Evidence of a thermonuclear runaway and proton-capture material on a white dwarf in a dwarf nova

Edward M. Sion, F. H. Cueno, Warren M. Sparks, Paula Szkody, Min Huang, Ivan Hubeny

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44 Scopus citations


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 Mwd = 0.86-0.32+0.18 M (this is only the second gravitational redshift determined for a cataclysmic variable white dwarf), white dwarf radius Rwd = 6.5-1.5+3.1 × 108 cm, and gravity log g = 8.43-0.54+0.31. Our best-fitting synthetic spectra yield white dwarf effective temperature Td = 22,000 K and a rotational velocity Vrot = 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 26Al 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.

Original languageEnglish (US)
Pages (from-to)L17-L20
JournalAstrophysical Journal
Issue number1 PART II
StatePublished - 1997


  • Novae, cataclysmic variables
  • Stars: individual (VW Hydri)
  • White dwarfs

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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