AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 53. Star Clusters Near and Far, Old and Young
Display, Tuesday, June 1, 1999, 10:00am-7:00pm, Southeast Exhibit Hall

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[53.07] Fundamental Properties of Star Clusters: A Fresh Look at ``Fitting'' Stellar Models to Color-Magnitude Diagrams

K.A. Janes, C.J. Law (Boston Univ.)

Star cluster color-magnitude diagrams often include photometry for a thousand or more stars. In principal, a tremendous amount of information should be available for deriving cluster parameters (distance, reddening, composition, age, etc.) or for investigating the quality of the fit between a given set of models and the observed clusters. This paper takes a fresh look at the process of comparing the models to clusters or one cluster to another. A procedure is described to replace the conventional ``eyeball fit'' of isochrones to the data with a statistical analysis that makes use of all the data in a systematic fashion. The analysis consists of calculating the magnitude and color differences between a target star in one cluster and a reference stellar model or a star in another cluster. If the target star and the reference star or model are similar in their properties, then the color and magnitude differences can be expressed as a set of simple difference equations in the fundamental stellar properties. These can be solved in the least squares sense to yield the cluster parameters. Furthermore (and perhaps more importantly) the method allows for a rigorous propagation of both observational and theoretical sources of uncertainty through the procedure, to produce meaningful error estimates. A series of sample analyses will illustrate the potential of the procedure. This work has been supported in part by NSF grant AST-9731656.

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