The Onset of Cluster Formation around Intermediate Mass Stars

We present the results of a recent near-infrared survey of the fields surrounding a large sample of intermediate-mass pre-main-sequence (Herbig Ae/Be) stars. While late-type Be and Ae stars are never associated with conspicuous groups of young lower mass stars, early-type Be stars are usually found within rich clusters. This finding has been tested against possible biases due to different mass sensitivity of the observations or dynamical dissipation of the clusters around older stars. Our results suggest that massive stars are preferentially produced in dense stellar clusters, possibly by dynamical interaction rather than by standard gas accretion as in the case of lower mass stars. The possibility that the observed correlation between maximum stellar mass and cluster richness could be the results of random sampling the cluster size spectrum and the stellar IMF is also discussed. Future observational tests capable of discriminating between these two competitive models are outlined.