Measuring starspots on magnetically active stars with the VLTI

We present feasibility studies to directly image stellar su rface features, which are caused by magnetic activity, with the Very Large Telescope Interferometer (VLTI). We con e trate on late type magnetically active stars, for which t he distribution of starspots on the surface has been inferred f rom photometric and spectroscopic imaging analysis. The st udy of the surface spot evolution during consecutive rotation c ycles will allow first direct measurements (apart from the Su n) of differential rotation which is the central ingredient of ma gnetic dynamo processes. The VLTI will provide baselines of up to 200 m, and two scientific instruments for interferometric st udies at nearand mid-infrared wavelengths. Imaging capab ilities will be made possible by closure-phase techniques. We concl ude that a realistically modeled cool surface spot can be det ected on stars with angular diameters exceeding ∼2 mas using the VLTI with the first generation instrument AMBE R. The spot parameters can then be derived with reasonable accuracy. We discuss that the lack of knowledge of magnetically active st ars of the required angular size, especially in the southern hem isphere, is a current limitation for VLTI observations of th ese surface features.