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 Larg e Magellanic Cloud from the Hubble Space Telescope archive to sample the lu minosity function below the detection limit of 2.8 M. previously reached. I n 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 recentl y computed evolutionary tracks indicates that these red objects are pre-mai n-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 a nd for our understanding of starburst galaxy formation and evolution in gen eral.