For testing 3-dimensional models of stellar atmospheres, spectroscopy across spatially resolved stellar surfaces would be desired with a spectral resolution of R=100,000 or more. Hydrodynamic models predict variations in line profile shapes, strengths, wavelength positions and asymmetries. These variations vary systematically between disk center and limb and as a function of line strength, excitation potential and wavelength region. However, except for a few supergiants and the Sun, current telescopes are not yet capable of resolving any stellar surfaces. One alternative method to resolve distant stellar surfaces, feasible already now, is differential spectroscopy of transiting exoplanet systems. By subtracting in-transit spectra from the spectrum outside of transit, the spectra from stellar surface portions temporarily hidden behind the planet can be disentangled. Since transiting planets cover only a small portion of the stellar surface, the method requires a very high signal-to-noise ratio, obtainable by averaging numerous similar spectral lines. We apply such differential spectroscopy on the 7.7 mag K1V star HD 189733 (‘Alopex’*); its transiting planet covers ∼ 3% of its host star’s surface, which is the deepest known transit among the brighter systems. Archival data from the ESO HARPS spectrometer are used to construct averaged profiles of photospheric Fe I lines, with the aim of comparing spatially resolved profiles to analogous synthetic line profiles computed from the 3-dimensional hydrodynamic CO5BOLD model.
* We refer to HD 189733 as ‘Alopex’ (from the Greek ‘αλεπού’), denoting a fox related to the one that gave name to its constellation of Vulpecula
