Is missing Fe I opacity in stellar atmospheres a significant problem?

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 Teff and log g. The comparison of these models with the standard ATLAS9 models has shown that there are no differences in the T-τRoss relations, while there are some changes in the energy distributions for Teff ≤ 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 Teff ≤ 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.