We present results on the analysis of background field stars found in HST WFPC2 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 of the field stars and use them to explore the LMC ’ s star formation history .
The photometry approaches V \sim 26 , well below the turnoff of an ancient ( \sim 14 Gyr ) LMC population of stars .
The field 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 .
( 1994 ) isochrones to the observed CMDs .
We constrain the models by the age-metallicity relation derived from LMC clusters , test four 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 \sim 0.15 in log age , implying a resolution of \sim 4 \times 10 ^ { 8 } years 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 while the NGC 1754 field 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 .