Spectropolarimetric measurements of the mean longitudinal magnetic field of chemically peculiar stars - On the light, spectral and magnetic variability

We have equipped the spectrograph of the Catania Astrophysical Observatory with a polarimetric module which gives simultaneous circularly right and le ft polarised radiation spectra. This facility has been used to perform time-resolved spectropolarimetric (S tokes V) measurements of the mean longitudinal (effective) magnetic field f or seven chemically peculiar stars. Since this class of stars is characteri sed by a periodically variable magnetic field, the monitoring of the Stokes V parameter is a fundamental step to recover the magnetic field topology. To better define the variation of the effective magnetic field, we have com bined our observations with data from the literature. Variability periods g iven in the literature have been verified using Hipparcos photometric data and, if necessary, we have re determined them. From Hipparcos absolute magnitudes, we have determined the stellar radii an d then, on the hypothesis of a rigid rotator, the inclination of the rotati onal axes with respect to the line of sight. On the hypothesis that the mag netic field presents a dominant dipolar component (that is, where the Stoke s Q and U parameters are not necessary to recover the magnetic configuratio n) we have determined the angle between the rotational and dipole axes and the polar strength of the magnetic field. Chemically peculiar stars show periodic anti-phase light variations short-w ard and long-ward of a constant wavelength, the null wavelength. We have pe rformed numerical computations of the expected flux distribution for metal- enhanced atmospheres with different effective temperature and gravity. From the behaviour of the null wavelength, we confirm the importance of the non -homogeneous distribution of elements on the stellar surface as origin of t he light variability. However to explain the photometric variability of som e stars, we suggest that the flux distribution is also influenced by the co ntribution of the magnetic field to the hydrostatic equilibrium.