Masses and Other Parameters of the PostCommon Envelope Binary BE Ursae Majoris

Abstract

The binary star BE Ursae Majoris is recently emerged from the common envelope phase; indeed, the hot sdO/DAO component is the central star of the associated planetary nebula. As such, BE UMa represents an important test case of stellar evolution theory. Using the Hubble Space Telescope (HST ) Goddard High Resolution Spectrograph (GHRS), we measured the radial velocity amplitude of the He II j1640 absorption line from the sdO/DAO component of this eclipsing system. Combining our results with those of Crampton, Cowley, & Hutchings, we determine stellar masses in units of solar mass as follows : for the sdO, the mass is 0.70^0.07, and that of the secondary star is 0.36^0.07, where we report the 1 p value for all errors. The separation between the component stars is 7.5 R and _^0.5 R_ is insensitive to small changes in inclination angle due to the near edge-on viewing angle of 84¡^1¡. Using these values, we modeled the eclipse light curve. Our results matched observed UBV R light curves of Wood and coworkers only if the modeled secondary star radius of 0.72 R has nearly _^0.05 R_ double the radius expected from the main-sequence mass-radius relation. The secondary star has thus not yet relaxed to thermal equilibrium since the common envelope phase ended D104 yr ago. Using the j1640 absorption-line proÐle and the surrounding continuum, we also were able to constrain the sdO helium abundance as log nHe\[1.1^0.2 and log nFe\1. Our results support the sdO/DAO log gD6.5 surface gravity and TeffD100,000 K values of Liebert et al. and are consistent with the post- AGB evolutionary track. Our best estimate of the distance to the BE UMa system is 2000 pc.