Far Ultraviolet Spectroscopic Explorer Observations of the Dwarf Nova Sw Ursae Majoris during Quiescence

We present spectroscopic observations of the short-period cataclysmic variable SW UMa obtained by the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite while the system was in quiescence. The data include the resonance lines of O vi at 1031.91 and 1037.61 8. These lines are present in emission, and they exhibit both narrow ( 150 km s ) and broad ( 2000 km s ) components. The narrow O vi emission lines exhibit unusual double-peaked and redshifted profiles. We attribute the source of this emission to a cooling flow onto the surface of the white dwarf primary. The broad O vi emission most likely originates in a thin, photoionized surface layer on the accretion disk. We searched for emission from H2 at 1050 and 1100 8, motivated by the expectation that the bulk of the quiescent accretion disk is in the form of cool, molecular gas. If H2 is present, then our limits on the fluxes of the H2 lines are consistent with the presence of a surface layer of atomic H that shields the interior of the disk. These results may indicate that accretion operates primarily in the surface layers of the disk in SW UMa. We also investigate the far-UV continuum of SW UMa and place an upper limit of 15,000 K on the effective temperature of the white dwarf. Subject headinggs: binaries: close — novae, cataclysmic variables — stars: individual (SW Ursae Majoris) — ultraviolet: stars