Superhumps in Cataclysmic Binaries. XVIII. IY Ursae Majoris

Joseph Patterson, John Thorstensen, Jonathan Kemp, Lasse Jensen, Tonny Vanmunster, Timothy Abbott, David Skillman, Brian Martin, and Robert Fried

Publications of the Astronomical Society of the Pacific

December 2000, Volume 112, Page 1567

We report photometric and spectroscopic observations of the eclipsing dwarf nova IY Ursae Majoris (= Takamizawa V85). During its January 2000 superoutburst, the star flashed superhumps with a period of 0.07583 d, 2.8% longer than the true orbital period. These waves rumbled through the light curve for ~20 days. They appeared compatible with their common interpretation as the lower precessional sideband of the orbital clock: at a frequency w_o-W, where w_o is the orbital frequency and W is the accretion disk's assumed precessional frequency. But the power spectrum of the superhumps showed additional complexity, including signals at 3w_o-W, 4w_o-W, and 5w_o-W. The latter probably arise from gravitational perturbation of more complex but still resonant (e.g., spiral) structures in the disk.

During eruption, the light curve across eclipse showed a large, bright accretion disk. But timings of minima and eclipse contacts revealed that the disk was quite eccentric, with e=0.29±0.06, and that this eccentric shape moved around with the putative precession period of 2.9 d. As the eruption faded, the eclipses began to reveal the signature of the white dwarf and the hot spot at the disk's edge. "Late" superhumps raged at high amplitude for another ~10 days, although the disk had seemingly contracted by ~40%. This is difficult to understand.

The travelling knife-edge of the secondary's limb made it possible to separate all the important light sources: white dwarf, secondary star, hot spot, accretion disk. The ingress and egress phases of the white-dwarf eclipse were very brief, declining from ~40 s to 25 s. The brightness of the white dwarf also varied, rapidly at first and then slowly. Eclipse measurements in quiescence lead to estimates for the fundamental parameters of the binary: M₂=0.08±0.03 M_sol, M₁=0.63±0.13 M_sol, i=87±2 °. We estimate a distance of 190±60 pc. With a small distance, a high inclination, vigorous superhumps, and clear lines of sight to the central object, IY UMa furnishes a promising new laboratory for studying accretion-disk physics.

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Copyright © 2000 Astronomical Society of the Pacific.