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Session 12 - Stellar Evolution - Theory.
Display session, Wednesday, January 07
Exhibit Hall,

[12.07] A Maximum Likelihood Multi-Isochrone Fitting Technique

M. Han (U. Wisconsin, NASA/GSFC, Hughes STX), J. G. Hoessel, J. S. Gallagher III (U. Wisconsin)

We present a fully quantitative and objective multi-isochrone fitting technique, which is able to simultaneously determine the distance, extinction and the star formation history of a single or multi-population stellar system from color-magnitude observations. The fitting process is constructed as a maximum likelihood problem and is solved using the Genetic Algorithms Global Optimization technique. Extensive simulations show that the fitting method is unbiased, self-consistent and remarkably accurate. With a typical dataset of two colors, our simulation shows that the method can recover distance and extinction (i.e., E(B-V)) within 0.05 and 0.01 mag, respectively; while the star formation history (expressed as a distribution of stars in the age-abundance space) can be determined within about ten percent with reasonable age-abundance resolutions. The performance of the method depends little on the presence of some familiar diagnostic features like the horizontal branch and the main-sequence turnoff in the dataset. It is also very robust against outliers. Application of this method in combination with the Padua stellar evolution tracks to a dataset of a stellar field in the Large Magellanic Cloud (LMC) yields a best fitting solution which shows two primary star formation periods from 10 to 5 Gyr and 2.5 Gyr to 300 Myr, respectively, with a clear trend that the later formed stars have progressively higher metallicities. The method also predicts a distance modulus of 18.50 \pm 0.05 mag, and a mean extinction of E(B-V) = 0.05 mag in the field.

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