Pulsations and Accretion Geometry in YY Draconis: A Study Based on Hubble Space Telescope Observations

We present 1 Å resolution fast ultraviolet spectroscopy of YY Dra, coordinated U, B, V, R, and I photometry, and Hα spectroscopy. The UV continuum is strongly pulsed: we found 16% semiamplitude pulses with period 264.7(1) s; there was no evidence for power at twice this period. The UBV pulses are in phase with the UV continuum pulsation. A color temperature of >~10⁵ K was found for the pulse spectrum, with a corresponding area <~0.5% of the white dwarf surface. We refine the white dwarf spin period, obtaining P_spin = 529.31 +/- 0.02 s. Optical pulses at 273(1) s were also detected; we attribute these to reprocessing in structures fixed in the orbital frame. Variations in the C IV line profile are apparent when the data are folded on P_spin. Faint, broad line wings extending to +/-3000 km s^-1 appear simultaneously with the continuum pulsation maxima. This implies that accretion was occurring onto both magnetic poles. Consequent constraints on the accretion geometry and white dwarf mass are derived, and a minimum white dwarf mass of 0.62 M_⊙ is obtained. A color temperature T <~ 30,000 K is derived for the unpulsed emission.

We determine a precise orbital period, 0.16537398[17] days, based on an unambiguous 14 yr cycle count. We describe a simple phase-matching technique used in the period search. The UV continuum and UBVR photometry show a single-humped orbital modulation; the I-band light curve shows a double-humped ``ellipsoidal'' shape. The deeper I-band minimum occurs at inferior conjunction of the mass donor, which suggests that heating of the L₁ point dominates over gravity darkening. I-band light-curve modeling suggests i <~ 50°. Orbital radial velocity variations of semiamplitude 91 +/- 10 km s^-1 were found in the C IV emission. Combining with the published value K_s = 202 +/- 3 km s^-1, this yields q = 0.45 +/- 0.05. Assuming a Roche lobe-filling main-sequence mass donor and adopting the empirical ZAMS mass-radius relation, we obtain M_s = 0.375 +/- 0.014 M_⊙, M_wd = 0.83 +/- 0.10 M_⊙, and i = 45° +/- 4°.

Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.