Ongoing massive star formation in the bulge of M51

We studied the HST-WFPC2 observations of the inner kpc of the interacting galaxy M51 in six bands from 2550 to 8140 Angstrom. The images show an oval shaped area ("bulge") of 11x16 arcsec or 450x650 pc around the nucleus, dominated by a smooth population of old stars with overimposed dust lanes. We found 30 bright point-like sources in the bulge of M51; many of these are bright in the UV. They are located in elongated "strings" which follow the general pattern of the dust lanes. The spectral energy distributions of the pointsources are compared with those of clusters or single stars. We present arguments that the pointsources are isolated massive stars or small groups of a few massive stars. The energy distributions are best fitted with those of single stars with 4000<Teff<50000 K, 4.2<log L/Lsun<7.2, and 12<Mstar<200 Msun. We have derived upper limits to the total mass of low mass stars that could be "hiding" within the point sources. For the bluest sources the upper limit is only a few hundred Msun. We conclude that massive stars are formed outside clusters (or in very low mass clusters) in the bulge of M51. The estimated star formation rate in the bulge is 1 to 2x10^(-3) Msun/yr. This star formation rate could be sustained for about 200 to 400 Myrs, suggesting that the ongoing massive star formation in the bulge of M51 is fed/triggered by the interaction with its companion about 400 Mrs ago. Theoretical cloud-models show that CO is destroyed in low extinction clouds near the bright nucleus of M51, so that H_2, [OI] and [CII] are the dominants coolants. This condition favors the formation of massive stars, in agreement with the observations. This mode of massive-star formation in the bulge of M51 may resemble the star formation in the early Universe, when the CO and dust contents were low due to the low metallicity.