AN ACCRETION COLUMN WITH A CENTRAL HOLE IN DQ HERCULIS BINARIES

We present a model for the accretion column of DQ Herculis (DQ Her) bi nary systems supposing that there is a cylindrical central hole in the accretion column. Taking into account the variations of the magnetic field (considered as a dipole with its center at the center of the whi te dwarf) and the perpendicular and parallel variations of the density and temperature of the accretion material, we examined the emitted ci rcular polarization as a function of the wavelength in the range of 30 00-16000 Angstrom. An extensive study of the influence of the free par ameters of the model is made. It is evident with this model that the t emperature and density of the accretion column is not homogeneous, bec ause in this case there is too much circular polarization in the optic al range. We also compared models of accretion columns with and withou t a central hole. Comparison also is made with the observations of the DQ Her system BG CMi (West et al 1987). Canalle & Opher 1991 studied an accretion column with gradients of temperature, density and magneti c field along and perpendicular to the axis of the column. They obtain ed almost null circular polarization in the optical range and did not investigate in detail the TR (infrared region). We use the same model to investigate the emitted circular polarization in both the optical a nd IR regions, but considering that there exists a central hole inside the accretion column, as might be expected in the presence of a disk. Frank & Chanmugam 1990 used an accretion ring model and suggested tha t the polar magnetic field of DQ Her system is low (similar to 4 MG), but no explicit comparison with the BG CMi observation was made. In th is paper we suggest that the polar magnetic field of DQ Her system is similar to 20 MG. The results are compared with the observations of BG CMi (West ct al 1987) and the fit is very good.