HD 89345: a bright oscillating star hosting a transiting warm Saturn-sized planet observed by K2

Abstract

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 one-minute time sampling. It exhibits solar-like oscillations. We conducted asteroseismology to determine the parameters of the star, finding the mass and radius to be $1.12_{-0.01}^{+0.04}{M}_{\odot }$ and $1.657_{-0.004}^{+0.020}{R}_{\odot }$, respectively. The star appears to have recently left the main sequence, based on the inferred age, $9.4_{-1.3}^{+0.4}\mathrm{Gyr}$, and the non-detection of mixed modes. The star hosts a "warm Saturn" ($P=11.8$~days, $R_p=6.86\pm 0.14R_{\oplus }$). Radial-velocity follow-up observations performed with the FIES, HARPS, and HARPS-N spectrographs show that the planet has a mass of $35.7\pm 3.3M_{\oplus }$. The data also show that the planet's orbit is eccentric ($e\approx 0.2$). 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 conform to the same patterns that have been observed for other sub-Saturns regarding planet mass and multiplicity, orbital eccentricity, and stellar metallicity.

arXiv commentSubmitted to MNRAS on 23 February 2018, accepted for publication, 4 May 2018

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