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Abstract. Recent advances in the computation of the Zeeman splitting of molecular lines have paved the way for their use as diagnostics of solar and stellar magnetic fields. A systematic study of their diagnostic capabilities had not been carried out so far, however. Here we investigate how molecular lines can be used to deduce the magnetic and thermal structure of sunspots, starspots and cool stars. First, we briefly describe the Stokes radiative transfer of Zeeman-split molecular lines. Then, we compute Stokes spectra of TiO, OH, CH and FeH lines and investigate their diagnostic capabilities. We also compare the synthetic profiles with observations. Spectra of TiO, OH and FeH are found to be interesting diagnostics of sunspot magnetic fields. This is also true for cool stars, where, however, the OH Stokes V profiles may require very high S/N data to be reliably employed. Finally we investigate the potential of various molecular bands for high-contrast imaging of the solar surface. The violet CN and CH bands turn out to be most promising for imaging the photosphere, the TiO bands are excellent for imaging sunspot umbrae, while the UV OH band can be used for imaging both the photosphere and sunspots.

Abstract. The magnetic, thermal and velocity structure of a regular sunspot, observed close to solar disk center is presented. Spectropolarimetric data obtained with the Tenerife Infrared Polarimeter (TIP) in two infrared Fe I lines at 15 648.5 Å and 15 652.8 Å are inverted employing a technique based on response functions to retrieve the atmospheric stratification at every point in the sunspot. In order to improve the results for the umbra, profiles of Zeeman split OH lines blending the Fe I 15 652.8 Å are also consistently fit. Thus we obtain maps of temperature, line-of-sight velocity, magnetic field strength, inclination, and azimuth, as a function of both location within the sunspot and height in the atmosphere. We present these maps for an optical depth range between log τ 5 = 0 and log τ 5 = −1.5, where these lines provide accurate results. We find decreasing magnetic field strength with increasing height all over the sunspot, with a particularly large vertical field gradient of ∼−4 G km −1 in the umbra. We also observe the so called "spine" structures in the penumbra, i.e. extended radial features with a stronger and more vertical magnetic field than the surroundings. Also we found that the magnetic field zenith angle increases with height. From the velocity map it is clear that the Evershed flow avoids the spines and mostly concentrates in the more inclined intervening field. The field inclination at a few locations in the outer penumbra in lower layers goes beyond 90• . These locations coincide with the strongest flows in the velocity map.

Properties of stellar granulation are obtained by inverting spectra of the late-type stars α Centauri A and B. Our inversions are based on a multi-component model of the stellar photosphere and take into account the center-to-limb variation and rotational broadening. The different atmospheric components describe the areas harboring up-, down-and horizontal flows. The inversions are constrained by fitting not only the flux profiles, but also their line bisectors, and by using a simple mass conservation scheme. The inversions return the properties of convection at the stellar surface, including the stratification of the thermodynamic parameters, as well as fundamental parameters such as the gravitational acceleration, v sin i and the element abundances. For α Cen A (G2V) the derived stratifications of the temperature and convective velocity are very similar to the Sun, while for α Cen B (K1V) we find similar up-and downflow velocities, but lower horizontal speeds and a reduced overshoot. The latter is consistent with the smaller scale height of the atmosphere, while mass conservation arguments taken with the lower horizontal speed imply that the granules on α Cen B are smaller than on the Sun. Both these properties are in good agreement with the hydrodynamic simulation of Nordlund & Dravins (1990, A&A, 228, 155). The inversions also return the fundamental parameters (T eff , log g, abundances, v sin i, etc.) of the two stars. These values are on the whole in good agreement with literature values. Also, most of them do not strongly depend on the details of the inversion. However, importantly, the element abundances are 0.1 to 0.15 dex lower when a 2-or 3-component inversion is carried out than with a 1-component inversion.

Abstract.A powerful method for the analysis of the structure of small scale magnetic elements in the solar photosphere is the inversion of Stokes spectra. In previous papers based on such inversions Bellot Rubio et al. (1997Rubio et al. ( , 1999 and Frutiger et al. (1999) have argued in favor of models with rather different dynamic properties. In this paper we return to this debate and compare results returned by inversions based on new multi-component models applied to several Fe i, Fe ii and C i spectral line profiles obtained in active region plage with a Fourier Transform Spectrometer. These inversions differ from earlier ones by the fact that mass conservation is strictly imposed both inside the magnetic elements and on the surrounding external flow field. These flux-tube models are not only able to reproduce the characteristic Stokes V asymmetries and line-shifts observed in active regions plages or network elements, but also the Stokes I line profiles, including line bisectors. It is confirmed that from the quality of the fits alone it is not possible to distinguish between the steady flow proposed by Bellot Rubio et al. (1997) and the oscillatory model of Frutiger & Solanki (1998). If, however, physical constraints are imposed (e.g. mass conservation or that the flow retains the same direction over height in the flux tube) then the oscillatory model is found to be superior. In addition, the current investigation also provides the first inversion-based model of abnormal granulation.