Accretion and magnetism - AM Herculies stars. Polars in the ROSAT era

Abstract

The accretion processes in Am Herculis binaries are reigned by the strong magnetic field of a mass-accreting white dwarf. In the first part of this article I describe the methods applicable to infer the strength and structure of the magnetic fields in these systems. These are based on direct measurements of Zeeman split Balmer lines and/or harmonic cyclotron emission. All well-studied systems show signs of complex field structures or at least deviations from the most simple model of a centered dipole. The observed range of B-values is 10 -70 MG. The evolution of the systems is responsible for this range being much narrower than that measured in single white dwarfs, 100 kG - 1000 MG (see Schmidt 1995, this volume). The following parts of this article are devoted to other processes intimately interwoven with the presence of magnetic fields. These are the accretion streams (instead of accretion disks), the different ways of energy release in the accretion spots and the synchronous rotation of both stars. Considerable progress was reached in various fields after the launch of ROSAT which detected a lot of new and some very remarkable systems and which made detailed observational studies of individual systems possible. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman