Measuring starspot temperature from line-depth ratios

Abstract

We present and apply to VY Ari, IM Peg and HK Lac a new method to determine spot temperatures (T sp ) and areas (A rel ) from the analysis of simultaneous light curves and temperature modulations deduced from line-depth ratios. A spot model, developed by us, has been applied to light and temperature curves. Grids of solutions with comparable χ -square have been found for a wide range of spot temperatures. The behavior of the solution grids for temperature and light curves in the T sp –A rel plane is very different and a rather small and unique intersection area can be found. In our spot-model we used spectral energy distributions (SEDs) based on the Planck law and on model atmospheres to evaluate the flux ratio between spots and unspotted photosphere and we found higher spot temperatures with SEDs based on model atmospheres than on the Planck law.