We hope we have made a step forward in the 1D-modelling of the photosphere. Our new photospheric model, slightly cooler than the widely used, for almost three decades, Holweger & Müller (1974) model, does reconcile results based on low and high excitation Fe Ilines without changing the result obtained from Fe IIlines. It allows to come to an end with the debate concerning the high versus low solar abundance of iron: the solar photospheric abundance of iron is low, = 7.50 0.05; it agrees with the very accurate meteoritic value .
The next step has however to abandon the 1D hypothesis in order to describe more realistically the photospheric structure. Actually, we do believe that the Holweger & Müller type photospheric models might possibly be slightly different if one wants to reproduce the behaviour of different tracers of the temperature, like infrared CO lines rather than Fe I lines. Each indicator of the temperature in the outer photospheric layers might lead to a slightly different homogeneous model because these different indicators have their own sensitivities to inhomogeneities and will react in a different way to the actual heterogeneous structure of the photospheric layers.
We know that the photospheric layers, just above the convection zone, are very heterogeneous and that matter motions extend very high through the photosphere up to about 1000 km above the top of the convection zone. Line shifts and asymmetries of profiles of unblended lines resulting from these matter motions are observed even on infrared CO lines (Grevesse & Sauval 1991; Blomme et al. 1994; Grevesse & Sauval 1994) which are formed very high into the so-called COmosphere (Ayres 1998). We do believe that we are now really facing the limitations of one-component model atmospheres and have now to turn to more realistic modelling (see e.g. Solanki 1998; Rutten 1998). But the effects on the solar iron abundance are still difficult to estimate (Holweger 1988, 1996; Holweger et al. 1990; Anstee et al. 1997; Gadun & Pavlenko 1997).
A last general comment has to be made concerning transition probabilities. Iron is by far the dominant species in the solar visible spectrum; this is true as well for a large number of stellar spectra. In each angström of the solar spectrum, we find an iron line. At the end of this Millennium, it is very disappointing to realize that accurate transition probabilities are known for only a minority of these lines .
© European Southern Observatory (ESO) 1999
Online publication: June 18, 1999