AAS 196th Meeting, June 2000
Session 3. Dwarf Stars
Display, Monday, June 5, 2000, 9:20am-6:30pm, Empire Hall South

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[3.03] A Possible Brown Dwarf Companion to the Hyades Eclipsing Binary V471 Tauri

E.F. Guinan (Villanova University), I. Ribas (U. de Barcelona), J.J. Bochanski, G.P. McCook (Villanova University)

The Hyades 9th mag dK0+DA eclipsing binary V471 Tauri (P = 12.5hrs) has been under intense study since its discovery over 30 yrs ago. V471 Tau plays key roles in deciphering binary star evolution as well providing a wealth of fundamental information on the structure and evolution of white dwarfs. Also V471 Tau serves as an important laboratory for studying chromospherically active stars since the dK component rotates rapidly and is very active. Here we show yet another interesting aspect of this little binary - i.e., V471 Tau appears to have a low mass tertiary companion that could likely be a brown dwarf.

We carried out an analysis of nearly 30 years of eclipse timings of V471 Tauri that shows a long-term quasi-sinusoidal modulation of its observed eclipse arrival times. The O-Cs have been analyzed using expressions given by Irwin (AJ 64, 149) for the "light time" effect that arises from the gravitational influence of a tertiary companion. The presence of a third body orbiting in an eclipsing system causes the relative distance of the eclipsing binary to change as it orbits the barycenter of the triple system producing the periodic variations in its light travel time. The magnitude of the "light time" effect depends primarily on the relative mass of the tertiary component and the size and inclination of its orbit. Early timings were obtained from Ibanoglu et al. AA,281,811). Eclipse times from 1994-2000 were obtained by us using APTs. The results of the analysis yield a light semi-amplitude of 118+/-1.5 sec and an orbital period of P' = 27.4 yrs and eccentricity of e' = 0.37. The minimum mass of the tertiary component is M'sini = 0.036\odot when the mass of 1.6\odot for V471 Tau is adopted. These results are in general agreement with the third body analysis made with less data by Ibanoglu et al. For orbital inclinations greater than i' ~30 deg, the mass of the third body would be too low to be star and it thus would be a brown dwarf. We will discuss the properties of the third body and also our plans to measure its orbital inclination and mass. This research is supported by NSF/RUI and NASA grants which we gratefully acknowledge.

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