1997A&A...318..429R

We present a careful analysis of 13 high-quality optical spectra of low to moderately active late-type dwarfs (G1-K5) aimed at determining their magnetic parameters. Among our sample only one star, ε Eri (spatially averaged field strength≃165±30G), exhibits the unambiguous signature of a magnetic field, a few are candidates and the remaining show no sign of a magnetic field in the observed spectra. Our analysis is based on an inversion of the spectra using detailed numerical solutions of the Unno-Rachkovsky equations, for multiple spectral lines at different positions on the stellar disk, and including magneto-optical effects. It gives results for ε Eri which are in good agreement with the detailed analysis of infrared spectra by Valenti et al. (1995ApJ...439..939V). However, the low value of the spatially averaged field strength of these recent analyses imply that most values of the magnetic flux determined previously for moderately active stars are probably too large, often by considerable amounts. We find that the magnetic flux can be reliably determined if considerable care is taken in the analysis, but the magnetic field strength and filling factor cannot be determined separately for moderately active stars with optical spectra of spectral resolution ≤10⁵ and S/N≤250. In the case of ε Eri we are able to constrain the temperature of the detected magnetic features, which we find to be similar to or hotter than the non-magnetic surroundings, providing the first direct evidence that the detected field is in the form of plages. We also find that if an inversion approach is used, which determines various line broadening parameters simultaneously in a self-consistent manner, the presence of a magnetic field is not as obvious as some previous analyses have suggested. In addition, we determine fundamental parameters of the stellar sample.