Interpreting the Spectra of Chemically Peculiar Stars

Similarly to many astrophysical studies, a spectroscopic analysis of chemically peculiar (CP) stars requires extensive, accurate sets of atomic data. Our purpose is to show here that the CP stars can also serve as a laboratory for checking the accuracy of the atomic data. Despite additional challenges toward modeling the atmospheres of the CP stars, they have a definite advantage as laboratories for atomic physics: their strong chemical anomalies make some spectral features especially prominent in their spectra, which can be better checked against theoretical predictions. This statement may be applied to both to stars exhibiting large over- or under- abundances. We discuss several examples how CP stars can be used along this idea.We will focus here on the chemically peculiar stars of the upper main-sequence, which encompass the classical Am and Ap stars. Chemical anomalies are widespread on the main sequence for spectral types B. to F. These stars mostly show weak helium lines, and abnormally strong lines from iron-peak and heavier (e.g. the lanthanides) elements. A classical characteristics of Am stars in their weak Ca lines, while Ap stars may exhibit either strong Si or Hg lines. CP stars can be grouped according to the most obvious line strength anomalies, which are related to the effective temperature and to the presence or the absence of a magnetic field. In magnetic CP stars, the chemical elements are not uniformly distributed over the stellar surface, but they seem to be concentrated (or depleted) in rings or polar caps following the magnetic geometry. Magnetic CP stars have also broad absorption features in their ultraviolet and visible spectra that can used photometric peculiarity criteria. Using the new extensive photoionization data from the Opacity Project (OP), the UV absorption features can be identified and the UV spectrum can be well reproduced.