A Method for Determining the Star Formation History of a Mixed Stellar Population

We present a method to determine the star formation history of a mixedstellar population from its photometry. We perform a chi-squared minimizationbetween the observed photometric distribution and a model photometricdistribution, based on theoretical isochrones. The initial mass function,distance modulus, interstellar reddening, binary fraction and photometricerrors are incorporated into the model, making it directly comparable to thedata. The model is a linear combination of individual synthetic color-magnitudediagrams (CMDs), each of which represents the predicted photometricdistribution of a stellar population of a given age and metallicity. We presentextensive tests of the algorithm, using both simulated and actual photometricdata. From a preliminary application of the algorithm to a subregion of theLarge Magellanic Cloud (LMC), we find that the that the lull in star formationobserved among the LMC's cluster population between 3 and 8 Gyr ago is alsopresent in the field population. The method was designed with flexibility andgenerality in mind, and we make the code available for use by the astronomicalcommunity.Comment: 58 pages, 23 figures; accepted to ApJ Supplement