Rare-earth systematics in Ap and Am stellar spectra: Are new theoretical developments required?

Abstract

The very large overabundances of rare earth elements found in Ap stars (>103 - 105) usually arise in cases where the observable second spectra does not represent the dominant ionization stage. The extreme overabundance factors are highly correlated with effective temperature in a way that is regarded with suspicion. In cool Ap stars, where one does observe the dominant ionization state, more modest overabundances (~ 102) occur. The typical cool Ap and Am stars must have abundances of the lighter lanthanides. It is argued that the sharp rise in typical Ap star abundances that parallels the predicted degree of ionization may be due to a breakdown in the Saha Equation or possibly to the existence of a cool upper photosphere caused by line blanketing in the hotter Ap stars.Attention is called to the observational fact that the cool Am stars have deeper line cores than chemically normal main sequence stars or cool Ap stars. These line cores resemble those of more luminous stars, a fact which comports well with some (though not all) low dispersion similarities between Am stars and evolved objects. While various studies (e.g. by Conti and Van't Veer Menneret) have shown that reduced gravity models cannot remove the abundance anomalies in the Am stars, some modification of the traditional models and/or line formation theory is indicated in order to explain the observations.