The Analysis of Isochrone Fitting Methods for Red Giant Branch Photometry, and Tip Red Giant Branch Distance Determination

We consider the procedure of isochrone fitting and its application to the study of red giant branch (RGB) photometry in old stellar populations. This is extended to consider the problems introduced by the inclusion of lower-magnitude regions of the Colour Magnitude Diagram (CMD). We refer especially to our previous paper Frayn & Gilmore (2002), where the details of our isochrone interpolation and fitting code are explained. We address the systematic errors inherent in the process of isochrone fitting, and investigate the extent to which simple stellar populations can be recovered from noisy photometric data. We investigate the effects caused by inaccurate distance estimates, isochrone model variation and photometric errors. We present results from two studies of approximately coeval stellar populations, those of the Milky Way Globular Cluster System (MWGCS), and the Ursa Minor dwarf spheroidal galaxy. In addition, we introduce a new method for estimating distances using photometry of the tip of the RGB which is significantly more robust than the standard edge-detection filter.