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

We study the association between embedded star clusters and young stellar o bjects believed to be precursors of intermediate mass stars (2 less than or similar to M less than or similar to 10 M.), within the Vela Molecular Rid ge D-cloud. A sample of 12 IRAS-selected Class I sources belonging to the r egion was imaged in the near infrared bands JHK and the photometry used in order to gain information on the stellar population around these objects. W e find a large fraction of sources with a NIR excess, particularly within f ields located towards higher luminosity protostars(L-bol greater than or si milar to 10(3) L., meaning M greater than or similar to 5 M. according to a ccretion models), indicative of the presence of a large number of less mass ive young stellar objects. An analysis of the K-source surface density conf irms that the higher luminosity Class I sources are embedded in young clust ers of sizes similar to 0.1-0.3 pc and volume densities greater than or sim ilar to 3000-12000 stars pc(-3). Conversely, the lower luminosity Class I s ources (L-bol < 10(3) L., i e., M less than or similar to 5 M.) are associa ted with small groups of young stellar objects or isolated. This indicates that intermediate mass star progenitors lie in clusters whose member richne ss 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 l ocated near the star surface density peaks, suggesting a mass and age segre gation which may be partly explained by models of competitive accretion. Th e K luminosity functions of the clusters are indicative of populations of c oeval stars 10(5)-10(6) yr old roughly distributed according to the field s tars initial mass function. A scenario in which clusters are formed by cont raction and fragmentation of molecular cores, with less massive stars first leaving the birth-line, is proposed.