MAIN-SEQUENCE STARS AND THE STAR-FORMATION HISTORY OF THE OUTER DISK IN THE LARGE MAGELLANIC CLOUD

Using the Wide Field Planetary Camera 2 on the Hubble Space Telescope, we have obtained a deep color-magnitude diagram in V- and I-band equi valents for more than 2000 stars in a patch of the outer disk of the L arge Magellanic Cloud (LMC). Aperture photometry is feasible from thes e data with good signal-to-noise ratio for stars with V less than or e qual to 25, which allows us for the first time to construct a color-ma gnitude diagram for LMC disk stars on the lower main sequence, extendi ng beyond the oldest main-sequence turnoff point. We analyze the struc ture of the main-sequence band and overall morphology of the color-mag nitude diagram to obtain a star formation history for the region. A co mparison between the distribution of stars across the main-sequence ba nd for M(V) less than or equal to 4 and a stellar population model con strains historical star formation rates within the past 3 Gyr. The ste llar populations in this region sample the outer LMC disk for stars wi th ages of 1 Gyr or older that have had time to spatially mix. The str ucture of the main-sequence band requires that star formation occurred at a roughly constant rate during most of the past approximate to 3 G yr. However, the distribution of subgiant stars indicate that a pronou nced peak in the star formation rate likely occurred about 2 Gyr ago, prior to which the star formation rate had not been enhanced for sever al Gyr. Studies over timescales of more than 3 Gyr require a separatio n of the effects of star formation history and the chemical evolution on the LMC color-magnitude diagrams, which is difficult to achieve wit hout additional constraints. If lower main-sequence stars in the LMC h ave moderate metallicities, then the age for most LMC disk stars is le ss than about 8 Gyr.