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

Abstract

We obtained three consecutive Hubble Space Telescope 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 C III (1175 Å), Lyα (1216 Å), N IV (1238 and 1242 Å), S II (1260-1565 Å), S III (1300 Å), C II (1335 Å), S IV (1394 and 1402 Å), and C IV (1548 and 1550 Å). 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 Space Telescope Imaging Spectrograph spectra. Using state-of-the-art accretion disk models by Wade & Hubeny, we have determined for all three spectra accretion rates for two white dwarf masses, 0.55 and 0.8 M_solar. For both white dwarf masses, the accretion rate during superoutburst decreased by a factor of 2 from early to late in the superoutburst. The average accretion rate during superoutburst is (3-6) × 10 ^-9 M_solar yr^-1, depending on the white dwarf mass.

Based on observations made with the NASA-CNES-CSA Hubble Space Telescope. HST is operated for NASA by AURA under NASA contract NAS 5-32985.