HAT-P-11b: A SUPER-NEPTUNE PLANET TRANSITING A BRIGHT K STAR IN THEKEPLERFIELD

Abstract

We report on the discovery of HAT-P-11b, the smallest radius transiting extrasolar planet (TEP) discovered from the ground, and the first hot Neptune discovered to date by transit searches. HAT-P-11b orbits the bright (V=9.587) and metal rich ([Fe/H] = +0.31 ± 0.05) K4 dwarf star GSC 03561-02092 with P = 4.8878162 ± 0.0000071 days and produces a transit signal with depth of 4.2 mmag; the shallowest found by transit searches that is due to a confirmed planet. We present a global analysis of the available photometric and radial-velocity data that result in stellar and planetary parameters, with simultaneous treatment of systematic variations. The planet, like its near-twin GJ 436b, is somewhat larger than Neptune (17M ⊕ , 3.8R ⊕ ) both in mass M p = 0.081 ± 0.009 M J (25.8 ± 2.9 M ⊕ ) and radius R p = 0.422 ± 0.014 R J (4.73 ± 0.16 R ⊕ ). HAT-P-11b orbits in an eccentric orbit with e = 0.198 ± 0.046 and ω = 355.2 ± 17.3 • , causing a reflex motion of its parent star with amplitude 11.6 ± 1.2 m s −1 , a challenging detection due to the high level of chromospheric activity of the parent star. Our ephemeris for the transit events is T c = 2454605.89132 ± 0.00032 (BJD), with duration 0.0957 ± 0.0012 d, and secondary eclipse epoch of 2454608.96 ± 0.15 d (BJD). The basic stellar parameters of the host star are M ⋆ = 0.809 +0.020 −0.027 M ⊙ , R ⋆ = 0.752 ± 0.021 R ⊙ and T eff⋆ = 4780 ± 50 K. Importantly, HAT-P-11 will lie on one of the detectors of the forthcoming Kepler mission; this should make possible fruitful investigations of the detailed physical characteristic of both the planet and its parent star at unprecedented precision. We discuss an interesting constraint on the eccentricity of the system by the transit light curve and stellar parameters. This will be particularly useful for eccentric TEPs with low amplitude RV variations in Kepler's field. We also present a blend analysis, that for the first time treats the case of a blended transiting hot Jupiter mimicing a transiting hot Neptune, and proves that HAT-P-11b is not such a blend.

Analysis of selected Kepler Mission planetary light curves

“…Our finding that the planet is about 4.23 times bigger than the Earth in radius agrees closely with the NEA value of 4.68 R ⊕ , and in apparent exact agreement with that of Bakos et al (2010). Bakos et al (2009Bakos et al ( , 2010 found an eccentricity of 0.198 to the planetary orbit, having a periastron longitude of ∼355 • with a formal error of about 15 • . In other words, the major axis of the elliptical orbit is not far from the plane of the sky, the planet exiting from the transit more quickly than its entrance.…”

The HARPS search for southern extra-solar planets

“…Dedicated surveys have proven to be more efficient for transit detections. Ground-based programs allow Jupiter-size planets to be detected with improving accuracies which now point toward planets with smaller radii (Bakos et al 2010), whereas space-based programs allow planets with even smaller radii to be detected (Léger et al 2009;Queloz et al 2009), as well as planets with longer periods (Moutou et al 2009b). If accepted, the ESA-proposed space mission PLATO (Catala et al 2009) could permit the detection of this kind of transiting planets in front of brighter stars.…”

The TRAPPIST survey of southern transiting planets

“…It is worth noticing that most of the baseline parameters are not jump parameters in the MCMC, they are deduced by least-square minimization from the residuals at each step of the chains, thanks to their linear nature in the baseline functions (Bakos et al 2009;). …”