The Low End of the Initial Mass Function in Young Large Magellanic Cloud Clusters. I. The Case of R136

Abstract

We report the result of a study in which we have used very deep broadband V and I Wide Field Planetary Camera 2 images of the R136 cluster in the Large Magellanic Cloud from the Hubble Space Telescope archive to sample the luminosity function below the detection limit of 2.8 M_solar previously reached. In these new deeper images, we detect stars down to a limiting magnitude of m_F555W=24.7 (~=1 mag deeper than previous works) and identify a population of red stars evenly distributed in the surrounding of the R136 cluster. A comparison of our color-magnitude diagram with recently computed evolutionary tracks indicates that these red objects are pre-main-sequence stars in the mass range 0.6-3 M_solar. We construct the initial mass function (IMF) in the 1.35-6.5 M_solar range and find that, after correcting for incompleteness, the IMF shows a definite flattening below ~=2 M_solar. We discuss the implications of this result for the R136 cluster and for our understanding of starburst galaxy formation and evolution in general. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA for NASA under contract NAS 5-26555, and observations obtained at the European Southern Observatory, La Silla.