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Determination of T$_{\rm eff}$ and $\log g$ in non standard atmosphere

In the attempt to study the chemical composition of stars, a very important task is the modeling of their atmosphere. A good model is a fundamental starting point for a detailed quantitative analysis of the chemical composition. This importance is stressed in the case of CP stars, for which the overabundances of metals change the structure of their atmospheres.

The most reliable codes used at the moment for such purposes are the Kurucz's ATLAS and SYNTHE. These programs have been extensively and successfully used to model optical spectra of stars whose spectral types lie between B and A.

Non evolved early-type stars play a key role in many aspects of modern astrophysics. Since their radiative atmospheres are particularly stable, their surface abundances reflect those of the formation regions from which they originated. Hence, a correct determination of these abundances is the primary task to fulfil in any study, from the chemical gradient in the Milky-Way to the evolution of external galaxies.

One of the most accurate methods commonly used to infer $T_{\rm eff}$ and $\log g$ is based on the comparison of the observed and theoretical profiles of Balmer lines. Several studies have shown that a non-standard chemical composition of the stellar atmosphere alters the flux distribution of the star or modifies the profiles of the Balmer lines.

Catanzaro, Leone and Dall () investigated how an extremely non standard atmosphere can influence the determination of the fundamental parameters. With this aim they extracted HR6000 from a sample of high resolution spectroscopic observations collected by means of FEROS@ESO.

Fig. 1.18 shows the importance of helium and metal abundances when we try to determine $T_{\rm eff}$ and $\log g$ matching balmer line profiles. Neglecting the correct He and metal abundances, two models, 400 K and 0.07 dex different in $T_{\rm eff}$ and gravity, are necessary.

After that, the authors studied the chemical pattern of this object and compared their results with those obtained by other authors both for the same star and for two other stars. The principal conclusion was that HR6000 seems to combine a number of abundance anomalies that usually appear separately in various classes of CP stars. It is very challenging to explain the presence of those anomalies all togheter in the same star, expecially if we consider its likely very young age. The next step toward the understanding of the real nature of this object will be the combined analysis of the UV and optical spectrum, in order to verify the possible stratification of the elements in the atmosphere.

Figure 1.18: Comparison between the observed and computed H$_{\delta}$ and H$_{\gamma}$ lines. Each profile has been calculated using models with different ODF tables.
\begin{figure*}\centerline{\hbox{
\psfig{file=stars/HR6000_hd.ps,width=8cm}
\psfig{file=stars/HR6000_hg.ps,width=8cm}}}\end{figure*}


next up previous contents index
Next: HgMn in binary systems Up: Chemical Peculiar stars of Previous: Magnetic Field   Contents   Index
Innocenza Busa' 2005-11-14