In the morning I had discussions with Gennaro (MPIA), Brandner (MPIA), and Stolte (Bonn) about their work on fitting the IMF in Milky Way young star cluster Westerlund 1. Dan Weisz and I were getting different results from them and I was interested in finding out why; there are significant methodological differences, but the same data. I think I see a path to resolving the discrepancies, but it depends on the method that Gennaro et al used to estimate their completeness as a function of position and magnitude.
In the afternoon, Lang and I worked on objects that don't match well between PHAT epochs. These are all likely to be either foreground stars with significant proper motions or else variable objects. By fitting out high-order distortions in the HST ACS camera, we can get few-milliarcsec per-source position uncertainties, so we are pretty sensitive even with PHAT's short internal time baseline.
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