*V471 Tau C again?

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*V471 Tau C again?

Post by Edasich on 13th October 2018, 10:09 am

The third body around V471 Tauri seems returning...

The Mystery of the Invisible Brown Dwarf Companion to the Eclipsing Binary V471 Tauri—Analysis of 45 Years of Eclipse Timings Including K2

Free-access version of the paper here.

The Hyades eclipsing binary (EB) V471 Tauri (BD +16°516) has been under intense study since its discovery by Nelson & Young (1970). V471 Tau is a detached EB (P = 12.51 hr) that consists of a hot DAZ white dwarf and a chromospherically active, spotted K2V star (Guinan & Sion 1984). V471 Tau is an important astrophysical laboratory for deciphering binary star evolution, particularly that of postcommon envelope binary (CEB) and pre-cataclysmic variable star systems, and providing fundamental information on white dwarfs and studying the magnetic activity and dynamos of cool stars (Guinan et al. 1986). The analyses of eclipse timings show periodic long-term (30+ year) sinusoidal-like variations in the observed minus computed (O–C) values. These eclipse timing variations (ETVs) have been interpreted as a "Light Travel Time Effect" (LTTE) arising from gravitational perturbations by a tertiary companion (e.g., Guinan & Ribas 2001; Hric & Kundra 2012; Vaccaro et al. 2015). The detailed study by Vaccaro et al. (2015) returns a period of P3 = 30.1 years, e3 = 0.39 and f (m) = 1.17 × 10−5(M⊙). Adopting an EB total mass of M1+2 = 1.875 M⊙, yields a mass of the tertiary component of M3(sin i3) = 0.035 M⊙—a mass appropriate for a brown dwarf if i3 > 30 deg.

We report on the analysis of over ~45 years of eclipse-timings including nearly 80 timings determined from short-cadence (~60 s) K2 mission photometry. The K2 observations were secured on 60 continuous days during 2016 February–April. A representative 5 day sample of the photometry is shown in Figure 1 (top panel) and below it a phased light curve is plotted. The light curves show asymmetries and ~50 minutes duration occultation-eclipses of the white dwarf. The observations were normalized and de-trended (example shown in the figure). The exonailer Python program (Espinoza et al. 2016) was used to determine the mid-eclipse times with precisions of ~5–8 s. The mean-weighted mid-eclipse timing from the K2 data is BJD 2457085.20090 with a corresponding O–C = +90.5 ± 1.5 s. The K2 timings were combined with the previous timings to calculate (O–C) points and carry out LTTE (Keplerian-orbital) least-squares fits. The (O–C) data are shown in the bottom panel of Figure 1 along with the computed system properties. The LTTE analysis of the timings returns a (O–C) semi-amplitude of 150.8 ± 1.8 s, P3 = 35.3 ± 0.4 years and e3 = 0.11 ± 0.02. Adopting the EB mass from Vaccaro et al. (2015), yields a minimum-mass of M3 > 0.043 M⊙ and semimajor axis (a3) ≈ 13.3 au. As shown in the figure, after the 35.3 years LTTE variation is removed, additional small O–C oscillations are present with a semi-amplitude of ~20 ± 2 s and a P ≈ 9.7 ± 1.2 years. These small oscillations could arise from an additional 4th body or from an Applegate-type mechanism (Applegate 1992) as discussed below.
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