2018MNRAS.478.4866V

We report the discovery and characterization of HD 89345b (K2-234b; EPIC 248777106b), a Saturn-sized planet orbiting a slightly evolved star. HD 89345 is a bright star (V = 9.3 mag) observed by the K2 mission with 1 min time sampling. It exhibits solar-like oscillations. We conducted asteroseismology to determine the parameters of the star, finding the mass and radius to be 1.12^+0.04_–0.01 M_☉and 1.657^+0.020_–0.004 R_☉, respectively. The star appears to have recently left the main sequence, based on the inferred age, 9.4^+0.4_–1.3 Gyr, and the non-detection of mixed modes. The star hosts a 'warm Saturn' (P = 11.8 d, R_p = 6.86 ± 0.14 R_⊕). Radial-velocity follow-up observations performed with the FIbre-fed Echelle Spectrograph, HARPS, and HARPS-N spectrographs show that the planet has a mass of 35.7 ± 3.3 M_⊕. The data also show that the planet's orbit is eccentric (e ≃ 0.2). An investigation of the rotational splitting of the oscillation frequencies of the star yields no conclusive evidence on the stellar inclination angle. We further obtained Rossiter-McLaughlin observations, which result in a broad posterior of the stellar obliquity. The planet seems to confirm to the same patterns that have been observed for other sub-Saturns regarding planet mass and multiplicity, orbital eccentricity, and stellar metallicity.