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

Giannini, T.; Elia, D.; Lorenzetti, D.; Molinari, S.; Motte, F.; Schisano, E.; Pezzuto, S.; Pestalozzi, M.; Di Giorgio, A. M.; André, P.; Hill, T.; Benedettini, M.; Bontemps, S.; Di Francesco, J.; Fallscheer, C.; Hennemann, M.; Kirk, J.; Minier, V.; Nguyn Lu’o’ng, Q.; Polychroni, D.; Rygl, K. L. J.; Saraceno, P.; Schneider, N.; Spinoglio, L.; Testi, L.; Ward-Thompson, D. and White, G. J. (2012). The Herschel view of the on-going star formation in the Vela-C molecular cloud. Astronomy & Astrophysics, 539, article no. A156.



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 μm and 500 μm 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 5σ over the local background. Out of this catalogue we selected a robust sub-sample of 268 sources, of which ~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 μm or at shorter wavelengths, and 218 as starless, because of non-detections at 70 μm. 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> = 12.8 K) and more compact (<diameter> = 0.040 pc) than starless sources (<T> = 10.3 K, <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)α 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.

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