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S. Marco (JHU), A. Nota (STScI/ESA), G. De Marchi (STScI/ESA), C. Leitherer (STScI), M. Clampin (STScI)
We present a new study of the low end of the stellar IMF of NGC 330, the richest young star cluster in the Small Magellanic Cloud (SMC). We obtained deep broadband V and I HST/WFPC2 images which have allowed us, for the first time, to detect stars down to a limiting magnitude of mF555W = 24.9. At the distance of the SMC, this corresponds to stellar masses of ~q 0.8 M\odot. We derived the cluster luminosity function, which we corrected for background contamination using an adjacent field, and we constructed the initial mass function in the 1-7 M\odot range. We found that, in the cluster center, the IMF is much flatter than Salpeter's. We also investigated the IMF slope as a function of radial distance from the center of the cluster, and noted that the very shallow IMF slope found in the inner regions of the cluster becomes uniformly steeper at increasing distances. A comparison of the cluster color-magnitude diagram with evolutionary tracks indicates that the cluster's age is ~ 30 Myr. We discuss the evidence for mass segregation and conclude that, since the age of NGC 330 is lower than the expected relaxation time, the observed mass segregation is most likely due to the way star formation occurs in the cluster.
This study is part of a larger program aimed to investigate the universality of the IMF at low masses in a number of young clusters in the LMC/SMC -- at different conditions of stellar density, age and metallicity -- to understand whether at small masses the IMF is constrained by local conditions. All the clusters in our project have been treated with a homogeneous data reduction and analysis procedure and a unique choice of models to minimise the uncertainties associated with data reduction and interpretation. NGC 330 is the cluster with the lowest metallicity in our sample.