AAS 207th Meeting, 8-12 January 2006
Session 70 Cataclysmic Variables and Novae
Poster, Tuesday, 9:20am-6:30pm, January 10, 2006, Exhibit Hall

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[70.15] The First Detection of a White Dwarf Primary in an AM CVn System: The 28 minute binary CP Eridani

E. M. Sion (Villanova Un.), J.-E. Solheim (Un. of Oslo), P. Szkody (Un. of Washington), B. T. Gaensicke (Un. of Warwick), S. B. Howell (NOAO)

The physical characteristics of the AM Canum Venaticorum binaries

are extreme. They have the shortest known orbital periods and closest separations of any binary, and along with the DB white dwarfs, have the purest helium chemical composition of any other known objects in the Galaxy. We report the first direct spectroscopic detection of the accreting white dwarf in these objects, the helium-rich white dwarf in a binary with a 28 minute orbital period, the system CP Eridani. We report the results of a synthetic spectral analysis of CP Eri obtained when the system was in quiescence. Our Hubble STIS FUV spectrum is best fitted by a nearly pure helium, hybrid white dwarf with Teff ~17,000K±1000K. We report the first surface gravity determination and chemical abundances for the white dwarf primary star in such a binary, as well as the first rotational velocity for the white dwarf in such a system. The implications of our analysis for evolutionary channels leading to the origin of AM CVn systems and for AM CVn binaries as supernova Type Ia progenitors are discussed.

This research was supported NASA/HST grant 8103.01 and by NSF grant AST05-07514. BTG was supported by a PPARC Advanced Fellowship

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