Two Warm, Low-density Sub-Jovian Planets Orbiting Bright Stars in K2 Campaigns 13 and 14

Abstract

We report the discovery of two planets transiting the bright stars HD 89345 (EPIC 248777106, V=9.376, K=7.721) in K2 Campaign 14 and HD 286123 (EPIC 247098361, V=9.822, K=8.434) in K2 Campaign 13. Both stars are G-type stars, one of which is at or near the end of its main-sequence lifetime, and the other is just over halfway through its main-sequence lifetime. HD 89345 hosts a warm sub-Saturn (0.66R J , 0.11M J , T eq =1100 K) in an 11.81 day orbit. The planet is similar in size to WASP-107b, which falls in the transition region between ice giants and gas giants. HD 286123 hosts a Jupiter-sized, low-mass planet (1.06R J , 0.39 M J , T eq =1000 K) in an 11.17 day, mildly eccentric orbit, with e=0.255±0.035. Given that they orbit relatively evolved main-sequence stars and have orbital periods longer than 10 days, these planets are interesting candidates for studies of gas planet evolution, migration, and (potentially) reinflation. Both planets have spent their entire lifetimes near the proposed stellar irradiation threshold at which giant planets become inflated, and neither shows any sign of radius inflation. They probe the regime where inflation begins to become noticeable and are valuable in constraining planet inflation models. In addition, the brightness of the host stars, combined with large atmospheric scale heights of the planets, makes these two systems favorable targets for transit spectroscopy to study their atmospheres and perhaps provide insight into the physical mechanisms that lead to inflated hot Jupiters.

The Aligned Orbit of the Eccentric Warm Jupiter K2-232b

“…The results, as listed in Table 2, provided solutions for transit and radial velocity parameters, as well as the sky-projected spin-orbit angle λ, the sky-projected stellar rotational velocity v i sin  , and the associated 1σ uncertainties. The radial-velocity and transit parameters obtained from our analysis are in good agreement with the values from Brahm et al (2018) and Yu et al (2018).…”

“…Uniform priors were adopted for all of these parameters. Initial guesses for P, T 0 , i cos , R P /R å , (R å + R P )/a, K, w e cos , and w e sin were set to the values reported by Yu et al (2018). The initial guess for each limb darkening coefficient was 0.5.…”

“…Here, we present observations of the Rossiter-McLaughlin effect for K2-232 b, a warm Jupiter of mass 0.39 M Jup and radius 1.06 R Jup on an 11.17 day orbit with an eccentricity of 0.26 (Brahm et al 2018;Yu et al 2018). In what follows, we describe our observations (Section 2), the parameterized model we used to determine the spin-orbit angle (Section 3), and the possible implications (Section 4).…”

K2-280 b – a low density warm sub-Saturn around a mildly evolved star

“…After Kepler-1656 b (Brady et al 2018), K2-280 b is the second most eccentric sub-Saturn known to date. In contrast to Kepler-1656 b, the mass of M b = 37.1 ± 5.6 M ⊕ , radius of R b = 7.50 ± 0.44 R ⊕ , and mean density of ρ b = 0.48 +0.13 −0.10 g cm −3 , make K2-280 b more similar to HD 89345 b (aka K2-234 b;Van Eylen et al 2018;Yu et al 2018b). The moderate eccentricity of K2-280 b suggests a formation pathway involving planet-planet gravitational interactions, and make this sub-Saturn planet a member of a relatively rare group of exoplanets and an interesting object for possible future follow-up.…”

“…Kepler-1656 b, a dense sub-Saturn with a high eccentricity of e = 0.84 transiting a relatively bright (V = 11.6 mag) solar-type star, was recently reported by Brady et al (2018). Three sub-Saturns were discovered and characterised by the KESPRINT consortium, two of them in K2 campaign 3 (K2-60 b, Eigmüller et al 2017) and campaign 14 (HD 89345 b, aka K2-234 b, Van Eylen et al 2018;Yu et al 2018b), and HD 219666 b (Esposito et al 2019) in TESS Sector 1. One sub-Saturn, GJ 3470 b (Bonfils et al 2012), orbiting an M1.5 dwarf was not included by Petigura et al (2017), but we add it to the current sample, adopting the parameters from Awiphan et al (2016).…”