Massive Young Clusters in the Disk of M31

We have studied the properties of a sample of 67 very blue and likely youngmassive clusters in M31 extracted from the Bologna Revised Catalog of globularclusters, selected according to their color [(B-V) < 0.45] and/or to thestrength of their Hbeta spectral index (Hbeta > 3.5 A). Their existence in M31has been noted by several authors in the past; we show here that these BlueLuminous Compact Clusters (BLCCs) are a significant fraction (>~ 15%) of thewhole globular cluster system of M31. Compared to the global properties of theM31 globular cluster system, they appear to be intrinsically fainter,morphologically less concentrated, and with a shallower Balmer jump andenhanced $H\beta$ absorption in their spectra. Empirical comparison with integrated properties of clusters with known age aswell as with theoretical SSP models consistently indicate that their typicalage is less than ~2 Gyr, while they probably are not so metal-poor as deducedif considered to be old. Either selecting BLCCs by their (B-V) colors or by thestrength of their Hbeta index the cluster sample turns out to be distributedonto the outskirts of M31 disc, sharing the kinematical properties of the thin,rapidly rotating disc component. If confirmed to be young and not metal-poor, these clusters indicate theoccurrence of a significant recent star formation in the thin disc of M31,although they do not set constraints on the epoch of its early formation.Comment: Submitted for publication in the Astronomical Journal. Aastex Latex file of 22 pages, 12 figures and 3 table