Star formation histories from Hubble Space Telescope color-magnitude diagrams of six fields of the Large Magellanic Cloud

We present results on the analysis of background held stars found in Hubble Space Telescope Wide Field Planetary Camera 2 observations of six of the old globular clusters of the Large Magellanic Cloud. Treated as contaminants by the globular cluster analysts, we produce (V - I, V) color-magnitude diagrams (CMDs) of the field stars and use them to explore the LMC's star formation history. The photometry approaches V similar to 26, well below the turnoff of an ancient (similar to 14 Gyr) LMC population of stars. The held star CMDs are generally characterized by an upper main sequence broadened by stellar evolution, an old red giant branch, a prominent red clump, and an unevolved lower main sequence. The CMDs also contain a few visual differences, the most obvious of which is the smeared appearance of the NGC 1916 field caused by heavy differential reddening. More subtly, the base of the subgiant branch near the old turnoff appears extended in V, and the red giant branch appears broad in V - I in four of the fields but not in the NGC 1754 field. We use a maximum likelihood technique to fit model CMDs drawn from Bertelli et al. isochrones to the observed CMDs. We constrain the models by the age-metallicity relation derived from LMC clusters, test four initial mass function (IMF) slopes, and fit for the reddening, distance modulus, and star formation rate. We find that we can just resolve structure in SFR(t) with time steps of similar to 0.15 in log(age), implying a resolution of similar to 4 x 10(8) yr at an age of 1 Gyr. For a Salpeter IMF, our derived star formation history for the NGC 1754 field is characterized by an enhanced star formation rate over the past 4 Gyr, qualitatively resembling that derived by others for a variety of LMC fields. The remaining four fields, however, appear to have had high levels of star formation activity as long as 5-8 Gyr ago; these fields lie in the LMC bar whereas the NGC 1754 held lies in the disk, suggesting that the inner regions of the LMC contain significantly more older stars than the outer regions. Examining the residuals of the models and observations, we find that the old red giant branches of the models provide a poor fit to the observations, which suggests an error in the model isochrones. The effect of the disagreement appears to be to underestimate the contribution of the old population.