THE SPECTACULAR EMISSION-LINE REFLECTION EFFECT OF BE URSAE-MAJORIS

BE UMa is a noninteracting eclipsing binary consisting of a similar to 100,000 K sdO primary star and a G2 V-K2 V secondary star. The powerful extreme ultraviolet (EUV) flux from the primary star incident on the facing secondary produces a stunning reflection effect that modulates with orbital phase. Preparatory to extensive modeling of the reflection effect, we identify and phase-normalize in self-consistent fashion the emission-line features from a variety of spectra with broad wavelength coverage. We present an analytic model of the reflection effect based on the graybody approximation that predicts a slight but potentially observable asymmetry in the visible light curve. We develop a digital model using the CLOUDY code in the upper regions of the reflection effect atmosphere, where hydrogen is photoionized, and approximate the higher density heated photosphere as a blackbody. The model accounts for angle of incidence of the EUV radiation across the facing secondary by dividing the hemisphere into concentric annuli and then co-adding results. A sensitivity study involving a wide variety of physical input values was performed, leading to a set of precise predictions describing the BE UMa system that differ substantially from existing estimates. The primary star mass and luminosity are 0.90 +/- 0.04 M,and (2.5 +/- 0.8) X 10(36) ergs s(-1), respectively. The secondary star mass, intrinsic temperature, and radius are 0.83 +/- 0.10 M., 5000 +/- 500 K, and 0.96 +/- 0.22 R., yielding a spectral type of G2 V-K2 V, although some enlargement of the secondary radius over main-sequence values cannot be ruled out. The helium abundance of the secondary star is log (He/H) = -1.0 +/- 0.25 by number. The secondary star metal abundance is from one-third solar to near-solar. The primary cause of the strong emission feature at 4650 Angstrom is recombinational O II with a nonnegligible C In component. The relative mix of metals appears solar-like, although only the relative abundance ratio of C/O similar to 0.6 could be quantitatively estimated. The binary star separation is 8.6 +/- 0.2 R.. Based on both estimates derived from the primary star and the reflection effect, BE UMa lies at a distance of 2.0 +/- 0.4 kpc, nearly perpendicular to the Galactic plane. Our model makes specific predictions regarding the luminosity of the primary star that have recently been confirmed independently through direct analysis of the primary star spectrum by Liebert et al. (1994). The methods we have developed are applicable to other EUV-produced reflection effects, such as those sometimes seen in planetary nebulae.