TheHerschelview of the on-going star formation in the Vela-C molecular cloud

Abstract

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 shorther 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) ∝ 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.

The Hi-GAL compact source catalogue – I. The physical properties of the clumps in the inner Galaxy (−71$_{.}^{\circ}$0 < ℓ < 67$_{.}^{\circ}$0)

“…The distributions of grey body temperatures of the sources are shown in Figure 5. As already found by Giannini et al (2012), Elia et al (2013), Giannetti et al (2013), Veneziani et al (2017) through Hi-GAL observations but also by Olmi et al (2009) through BLAST observations, the distributions of pre-and proto-stellar sources show some relevant differences, the latter being found towards warmer temperatures with respect to the former. A quantitative argument is represented by the average valuesT pre = 12.0 K andT prt = 16.0 K for pre-and proto-stellar sources, and the median values T pre = 11.7 K andT prt = 15.1 K, respectively.…”

“…Here we can confirm, based on observational arguments, that the slopes reported across various panels of Figure 10 do not show any particular trend with distance. Therefore, whereas a clear distinction is found between the mass spectrum of cores (typically resolved by Herschel if located at d 1 kpc, Giannini et al 2012;Baldeschi et al 2017), and that of clumps, with the former being steeper than the latter, no further systematic steepening is found for clumps observed at increasing distances. This might be also regarded as an indirect evidence of the self-similarity of molecular clouds (Stutzki et al 1998;Smith et al 2008;Elia et al 2014) over the investigated range of physical scales, which, in this case, is the range of the linear sizes of compact sources located between d = 1.5 and 13.5 kpc.…”

“…recently used, e.g., in Giannini et al (2012), where F ν is the observed flux density at the frequency ν, B ν (T d ) is the Planck function at the dust temperature T d and Ω is the source solid angle in the sky. The optical depth can be parametrised in turn as…”

“…Second, a higher degree of overlap is seen between the pre-and proto-stellar source populationsthe former being found, at M > 10 M , to be overlapped with the accretion portion of the evolutionary tracks, also populated by the proto-stellar sources. This does not mean necessarily that in the outer Galaxy a clearer segregation of the pre-vs protostellar clump populations is seen through this diagnostic tool (see also Giannini et al 2012, for another example of analysis of an outer Galaxy region, namely Vela-C), since distance effects must also be taken into account. On average, the sources of Giannini et al (2012) and Elia et al (2013) are much closer (d = 700 pc and d 2200 pc, respectively) than most sources of our sample, and larger distances might introduce ambiguities in the pre-vs proto-stellar classification (see Appendix C).…”

The G305 star-forming complex: embedded massive star formation discovered byHerschelHi-GAL

“…The majority of sources, with no 21, 24 or 70 μm emission, have fitted SEDs that peak at wavelengths λ ≥ 160 μm and are found to have an averaged temperature of ≈14 K. Bontemps et al () characterize YSOs within the Aquila rift complex from pre‐stellar sources on the presence of either a 24 or 70 μm counterpart, with this emission originating from warm dust within the inner regions of the YSO envelope. Moreover, such emission is unlikely the result of external heating from the interstellar radiation field producing a detectable 70 μm counterpart (Giannini et al ), making sources with 21, 24 or 70 μm emission likely protostellar sources. Indeed, Dunham et al () show that 70 μm emission is closely correlated to the overall luminosity of protostellar sources.…”