The FeH F–X system
Creating a valuable diagnostic tool to explore solar and stellar magnetic fields
Institute of Astronomy, ETH Zurich, 8092 Zurich, Switzerland e-mail: firstname.lastname@example.org
2 Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
3 Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany
Accepted: 16 January 2008
Context. Lines of diatomic molecules are ideal tools for studying cool stellar atmospheres and the internal structure of sunspots and starspots, given their temperature and pressure sensitivities, which are typically higher than in atomic lines. The Wing-Ford FeH F–X system represents such a diatomic molecule that is, in addition, highly sensitive to magnetic fields. The current theoretical description of those transitions that include the involved molecular constants, however, are only based on intensity measurements because polarimetric observations have not been available until now, which limits their diagnostic value. Furthermore, the theory has so far been optimized to reproduce energy levels and line strengths without taking magnetic sensitivities into account.
Aims. The FeH F–X system is produced by transitions between two electronic states with the coupling of the angular momenta that is intermediate between limiting Hund's cases (a) and (b). Our goal is to investigate the diagnostic capabilities of the current theoretical description of the molecule FeH.
Methods. Using the most precise available Hamiltonian, we carried out the perturbation calculation of the molecular Zeeman effect for this transition and computed the Landé factors of the energy levels and of transitions. We extracted Landé factors from a comparison of observed and calculated Stokes I and V profiles. Certain spectral lines, most frequently with high magnetic sensitivity, exhibited discrepancies between the theory and observations. We extended the theoretical model with a semi-empirical approach to obtain a diagnostic tool that is able to reproduce many of the interesting spectral lines.
Results. We find that the current theory successfully reproduces the magnetic properties of a large number of lines in the FeH F–X system and that the modified Hamiltonian allows us to synthesize and successfully reproduce the most sensitive lines. Thus, our observations have provided valuable constraints for determining empirical molecular constants and Landé factors.
Conclusions. The FeH F–X system is found to be a very sensitive magnetic diagnostic tool. Polarimetric data of these lines, in contrast to intensity measurements, provide us with more direct and detailed information to study the coolest parts of sunspot and starspot umbrae, as well as cool active dwarfs.
Key words: molecular processes / Sun: magnetic fields / polarization / radiative transfer / line: formation / stars: magnetic fields
© ESO, 2008