Expulsion of Dust from Young Stellar Clusters

Abstract

Young stellar clusters were identified on deep near-infrared images of 6 nearby, grand-design spirals observed with HAWK-I/VLT. A 90% completeness was reached for cluster complexes with M_K = −11.5m (corresponding to masses around 10⁴ M_⊙) while the linear resolution was around 40 pc. The distribution of clusters in the (H-K)–(J-H) diagrams revealed two groups of clusters. Comparing with Starburst99 model tracks, the groups could be interpreted as one old population of clusters with low extinction and one consisting of young clusters with visual extinction as high as A_V = 7^m. The clear separation between the two groups suggests a rapid expulsion of dust from the young clusters.

Monte-Carlo models were made assuming a cluster distribution function g(M_c, τ) = M_c^−a τ^−γ where M_c and τ are cluster mass and age, respectively. Fitting such models to the observed NIR color distributions, it was concluded that the cluster mortality had to be significant with γ = 0.8-1.0. Further, the star formation phase of clusters has to extend over several Myr. Young clusters have high extinctions at least the first 3 Myr but then lose their reddening over a period of around 5 Myr, depending on the exact evolutionary tracks used.