CBA Center for Backyard Astrophysics

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

Carole Haswell, Joseph Patterson, John Thorstensen, Coel Hellier, and David Skillman

Astrophysical Journal

20 February 1997, Volume 476, Page 847

We present 1 Å 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% semi-amplitude pulses with period 264.7(1) seconds; 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 kelvin 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 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 Msolar is obtained. A color temperature T <~ 30,000 kelvin is derived for the unpulsed emission.

We determine a precise orbital period, 0.16537398[17] d, based on an unambiguous 14-year 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, suggesting that heating of the L1 point dominates over gravity darkening. I band light curve modelling suggests i <~ 50°. Orbital radial velocity variations of semi-amplitude 91 ± 10 km/s were found in the C IV emission. Combining with the published value Ks = 202 ± 3 km/s, 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 Msolar, Mwd = 0.83 ± 0.10 Msolar, and i = 45° ± 4°.



Copyright © 1997 American Astronomical Society.