AAS 206th Meeting, 29 May - 2 June 2005
Session 23 Stars and Observing Them
Oral, Monday, 2:00-3:30pm, May 30, 2005, 102 B

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[23.02] The W Ursae Majoris Binary V781 Tauri: A Close Binary in Shallow Contact

E.F. Milone (U Calgary), J Kallrath (U Florida, BASF), R.A. Breinhorst (U Bonn), A. Schnell (U Wien), A. Purgathofer (U. Wien, (now deceased))

V781 Tau [= BD+26 971 = HD 248087 = SAO 077615 (G0) is a W UMa system that was among those studied by Zwitter et al., in a larger program to examine the capability of ESO's GAIA emission to produce fundamental data from spectroscopic and eclipsing binary stars. Zwitter, et al. used new ground-based RV data taken near the resolution expected for GAIA (~12,000), and light curves from the Hipparcos and Tycho missions. Previously unpublished photometry by RAB, AS, and AP were subsequently analyzed along with all existing RV- and light-curves to produce a new solution, which we discuss here. The system has a variable O'Connell Effect. The analyses, carried out mainly by JK, with the WD2002 light curve modeling package, reveal that the apparently cooler component is the larger and more massive; this circumstance and the variable light curve asymmetries are suggestive of W-type W UMa systems. The system has a decreasing period and a low contact parameter (0.205) that indicates that it is near, but not currently in, a broken contact phase. The properties of the system are self-consistent if the secondary (more massive and ostensibly the cooler) is, in fact, also the star with greater surface brightness but heavily obscured presumably with numerous or extensive spot regions, a verifiable hypothesis that we intend to test. We also compare its properties with those of CN And, thought to be at a broken contact phase, by Van Hamme et al. We conclude that V781 Tau may be nearing the end of the primary mass donor stage of thermal relaxation oscillations, and CN And, with properties of an A-Type W UMa system, is near the beginning of this stage. This work has been supported in part by Canadian NSERC grants to EFM.

The author(s) of this abstract have provided an email address for comments about the abstract: milone@ucalgary.ca

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