Photometric constraints on microlens spectroscopy of EROS-BLG-2000-5

We apply EROS photometric data to interpret previously published Keck and VLT spectra of the binary-microlens caustic-crossing event EROS-BLG-2000-5. We show that the VLT data imply that the outer ${\sim} 4\%$ of the limb of the K3-giant source is strongly in emission in H$\alpha$, in contradiction to available models of the photosphere. This conflict could be resolved if the integrated H$\alpha$ emission from the chromosphere were equal to 8% of the integrated H$\alpha$ absorption from the source as a whole. These conclusions regarding the extreme limb are almost completely model-independent. We also present a general method for using the photometric data to facilitate direct comparison between the atmospheric model and the spectroscopic data. While this method has some model-dependent features, it is fairly robust and can serve to guide the analysis of spectra while more detailed models of the lens geometry are being developed. In addition, we find that the color of the limb of the source (outer 5.5% by radius) is $\Delta(V-I)\sim 0.37$ redder than the source as a whole, so that it has the color of a M0 giant.