The Ultraviolet Spectrum of the High-Field Magnetic Cataclysmic Variable AR Ursae Majoris

Abstract

We have obtained an ultraviolet spectrum covering the range 917-1182 Å of the high-field magnetic cataclysmic variable AR Ursae Majoris when the binary was in its typical low-accretion state. The data reveal an array of deep Zeeman absorption features due to hydrogen Lyα-Lyγ, including several transitions that are normally forbidden but become enabled in the strong electric fields that exist in the atmospheres of highly magnetic white dwarfs. Spectral models indicate a magnetic field strength and structure consistent with that determined from longer wavelength Hubble Space Telescope Space Telescope Imaging Spectrograph UV observations; that is, a dipole of strength ~235 MG offset along its axis by ~0.15 of the stellar radius.

This research is based on observations with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer, which is operated for NASA by Johns Hopkins University under NASA contract NAS 5-32985, and on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysics Research Consortium.