Young stellar clusters in the Vela D molecular cloud

It is now well established by means of direct and indirect observations that most, if not all, stars are formed in groups rather than in isolation (Clarke, Bonnell & Hillenbrand 2000). An important result that strongly constrains theories of massive star and stellar cluster formation is that the stellar density of young stellar clusters seems to depend on the mass of the most massive star in the cluster. Low-mass stars are usually found to form in loose groups with typical densities of a few stars per cubic parsec (Gomez et al. 1993), while high-mass stars are found within dense stellar clusters of up to 104 stars per cubic parsec (e.g. the Orion Nebula Cluster, Hillenbrand & Hartmann 1998). To explain this different behaviour, it has been proposed that massive stars may form with a process that is drastically different from the standard accretion picture, e.g. by coalescence of lower mass seeds in a dense cluster environment. The transition between these two modes of formation should occur in the intermediate-mass regime, namely 2 <= M/M0 <= 15.