obtained using the Large Magellanic Cloud (LMC) extinction curves. Results. The stellar system ages range between 2 and 15 Myr, masses are between 3x10(2) and 4x10(4) M-circle dot, and the intrinsic extinction, AV, varies from 0.3 to 1 mag. We find a correlation between age and extinction (young clusters being more reddened than older ones), and between the cluster mass and size. From the MIR emission we infer the presence of a dust component around the clusters whose fractional luminosity at 24 mu m, L-24/L-Bol, decreases with the galactocentric distance. However, the total infrared luminosity inferred from L-24 is smaller than what we derive from the extinction corrections. We also find that the H alpha luminosity predicted by population synthesis models is higher than the observed one, especially for low-mass systems (M < 10(4) M-circle dot). This difference is reduced, but not erased, when the incomplete sampling of the initial mass function (IMF) at the high-mass end is taken into account. Conclusions. Our results suggest that a non-negligible fraction of UV ionising and non-ionising radiation is leaking into the ISM outside the HII regions. This agrees with the large UV and H alpha diffuse fractions observed in M33, but it implies that stellar systems younger than 3 Myr retain, on average, only 30% of their Lyman-continuum photons. However, the uncertainties in cluster ages and the stochastic fluctuations of the IMF do not allow us to accurately quantify this issue. We also consider the possibility that this discrepancy is the consequence of a suppressed or delayed formation of the most massive stars, given that it mainly affects the young and less massive clusters. We do not find any clear correlation between the cluster mass and the gas surface density or metallicity.