Young Stellar Populations around SN 1987A

We present the results of a study of the stellar population in a region of 30pc radius around SN1987A, based on an analysis of multi-band HST-WFPC2 images. The effective temperature, radius and, possibly, reddening of each star weredetermined for 21,955 stars by fitting the measured broad band magnitudes tothe ones calculated with model atmospheres. In addition, we have identified allstars with Halpha equivalent widths in excess of 8 A, amounting to a total of492 stars. An inspection to the HR diagram reveals the presence of several generationsof young stars, with ages between 1 and 150 Myrs, superposed on a much olderfield population (0.6-6 Gyrs). A substantial fraction of young stars have agesaround 12 Myrs which is the stellar generation coeval to SN 1987A progenitor.The youngest stars in the field appear to be strong-line T Tauri stars. Thisconstitute the first positive detection of low mass (about 1-2 Mo) Pre MainSequence (PMS) stars outside the Milky Way. Their positions in the HR diagramappear to require that star formation in the LMC occurs with accretion ratesabout 10 times higher than in the Milky Way, i.e. ~10^{-4} Mo/yr. SN1987A appears to belong to a loose, young cluster 12+/-2 Myrs old, in whichthe slope of the present mass function is almost identical to Salpeter's formasses above 3 Mo, but becomes much flatter for lower masses. On a large scale,we find that the spatial distribution of massive stars and low-mass PMS starsare conclusively different. This results casts doubts on the validity of an IMFconcept on a small scale (say, less than 10 pc). A preliminary analysis, donefor the whole field as a single entity, shows that the IMF slope for the youngpopulation present over the entire region is steeper than Gamma ~ -1.7.Comment: 31 pages, 12 figures. Accepted for publication in the Astrophysical Journal. Please contact mromanie@eso.org for the full color version of the figures. Minor typos fixed, suffix of Figure 7b changed from ps to gi