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 mixed stel lar population from its photometry. We perform a chi (2) minimization betwe en the observed photometric distribution and a model photometric distributi on, based on theoretical isochrones. The initial mass function, distance mo dulus interstellar reddening, binary fraction, and photometric errors are i ncorporated into the model, making it directly comparable to the data. The model is a linear combination of individual synthetic color-magnitude diagr ams (CMDs), each of which represents the predicted photometric distribution of a stellar population of a given age and metallicity. While the method i s similar to existing synthetic CMD algorithms, we describe several key imp rovements in our implementation. In particular, we focus on the derivation of accurate error estimates on the star formation history to enable compari sons between such histories, either from different objects or from differen t regions of a single object. We present extensive tests of the algorithm, using both simulated and actual photometric data. From a preliminary applic ation of the algorithm to a subregion of the Large Magellanic Cloud (LMC), we find that the that the lull in star formation observed among the LMC's c luster population between 3 and 8 Gyr ago is also present in the field popu lation. The method was designed with flexibility and generality in mind, an d we make the code available for use by the astronomical community.