A Self-occulting Accretion Disk in the SW Sextantis Star DW Ursae Majoris

Abstract

We present the ultraviolet spectrum of the SW Sextantis star and nova-like variable DW Ursae Majoris in an optical low state, as observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). The data are well described by a synthetic white dwarf (WD) spectrum with T_eff=46,000+/-1000 K, logg=7.60+/-0.15, vsini=370+/-100 km s^-1, and Z/Z_solar=0.47+/-0.15. For this combination of T_eff and logg, WD models predict M_WD=0.48+/-0.06 M_solar and R_WD=(1.27+/-0.18)×10⁹ cm. Combining the radius estimate with the normalization of the spectral fit, we obtain a distance estimate of d=830+/-150 pc. During our observations, DW UMa was approximately 3 mag fainter in V than in the high state. A comparison of our low-state HST spectrum with a high-state spectrum obtained with the International Ultraviolet Explorer shows that the former is much bluer and has a higher continuum level shortward of 1450 Å. Since DW UMa is an eclipsing system, this suggests that an optically thick accretion disk rim blocks our view of the WD primary in the high state. If self-occulting accretion disks are common among the SW Sex stars, we can account for (1) the preference for high-inclination systems within the class and (2) their V-shaped continuum eclipses. Moreover, even though the emission lines produced by a self-obscured disk are generally still double-peaked, they are weaker and narrower than those produced by an unobscured disk. This may allow a secondary line emission mechanism to dominate and produce the single-peaked, optical lines that are a distinguishing characteristic of the SW Sex stars. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.