Masses and Other Parameters of the Post–Common Envelope Binary BE Ursae Majoris

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 rep- resents 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 and R _ ^ 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 has nearly R _ ^ 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 pro—le and the surrounding continuum, we also were able to constrain the sdO helium abundance as and Our results support the sdO/DAO log n He \( 1.1 ^ 0.2 log n Fe \ 1. log g D 6.5 surface gravity and K values of Liebert et al. and are consistent with the post- T eff D 100,000 AGB evolutionary track. Our best estimate of the distance to the BE UMa system is 2000 pc. Subject headings: binaries: eclipsingstars: abundancesstars: fundamental parameters ¨ stars: individual (BE Ursae Majoris) ¨ stars: white dwarfsultraviolet: stars