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Astron. Astrophys. 320, 469-477 (1997)


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An accretion column with a central hole in DQ Herculis binaries

J.B.G. Canalle 1 and R. Opher 2

1 Instituto de Física, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier, 524, sala 3002-D, CEP 20550-013, Rio de Janeiro-RJ, Brazil (e-mail: canalle@vmesa.uerj.br)
2 Instituto Astronômico e Geofísico, Universidade de São Paulo, Av. Miguel Stéfano, 4200, Água Funda, CEP 04301-904, São Paulo - SP, Brazil (e-mail: opher@vax.iagusp.usp.br)

Received 21 March 1996 / Accepted 30 August 1996

Abstract

We present a model for the accretion column of DQ Herculis (DQ Her) binary 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 white dwarf) and the perpendicular and parallel variations of the density and temperature of the accretion material, we examined the emitted circular polarization as a function of the wavelength in the range of 3000-16000 Å. An extensive study of the influence of the free parameters of the model is made. It is evident with this model that the temperature and density of the accretion column is not homogeneous, because in this case there is too much circular polarization in the optical range. We also compared models of accretion columns with and without 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 magnetic field along and perpendicular to the axis of the column. They obtained almost null circular polarization in the optical range and did not investigate in detail the IR (infrared region). We use the same model to investigate the emitted circular polarization in both the optical and 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 that the polar magnetic field of DQ Her system is low ([FORMULA] 4 MG), but no explicit comparison with the BG CMi observation was made. In this paper we suggest that the polar magnetic field of DQ Her system is [FORMULA] 20 MG. The results are compared with the observations of BG CMi (West et al 1987) and the fit is very good.

Key words: white dwarfs – accretion – polarization – novae, cataclysmic variables – stars: magnetic fields

Send offprint requests to: J.B.G.Canalle

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998

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