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

HD 89345: a bright oscillating star hosting a transiting warm Saturn-sized planet observed by K2
See arXiv version
Submitted to MNRAS on 23 February 2018, accepted for publication, 4 May 2018

### 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 $$1.12^{+0.04}_{-0.01}~M_\odot$$ and $$1.657^{+0.020}_{-0.004}~R_\odot$$, respectively. The star appears to have recently left the main sequence, based on the inferred age, $$9.4^{+0.4}_{-1.3}~\mathrm{Gyr}$$, and the non-detection of mixed modes. The star hosts a “warm Saturn” ($$P = 11.8$$~days, $$R_p = 6.86 \pm 0.14~R_\oplus$$). Radial-velocity follow-up observations performed with the FIES, HARPS, and HARPS-N spectrographs show that the planet has a mass of $$35.7 \pm 3.3~M_\oplus$$. 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.