Evidence of a Thermonuclear Runaway and Proton-Capture Material on a White Dwarf in a Dwarf Nova

We present Hubble Space Telescope Goddard High-Resolution Spectrograph G160M spectra of the white dwarf in VW Hydri, exposed during quiescence, 1 month after the end of a normal dwarf nova outburst. Our spectra, covering the wavelength interval 1236-1272 Å, were obtained at orbital phase 0.06-1.60; they reveal strong photospheric Si II λλ1260, 1265 absorption features and a previously unidentified broad feature centered around 1250 Å. This feature is due to a blend of phosphorus lines. From line-shift measurements we determine a gravitational redshift of 58 ± 33 km s -1 , yielding a white dwarf mass M wd = 0.86 + 0.18 −0.32 M ☉ (this is only the second gravitational redshift determined for a cataclysmic variable white dwarf), white dwarf radius R wd = 6.5 + 3.1 −1.5 × 10 8 cm, and gravity log g = 8.43 + 0.31 −0.54 . Our best-fitting synthetic spectra yield white dwarf effective temperature T wd = 22,000 K and a rotational velocity V rot = 400 km s -1 . The chemical abundances, in number relative to solar, are, for C, 0.5; N, 5.0; O, 2.0; Fe, 0.5; Si, 0.1; P, 900; and all other metals, 0.3. The abundance of phosphorus being 900 solar, coupled with the elevated aluminum abundance reported by Sion and coworkers, suggest nucleosynthetic production of these odd-numbered nuclei from proton capture on the even-numbered nuclei during a CNO thermonuclear runaway (TNR) on the white dwarf. It is clear that the white dwarf has undergone a runaway sometime in the past, the first such evidence of a TNR in a dwarf nova. A TNR on a slowly accreting 0.86 M ☉ white dwarf should produce a classical nova explosion. If our interpretation is correct, then we have found the first direct spectroscopic link between a dwarf nova and a classical nova by using the white dwarf surface chemical abundance. This is also the first direct evidence of proton capture-processed material in the atmosphere of a white dwarf. Nova explosions on more numerous, lower mass C-O white dwarfs may therefore account for some fraction of the short-lived radionuclide 26 Al in the Galaxy. This nuclide is observed from its Galactic gamma-ray line emission and is postulated to have an important role in the heating of small bodies in the solar system.