The ultraviolet spectrum of the high-field magnetic cataclysmic variable AR Ursae Majoris

摘要

We have obtained an ultraviolet spectrum covering the range 917 - 1182 Angstrom 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 Lyalpha-Lygamma, 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 similar to 235 MG offset along its axis by similar to 0.15 of the stellar radius.