The Herschel view of the on-going star formation in the Vela-C molecular cloud

Aims. As part of the Herschel guaranteed time key programme "HOBYS", we present the PACS and SPIRE photometric survey of the star-forming region Vela-C, one of the nearest sites of low-to-high-mass star formation in the Galactic plane. Our main objectives are to take a census of the cold sources and to derive their mass distribution down to a few solar masses. Methods. Vela-C was observed with PACS and SPIRE in parallel mode at five wavelengths between 70 mu m and 500 mu m over an area of about 3 square degrees. A photometric catalogue was extracted from the detections in each of the five bands, using a threshold of 5 sigma over the local background. Out of this catalogue we selected a robust sub-sample of 268 sources, of which similar to 75% are cloud clumps (diameter between 0.05 pc and 0.13 pc) and 25% are cores (diameter between 0.025 pc and 0.05 pc). Their spectral energy distributions (SEDs) were fitted with a modified black body function. We classify 48 sources as protostellar, based on their detection at 70 mu m or at shorther wavelengths, and 218 as starless, because of non-detections at 70 mu m. For two other sources, we do not provide a secure classification, but suggest they are Class 0 protostars. Results. From the SED fitting we derived key physical parameters (i.e. mass, temperature, bolometric luminosity). Protostellar sources are in general warmer (< T > = 12.8K) and more compact (< diameter > = 0.040 pc) than starless sources (< T > = 10.3K, < diameter > = 0.067 pc). Both these findings can be ascribed to the presence of an internal source(s) of moderate heating, which also causes a temperature gradient and hence a more peaked intensity distribution. Moreover, the reduced dimensions of protostellar sources may indicate that they will not fragment further. A virial analysis of the starless sources gives an upper limit of 90% probability for the sources to be gravitationally bound and therefore prestellar in nature. A luminosity vs. mass diagram of the two populations shows that protostellar sources are in the early accretion phase, while prestellar sources populate a region of the diagram where mass accretion has not started yet. We fitted a power law N(log M) proportional to M-1.1 +/- 0.2 to the linear portion of the mass distribution of prestellar sources. This is in between that typical of CO clumps and those of cores in nearby star-forming regions. We interpret this as a result of the inhomogeneity of our sample, which is composed of comparable fractions of clumps and cores.