v781tau - Astronomy & Astrophysics A&A 452, 959967...

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A&A 452, 959–967 (2006) DOI: 10.1051 / 0004-6361:20053270 c ± ESO 2006 Astronomy & Astrophysics V781 Tauri: a W Ursae Majoris binary with decreasing period ? J. Kallrath 1 , 4 ,E.F .Milone 2 , R. A. Breinhorst 3 ,R.E.W ilson 4 , 6 ,A.Schnell 5 , and A. Purgathofer 5 ,?? 1 BASF-AG, GVC / S-B009, 67056 Ludwigshafen, Germany e-mail: josef.kallrath@t-online.de 2 Physics and Astronomy Department, The University of Calgary, 2500 University Drive, Calgary, Alberta, T2N 1N4, Canada e-mail: milone@ucalgary.ca 3 Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany 4 Astronomy Department, University of Florida, Gainesville, 32611 FL, USA 5 Institut für Astronomie, Universität Wien, Türkenschanzstraße 17, 1180 Wien, Austria 6 Maria Mitchell Observatory, Nantucket, MA 02554 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binary V781 Tauri from three epochs and radial velocity curves from two epochs simultaneously, including previously unpublished B and V data.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 so- lution is done coherently in time (not phase) with ±ve light curves and two sets of primary and secondary velocity curves. Minor systematic di f erences among the individual light curves are not large enough to undermine the value of a coherent solution that rep- resents 18 years of observations. Times of minima con±rm a period of 0 . d 34491 and the general solution ±nds a small period change, d P / P ,of(5 . 08 ² 04) × 10 11 that represents recent behavior. The eclipse timings cover the last half-century and ±nd d P / P about four times smaller, corresponding to a period change time scale, P / (d P / d t ) of about 6 million years. The system is over-contact with a ±lling factor of 0.205. The solution produces a temperature di f erence of about 260 K between the components, an inclination of 65 . 9, and a mass ratio M 2 / M 1 = 2 . 47. Separate solutions of the several light curves that incorporate dark spots ±nd parameters that di f er 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 ²ow that one can infer from d P / d t 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. Key words. binaries: eclipsing – stars: individual: V781 Tauri 1. Introduction V781 Tau [ = BD + 26 971 = HD 248087 = SAO 077615 (spec- tral type G0)] was found to be variable by Harris (1979) while se- lecting comparison stars for asteroid photometry (Harris 1982), but he was unable to distinguish between periods of 0 . d 3394 and 0 . d 3449. Limited numbers of photographic (Berthold 1981, 1983) and photoelectric measurements (Diethelm 1981) indi- cated the longer period to be more likely, but both data sets are very sparse. V781 Tau was originally classi±ed as an A- type W UMa eclipsing binary (EB) by Rucinski (1974) but is now considered to be W-type. Cereda et al. (1988, hereafter
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v781tau - Astronomy & Astrophysics A&A 452, 959967...

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