The low end of the initial mass function in young Large Magellanic Cloud clusters. I. The case of R136

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. previously reached. In these new deeper images, we detect stars down to a limiting magnitude of m(F555W) = 24.7 (similar or equal to 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.. We construct the initial mass function (IMF) in the 1.35-6.5 M. range and find that, after correcting for incompleteness, the IMF shows a definite flattening below similar or equal to 2 M.. We discuss the implications of this result for the R136 cluster and for our understanding of starburst galaxy formation and evolution in general.