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Session 27 - Variable Stars, Novae, & Supernovae.
Display session, Tuesday, June 10
South Main Hall,

[27.04] Evidence of a Thermonuclear Runaway and Proton Capture Material on a White Dwarf in a Dwarf Nova

E. M. Sion, F. H. Cheng (Villanova U.), W. M. Sparks (Los Alamos Nat. Lab.), P. Szkody (U. Washington), M. Huang (Villanova U.), I. Hubeny (NASA/GSFC)

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.

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