The IMF and star formation history of the stellar clusters in the Vela D cloud

Aims.We present the results of a Near-Infrared deep photometric survey of a sample of six embedded star clusters in the Vela-D molecular cloud, all associated with luminous (∼ 103 L⊙) IRAS sources. The clusters are unlikely to be older than a few 106 yrs, since all are still associated with molecular gas.Methods.We employed the fact that all clusters lie at the same distance and were observed with the same instrumental setting to derive their properties in a consistent way, being affected by the same instrumental and observational biases. We extracted the clusters' K Luminosity Functions and developed a simple method to correct them for extinction, based on colour-magnitude diagrams. The reliability of the method has been tested by constructing synthetic clusters from theoretical tracks for pre-main sequence stars and a standard Initial Mass Function. The clusters' Initial Mass Functions have been derived from the dereddened K Luminosity Functions by adopting a set of pre-main sequence evolutionary tracks and assuming coeval star formation.Results.All clusters are small (∼ 100 members) and compact (radius ∼ 0.1-0.2 pc); their most massive stars are intermediate-mass (∼ 2-10 M⊙) ones. The dereddened K Luminosity Functions are likely to arise from the same distribution, suggesting that the selected clusters have quite similar Initial Mass Functions and star formation histories. The Initial Mass Functions are consistent with those derived for field stars and clusters. Adding them together we found that the "global" Initial Mass Function appears steeper at the high-mass end and exhibits a drop-off at ∼ 10 M⊙. In fact, a standard Initial Mass Function would predict a star with M > 22.5 M⊙ within one of the clusters, which is not found. Hence, either high-mass stars need larger clusters to be formed, or the Initial Mass Function of the single clusters is steeper at the high-mass end because of the physical conditions in the parental gas.