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Abstract.We present an empirical model-atmosphere investigation of missing Fe I opacity. Houdashelt et al. (2000) estimated that if Dragon & Mutschlecner (1980) Fe I cross sections used in the MARCS model atmospheres (Gustafsson et al. 1975) were replaced by the Bautista (1997) cross sections the solar continuous flux would be reduced by 15% in the near ultraviolet. That would imply systematic errors in models for F, G, and K stars. As a consequence, since ATLAS9 (Kurucz 1993a) uses an approximation to the same Dragon & Mutschlecner (1980) opacities, there should also be similar systematic errors in ATLAS9 models that required this investigation. Bound-free Fe I cross sections computed by Bautista (1997) in the framework of the IRON Project were used to generate the continuous Fe I absorption coefficient. It was incorporated in the Kurucz (1993a) ATLAS9 code, in place of that currently used, which is based on approximate cross sections by Kurucz. By combining Opacity Distribution Functions (ODFs) computed without the contribution of Fe I autoionization lines with the new Fe I absorption coefficient which is crowded with autoionization resonances, we obtained solar metallicity model atmospheres and energy distributions for several combinations of T eff and log g. The comparison of these models with the standard ATLAS9 models has shown that there are no differences in the T -τ Ross relations, while there are some changes in the energy distributions for T eff ≤ 7000 K, but limited to small wavelength regions around 2150 Å, where Kurucz has less opacity, and 3350 Å, where Bautista has less opacity. The differences are of the order of 25% and less than 10%, respectively. That around 2150 Å disappears for T eff ≤ 5500 K owing to the fall of the emergent flux at these wavelengths in cool stars. This behaviour is independent of the gravity. The explanation is that our line list actually has more autoionizing opacity than Bautista's but it is treated as bound-bound line opacity rather than as bound-free opacity.

Vienna atomic line database (VALD) is a collection of critically evaluated laboratory parameters for individual atomic transitions, complemented by theoretical calculations. VALD is actively used by astronomers for stellar spectroscopic studies-model atmosphere calculations, atmospheric parameter determinations, abundance analysis etc. The two first VALD releases contained parameters for atomic transitions only. In a major upgrade of VALD-VALD3, publically available from spring 2014, atomic data was complemented with parameters of molecular lines. The diatomic molecules C 2 , CH, CN, CO, OH, MgH, SiH, TiO are now included. For each transition VALD provides species name, wavelength, energy, quantum number J and Landé-factor of the lower and upper levels, radiative, Stark and van der Waals damping factors and a full description of electronic configurarion and term information of both levels. Compared to the previous versions we have revised and verify all of the existing data and added new measurements and calculations for transitions in the range between 20 Å and 200 microns. All transitions were complemented with term designations in a consistent way and electron configurations when available. All data were checked for consistency: listed wavelength versus Ritz, selection rules etc. A new bibliographic system keeps track of literature references for each parameter in a given transition throughout the merging process so that every selected data entry can be traced to the original source. The query language and the extraction tools can now handle various units, vacuum and air wavelengths. In the upgrade process we had an intensive interaction with data producers, which was very helpful for improving the quality of the VALD content.

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We present semiempirical model atmospheres for the darkest parts of large sunspot umbrae, regions we call umbral cores. Our approach is based on general-purpose computational procedures that are applicable to different types of stellar atmospheres. We show that recent umbral intensity measurements of the spectral energy distribution may be accounted for by an umbral core atmospheric model that varies with time during the solar cycle; the observed center-limb variation can be accounted for by the properties of the model. Three umbral core models are presented, corresponding to the early, middle, and late phases of the solar cycle. These three models also may be regarded as having the properties of dark, average, and bright umbral cores respectively. The effects of atomic, opacity, and abundance data uncertainties on the model calculations are briefly discussed. We also give for comparison a new reference model for the average quiet solar photosphere.

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