Hubble Space Telescope Far Ultraviolet Spectroscopy of the Dwarf Nova VW Hyi in Superoutburst

Abstract

We obtained three consecutive HST spectroscopic observations of a single superoutburst of the dwarf nova VW Hydri. The spectra cover the beginning, middle, and end of the superoutburst. All of the spectra are dominated by strong absorption lines due to CIII (1175 \AA), Lyman alpha (1216 \AA), NIV (1238 \AA, 1242 \AA), SII (1260-65 \AA), SIII (1300 \AA), CII (1335 \AA), SIV (1394 \AA, 1402 \AA) and CIV (1548 \AA, 1550 \AA). We discuss the evolution of the far UV energy distribution and line structure during the superoutburst. We note the absence of any P Cygni line structure in the STIS spectra. Using state of the art accretion disk models by Wade and Hubeny, we have determined accretion rates for all three spectra, for two white dwarf masses, 0.55 M$_{\sun}$ and 0.8 M$-{\sun}$. For both white dwarf masses the accretion rate during superoutburst decreased by a factor of two from early to late in the superoutburst. The average accretion rate during superoutburst is $3 - 6 \times 10^{-9}$M$_{\sun}$/yr depending on the white dwarf mass.