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Session 27 - Variable Stars, Novae, & Supernovae.
Display session, Tuesday, June 10
South Main Hall,
We present Hubble Space Telescope GHRS G160M spectra of the white dwarf in VW Hydri, exposed during quiescence, one month after the end of a normal dwarf nova outburst. Our spectra reveal strong photospheric Si II 1260ÅÅabsorption features, and a previously unidentified broad feature centered around 1250ÅThis feature is due to a blend of phosphorus lines. From line-shift measurements we determine a gravitational redshift of 58 \pm 33 km s^-1 yielding a white dwarf mass, M = 0.86 (+0.18,-0.32) M_ødot, white dwarf radius R = 6.5 (+3.1,-1.5) \times 10^8 cm, and gravity log g = 8.43 (+0.31,-0.54). Our best fitting synthetic spectra yield white dwarf effective temperature T = 22,000 K, a rotational velocity v \sin i = 400 km s^-1. The chemical abundances in number relative to solar are: C = 0.5, N = 5.0, O = 2.0, Fe - 0.5, Si = 0.1, P = 900, and all other metals Z = 0.3. The abundance of phosphorus being 900 times 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_ødot 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.
We acknowledge with gratitude the support of this work by NASA through grant GO6084.01-95A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
