THE sdB+M ECLIPSING SYSTEM HW VIRGINIS AND ITS CIRCUMBINARY PLANETS

Abstract

For the very short-period sdB eclipsing binary HW Vir, we present new CCD photometry made from 2000 through 2008. In order to obtain consistency of the binary parameters, our new light curves, showing sharp eclipses and a striking reflection effect, were analyzed simultaneously with previously published radial-velocity data. The secondary star parameters of M 2 =0.14 M ⊙ , R 2 =0.18 R ⊙ , and T 2 =3,084 K are consistent with those of an M6-7 main sequence star. A credibility issue regarding bolometric corrections is emphasized. More than 250 times of minimum light, including our 41 timings and spanning more than 24 yrs, were used for a period study. From a detailed analysis of the O-C diagram, it emerged that the orbital period of HW Vir has varied as a combination of a downward-opening parabola and two sinusoidal variations, with cycle lengths of P 3 =15.8 yr and P 4 =9.1 yr and semi-amplitudes of K 3 =77 s and K 4 =23 s, respectively. The continuous period decrease with a rate of −8.28×10 −9 d yr −1 may be produced by angular momentum loss due to magnetic stellar wind braking but not by gravitational radiation. Of the possible causes of the cyclical components of the period change, apsidal motion and magnetic period modulation can be ruled out. The most reasonable explanation of both cyclical variations is a pair of light-travel-time effects driven by the presence of two substellar companions with projected masses of M 3 sin i 3 =19.2 M Jup and M 4 sin i 4 =8.5 M Jup . The two objects are the first circumbinary planets known to have been formed in a protoplanetary disk as well the first ones discovered by using the eclipse-timing method. The detection implies that planets could be common around binary stars just as are planets around single stars and demonstrates that planetary systems formed in a circumbinary disk can survive over long time scales.Depending on the thermal inertia of their massive atmospheres, the hemispheres of the planets turned toward the stars can experience substantial reciprocating temperature changes during the minutes-long primary eclipse intervals.

Line-depth-ratio temperatures for the close binary ν Octantis: new evidence supporting the conjectured circumstellar retrograde planet

“…The majority of planets found in binary systems are circumstellar but a few have been identified in circumbinary (P-type) geometries e.g. HW Vir (Lee et al 2009). More recent studies, such as SPOTS (Search for Planets Orbiting Two Stars), are specifically targeting close binary systems (Thalmann et al 2014).…”

New Insights into the Dynamics of Planets in P-Type Motion Around Binaries

“…The initial conditions used can be found in Beuermann et al (2010) for the system NN Ser, in Qian et al (2011) for the system HU Aqr, and in Lee et al (2009) for the system HW Vir. In all cases, the motion of the planets were considered in the framework of purely Newtonian forces and all the celestial bodies involved were regarded as point masses.…”

The Dwarf project: Eclipsing binaries – precise clocks to discover exoplanets

“…With the continuing discovery of extrasolar planets and an expectation that the majority of solar-type stars reside in binary or multiple systems (Duquennoy & Mayor 1991), planetary formation in binary systems has become an important issue (Lee et al 2009). In general, we can consider planetary companions to binary stars in two types of hierarchical planet-binary configurations: first "S-type" planets which orbit just one of the stars, with the binary period being much longer than that of the planet; second, "P-type" or circumbinary planets, where the planet simultaneously orbits both stars, and the planetary orbital period is much longer than that of the binary (Muterspaugh et al 2007).…”

“…Characterization of such planets is potentially of great interest because they can lead to a better understanding of the formation and evolution of planetary systems around close binary stars, which can be rather different from the case of single stars. (Lee et al 2009). Hereafter we will consider the "P-type" or circumbinary planets only.…”

“…Three principal techniques are: (i) precise radialvelocity (hereafter RV) measurements to detect the wobble of the binary mass center (Konacki et al 2009), (ii) photometric detection of transits of the planet(s) across the disks of the components of the inner binary (Doyle et al 2011), and (iii) timing of the inner binary eclipses (Lee et al 2009). …”