Observations and Implications of the Star Formation History of the Large Magellanic Cloud

We present derivations of star formation histories based on color-magnitudediagrams of three fields in the LMC from HST/WFPC2 observations. A significantcomponent of stars older than 4 Gyr is required to match the observedcolor-magnitude diagrams. Models with a dispersion-free age-metallicityrelation are unable to reproduce the width of the observed main sequence;models with a range of metallicity at a given age provide a much better fit.Such models allow us to construct complete ``population boxes'' for the LMCbased entirely on color-magnitude diagrams; remarkably, these qualitativelyreproduce the age-metallicity relation observed in LMC clusters. We discusssome of the uncertainties in deriving star formation histories. We find,independently of the models, that the LMC bar field has a larger relativecomponent of older stars than the outer fields. The main implications suggestedby this study are: 1) the star formation history of field stars appears todiffer from the age distribution of clusters, 2) there is no obvious evidencefor bursty star formation, but our ability to measure bursts shorter induration than $\sim$ 25% of any given age is limited by the statistics of theobserved number of stars, 3) there may be some correlation of the starformation rate with the last close passage of the LMC/SMC/Milky Way, but thereis no dramatic effect, and 4) the derived star formation history is probablyconsistent with observed abundances, based on recent chemical evolution models.Comment: Accepted by AJ, 36 pages including 12 figure