IMF in Small Young Embedded Star Clusters

Abstract

Using near-infrared photometry, we have constrained the IMFs of 6 young embedded star clusters in the cloud D of the Vela Molecular Ridge. We show that they are consistent with that of field stars. We selected a sample of 6 clusters, all associated with a CO(1–0) emission peak in the cloud D of the Vela Molecular Ridge. Massi et al. (2000) found them to be compact (∼ 0.1 pc) structures with < 100 members whose most massive objects are intermediate-mass (proto)stars (M ∼2–10 M ). Most of them are embedded in molecular gas, suggesting an age < 3 Myrs (Lada & Lada 2003). The estimated distance is 700 pc. The observations were carried out through the JHKs near-Infrared bands using SOFI at the NTT (ESO, La Silla, Chile). The completeness limit is Ks ∼ 17–18, i. e., more than 1.5 mag deeper than Massi et al. (2000). This allows us to probe pms stars of 106 yrs old down to 0.1 M at AV ∼ 30 mag. More details on the photometry can be found in Testi et al. (2001). Our aim was to constrain the clusters’ IMFs based on their K Luminosity Functions (KLFs). Hence, for each field we constructed both a cluster KLF, by using the photometry of sources within a radius of 2 arcmin from the cluster centre, and a reference KLF, by using all the photometry outside such radius. Noting that the clusters are likely to be younger than 3 Myrs and that the isochrones of pms stars in a Ks vs. H −Ks diagram all lie in a strip only few tenths of a magnitude in colour, we constructed dereddened KLFs by projecting each point in the Ks vs. H −Ks diagram back along the reddening vector onto a mean locus (see Fig. 1). Once normalized each outer KLF to the area of the corresponding inner one, it was subtracted from the latter to obtain a KLF free from field star contamination. We performed several tests to check that the dereddened KLFs (see Fig. 2) are good representations of the real ones, in spite of the presence of near-infrared excess emission.