Star formation in the Vela molecular clouds IV. Young embedded star clusters towards D-cloud class I sources

We study the association between embedded star clusters and young stellar objects believed to be precursors of intermediate mass stars (2 <∼ M <∼ 10 M ), within the Vela Molecular Ridge D-cloud. A sample of 12 IRAS-selected Class I sources belonging to the region was imaged in the near infrared bands JHK and the photometry used in order to gain information on the stellar population around these objects. We find a large fraction of sources with a NIR excess, particularly within fields located towards higher luminosity protostars (Lbol >∼ 103 L , meaning M >∼ 5 M according to accretion models), indicative of the presence of a large number of less massive young stellar objects. An analysis of the K-source surface density confirms that the higher luminosity Class I sources are embedded in young clusters of sizes ∼ 0.1–0.3 pc and volume densities >∼ 3000–12000 stars pc−3. Conversely, the lower luminosity Class I sources (Lbol < 103 L , i. e., M <∼ 5 M ) are associated with small groups of young stellar objects or isolated. This indicates that intermediate mass star progenitors lie in clusters whose member richness increases with the progenitor mass itself. The Class I sources appear as the most massive and less evolved objects in the clusters and tend to be located near the star surface density peaks, suggesting a mass and age segregation which may be partly explained by models of competitive accretion. The K luminosity functions of the clusters are indicative of populations of coeval stars 105–106 yr old roughly distributed according to the field stars initial mass function. A scenario in which clusters are formed by contraction and fragmentation of molecular cores, with less massive stars first leaving the birthline, is proposed.