Star formation in the Vela molecular ridge

Context.The Vela molecular ridge is one of the nearest intermediate-mass star forming regions, located within the galactic plane and outside the solar circle. Cloud D, in particular, hosts a number of small embedded young clusters. Aims.We present the results of a large-scale map in the dust continuum at 1.2 mm of a ~$ 1\degr \times 1\degr$ area within cloud D. The main aim of the observations was to obtain a complete census of cluster-forming cores and isolated (both high- and low-mass) young stellar objects in early evolutionary phases. Methods.The bolometer array SIMBA at SEST was used to map the dust emission in the region with a typical sensitivity of ~$ 20$ mJy/beam. This allows a mass sensitivity of ~$ 0.2$ $M_{\odot}$. The resolution is $24\arcsec$, corresponding to ~$ 0.08$ pc, roughly the radius of a typical young embedded cluster in the region. The continuum map is also compared to a large scale map of CO(1–0) integrated emission. Results.Using the CLUMPFIND algorithm, a robust sample of 29 cores has been obtained, spanning the size range 0.03–0.25 pc and the mass range 0.4–88 $M_{\odot}$. The most massive cores are associated both with red IRAS sources and with embedded young clusters, and coincide with CO(1–0) integrated emission peaks. The cores are distributed according to a mass spectrum ~$ M^{-\alpha}$ and a mass-versus-size relation ~ Dx, with $\alpha \sim 1.45$–1.9 and $x \sim 1.1$–1.7. They appear to originate in the fragmentation of gas filaments seen in CO(1–0) emission and their formation is probably induced by expanding shells of gas. The core mass spectrum is flatter than the Initial Mass Function of the associated clusters in the same mass range, suggesting further fragmentation within the most massive cores. A threshold $A_{\rm V} \sim 12$ mag seems to be required for the onset of star formation in the gas