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CONTEXT: High-precision stellar analyses require hydrodynamic modeling tointerpret chemical abundances or oscillation modes. Exoplanet atmospherestudies require stellar background spectra to be known along the transit pathwhile detection of Earth analogs require stellar microvariability to beunderstood. Hydrodynamic 3-D models can be computed for widely different starsbut have been tested in detail only for the Sun with its resolved surfacefeatures. Model predictions include spectral line shapes, asymmetries, andwavelength shifts, and their center-to-limb changes across stellar disks. AIMS:To observe high-resolution spectral line profiles across spatially highlyresolved stellar surfaces, which are free from the effects of spatial smearingand rotational broadening present in full-disk spectra, enabling comparisons tosynthetic profiles from 3-D models. METHODS: During exoplanet transits,successive stellar surface portions become hidden and differential spectroscopybetween various transit phases provides spectra of small surface segmentstemporarily hidden behind the planet. Planets cover no more than about 1% ofany main-sequence star, enabling high spatial resolution but demanding veryprecise observations. Realistically measurable quantities are identifiedthrough simulated observations of synthetic spectral lines. RESULTS: In normalstars, line profile ratios between various transit phases may vary by some0.5%, requiring S/N ratios of 5,000 or more for meaningful spectralreconstruction. While not yet realistic for individual spectral lines, this isachievable for cool stars by averaging over numerous lines with similarparameters. CONCLUSIONS: For bright host stars of large transiting planets,spatially resolved spectroscopy is currently practical. More observable targetsare likely to be found in the near future by ongoing photometric searches.

Spatially resolved spectroscopy across stellar surfaces. I. Using exoplanet transits to analyze 3-D stellar atmospheres