Accretion in dipole magnetic fields: flow structure and X-ray emission of accreting white dwarfs

¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK e-mail: cjs2@mssl.ucl.ac.uk
² State University of Rio de Janeiro, Rua São Francisco Xavier, 524/3023-D, CEP 20559-900, Rio de Janeiro, RJ, Brazil
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 16 January 2005
Accepted: 12 March 2005

Abstract

Field-channelled accretion flows occur in a variety of astrophysical objects, including T Tauri stars, magnetic cataclysmic variables and X-ray pulsars. We consider a curvilinear coordinate system and derive a general hydrodynamic formulation for accretion onto stellar objects confined by a stellar dipole magnetic field. The hydrodynamic equations are solved to determine the velocity, density and temperature profiles of the flow. We use accreting magnetic white-dwarf stars as an illustrative example of astrophysical applications. Our calculations show that the compressional heating due to the field geometry is as important as radiative cooling and gravity in determining the structure of the post-shock flow in accreting white-dwarf stars. The generalisation of the formulation to accretion flows channelled by higher-order fields and the applications to other astrophysical systems are discussed.