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

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

Feb 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...476..847h&link_type=abstract

Astrophysical Journal v.476, p.847

Astronomy and Astrophysics

Astronomy

33

Stars: Oscillations, Stars: Individual Constellation Name: Yy Draconis, Ultraviolet: Stars, Stars: White Dwarfs

Scientific paper

We present 1 Angstroms resolution fast ultraviolet spectroscopy of YY Dra, coordinated U, B, V, R, and I photometry, and H alpha 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 Pspin = 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 Pspin. 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&sun; 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 deg. Orbital radial velocity variations of semiamplitude 91 +/- 10 km s-1 were found in the C IV emission. Combining with the published value Ks = 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 Ms = 0.375 +/- 0.014 M&sun;, Mwd = 0.83 +/- 0.10 M&sun;, and i = 45 deg +/- 4 deg.

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