Magnesium abundance in main sequence B-type and magnetic chemically peculiar stars

The abundance of magnesium for a sample of 19 main sequence B-type and 41 magnetic chemically peculiar stars has been derived by spectrum synthesis analysis of the Mgii 448.1 nm line under the LTE assumption. The logarithm of the average Mg abundance for the main sequence stars is log (N(Mgii)/N(Tot)) =−4.28±0.19. Comparing magnetic chemically peculiar (Cp) and main sequence stars with equal effective temperature and gravity, one finds that the magnesium abundance tends to be lower in peculiar stars with the exception of helium rich stars where this element can be overabundant. In Cp stars with effective temperature of about 14000 K, the magnesium abundance does not depend on gravity, microturbulent velocity or rotational period. There appears to exist a correlation between the magnesium abundance and the surface magnetic field, with the stars poorest in magnesium presenting the strongest magnetic fields. In accord with the theory of magnetically controlled diffusion – which predicts a non-homogeneous distribution of magnesium over the stellar surface and stratification in the photosphere – some peculiar stars show evidence of spectral variability with the rotational phase; the respective magnesium abundances of the HgMn stars HD49606 and HD78316 depend on optical depth.