Young stellar populations around SN 1987A

We present the first results of a study of the stellar population in a regi on of 30 pc radius around SN 1987A, based on an ana ysis of multiband Hubbl e Space Telescope (HST) WFPC2 images. The effective temperature, radius and , possibly, reddening of each star were determined by fitting the measured broadband magnitudes to the ones calculated with model atmospheres. In part icular, we have determined effective temperatures and bolometric luminositi es for 21,995 stars, and for a subsample of 2510 stars we also determined i ndividual reddening corrections. In addition, we have identified all stars with Ha equivalent widths in excess of 8 Angstrom a total of 492 stars. An inspection of the H-R diagram reveals the presence of several generations o f young stars, with ages between 1 and 150 Myr, superposed on a much older field population (0.6-6 Gyr). A substantial fraction of young stars with ag es around 12 Myr make up the stellar generation coeval to SN 1987A progenit or. The youngest stars in the held appear to be strong-line T Tauri stars, identified an the basis of their conspicuous (W-eq > 8 Angstrom) H alpha ex cesses. This constitute the first positive detection of low-mass (about 1-2 M.) pre-mam-sequence (PMS) stars outside the Milky Way. Their positions in the H-R diagram appear to require that star formation in the LMC occurs wi th accretion rates about 10 times higher than in the Milky Way, i.e., simil ar to 10(-4) M. yr(-1). SN 1987A appears to belong to a loose, young cluste r 12 +/- 2 Myr old, in which the slope of the present mass function is almo st identical to Salpeter's, i.e., Gamma = d log N/d log M similar or equal to - 1.25 for masses above 3 M., but becomes much Batter for lower masses, i.e., Gamma similar or equal to -0.5. On a large scale, we find that the sp atial distributions of massive r tars and low-mass PMS stars are conclusive ly different, indicating that different star formation processes operate fo r high- and low-mass stars. This results casts doubts on the validity of an initial mass function (IMF) concept on a small scale (say, less than 10 pc ). Moreover, it appears that a determination of the low-mass end IMF in the LMC requires an explicit identification of PMS stars. A preliminary analys is, done for the whole held as a single entity, shows that the IMF slope fo r the young population present over :he entire region is steeper than Gamma similar or equal to -1.7.