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

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

Jun 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...518..866f&link_type=abstract

The Astrophysical Journal, Volume 518, Issue 2, pp. 866-872.

Astronomy and Astrophysics

Astronomy

23

Stars: Binaries: Eclipsing, Stars: Abundances, Stars: Fundamental Parameters, Stars: Individual (Be Ursae Majoris), Stars: White Dwarfs, Ultraviolet: Stars

Scientific paper

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 lambda1640 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 sigma value for all errors. The separation between the component stars is 7.5 R_solar+/-0.5 R_solar and is insensitive to small changes in inclination angle due to the near edge-on viewing angle of 84^deg+/-1^deg. Using these values, we modeled the eclipse light curve. Our results matched observed UBVR light curves of Wood and coworkers only if the modeled secondary star radius of 0.72 R_solar+/-0.05 R_solar has nearly 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 ~10^4 yr ago. Using the lambda1640 absorption-line profile and the surrounding continuum, we also were able to constrain the sdO helium abundance as logn_He=-1.1+/-0.2 and logn_Fe<1. Our results support the sdO/DAO logg~6.5 surface gravity and T_eff~100,000 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.

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