  CONTEXT: High-precision stellar analyses require hydrodynamic modeling to
interpret chemical abundances or oscillation modes. Exoplanet atmosphere
studies require stellar background spectra to be known along the transit path
while detection of Earth analogs require stellar microvariability to be
understood. Hydrodynamic 3-D models can be computed for widely different stars
but have been tested in detail only for the Sun with its resolved surface
features. Model predictions include spectral line shapes, asymmetries, and
wavelength shifts, and their center-to-limb changes across stellar disks. AIMS:
To observe high-resolution spectral line profiles across spatially highly
resolved stellar surfaces, which are free from the effects of spatial smearing
and rotational broadening present in full-disk spectra, enabling comparisons to
synthetic profiles from 3-D models. METHODS: During exoplanet transits,
successive stellar surface portions become hidden and differential spectroscopy
between various transit phases provides spectra of small surface segments
temporarily hidden behind the planet. Planets cover no more than about 1% of
any main-sequence star, enabling high spatial resolution but demanding very
precise observations. Realistically measurable quantities are identified
through simulated observations of synthetic spectral lines. RESULTS: In normal
stars, line profile ratios between various transit phases may vary by some
0.5%, requiring S/N ratios of 5,000 or more for meaningful spectral
reconstruction. While not yet realistic for individual spectral lines, this is
achievable for cool stars by averaging over numerous lines with similar
parameters. CONCLUSIONS: For bright host stars of large transiting planets,
spatially resolved spectroscopy is currently practical. More observable targets
are likely to be found in the near future by ongoing photometric searches.
