Stellar Populations in Three Outer Fields of the Large Magellanic Cloud

We present HST photometry for three fields in the outer disk of the LMCextending approximately four magnitudes below the faintest main sequenceturnoff. We cannot detect any strongly significant differences in the stellarpopulations of the three fields based on the morphologies of thecolor-magnitude diagrams, the luminosity functions, and the relative numbers ofstars in different evolutionary stages. Our observations therefore suggestsimilar star formation histories in these regions, although some variations arecertainly allowed. The fields are located in two regions of the LMC: one is inthe north-east field and two are located in the north-west. Under theassumption of a common star formation history, we combine the three fields withground-based data at the same location as one of the fields to improvestatistics for the brightest stars. We compare this stellar population withthose predicted from several simple star formation histories suggested in theliterature, using a combination of the R-method of Bertelli et al (1992) andcomparisons with the observed luminosity function. The only model which weconsider that is not rejected by the observations is one in which the starformation rate is roughly constant for most of the LMC's history and thenincreases by a factor of three about 2 Gyr ago. Such a model has roughly equalnumbers of stars older and younger than 4 Gyr, and thus is not dominated byyoung stars. This star formation history, combined with a closed box chemicalevolution model, is consistent with observations that the metallicity of theLMC has doubled in the past 2 Gyr.Comment: 30 pages, includes 10 postscript figures. Figure 1 avaiable at ftp://charon.nmsu.edu/pub/mgeha/LMC. Accepted for publication in Astronomical Journa