V781 Tauri: a W Ursae Majoris binary with decreasing period

¹ BASF-AG, GVC/S-B009, 67056 Ludwigshafen, Germany e-mail: josef.kallrath@t-online.de
² Astronomy Department, University of Florida, Gainesville, 32611 FL, USA
³ Physics and Astronomy Department, The University of Calgary, 2500 University Drive, Calgary, Alberta, T2N 1N4, Canada e-mail: milone@ucalgary.ca
⁴ Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁵ Maria Mitchell Observatory, Nantucket, MA 02554
⁶ Institut für Astronomie, Universität Wien, Türkenschanzstraße 17, 1180 Wien, Austria

Received: 20 April 2005
Accepted: 3 March 2006

Abstract

We analyze light curves of the W UMa type eclipsing binary V781 Tauri from three epochs and radial velocity curves from two epochs simultaneously, including previously unpublished B and V data. The overall time span is from 1983 to 2000 and the solution is done coherently in time (not phase) with five light curves and two sets of primary and secondary velocity curves. Minor systematic differences among the individual light curves are not large enough to undermine the value of a coherent solution that represents 18 years of observations. Times of minima confirm a period of and the general solution finds a small period change, , of that represents recent behavior. The eclipse timings cover the last half-century and find about four times smaller, corresponding to a period change time scale, of about 6 million years. The system is over-contact with a filling factor of 0.205. The solution produces a temperature difference of about 260 K between the components, an inclination of , and a mass ratio . Separate solutions of the several light curves that incorporate dark spots find parameters that differ little from curve to curve. Absolute masses, luminosities, radii and the distance are derived, with luminosities and distance based on star 1 being of type G0V. The orbital angular momentum is compared with those of other W UMa type binaries and is normal. The star to star mass flow that one can infer from is opposite to that expected from TRO (Thermal Relaxation Oscillator) theory, but pertains to a time span that is very short compared to the time scale of TRO oscillations.