Photospheric Signatures of Stellar Activity

Differences in the photospheric structure of chromospherically active and inactive stars are studied by means of detailed line profile modeling of both echelle and infrared FTS spectra. Six stellar parameters (effective temperature, gravity, iron abundance, microturbulence, macroturbulence, and projected equatorial rotational velocity) are deduced from echelle spectra for a sample of 53 late-type stars. Small, but statistically significant, differences are seen in the parameters for active and inactive stars, probably due to the inappropriate use of a single family of scaled solar atmospheres, regardless of activity level. This interpretation is further supported by systematic differences in the model profile residuals as a function of residual intensity for active and inactive stars. This provides the clearest evidence to date for enhanced nonradiative heating in the photospheres of active stars. Infrared FTS spectra are also modeled to obtain the best measurement of magnetic field strength and covering fraction to date for an active main-sequence star. The fraction of the surface of epsilon Eri covered by magnetic fields is found to be a factor of three below the results of previous optical analyses, which were considerably less sensitive. Special topics also covered include procedures for reducing echelle spectra, a new technique for recovering spatially variable instrumental profiles in echelle spectrographs, a new method of efficiently integrating intensity profiles over the stellar surface, and optimal techniques for simultaneously determining all stellar parameters using line profile information from many lines.