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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 478; no. 4; pp. 4866 - 4880
• Main Authors: Van Eylen, V, Dai, F, Mathur, S, Gandolfi, D, Albrecht, S, Fridlund, M, García, R A, Guenther, E, Hjorth, M, Justesen, A B, Livingston, J, Lund, M N, Pérez Hernández, F, Prieto-Arranz, J, Regulo, C, Bugnet, L, Everett, M E, Hirano, T, Nespral, D, Nowak, G, Palle, E, Silva Aguirre, V, Trifonov, T, Winn, J N, Barragán, O, Beck, P G, Chaplin, W J, Cochran, W D, Csizmadia, S, Deeg, H, Endl, M, Heeren, P, Grziwa, S, Hatzes, A P, Hidalgo, D, Korth, J, Mathis, S, Montañes Rodriguez, P, Narita, N, Patzold, M, Persson, C M, Rodler, F, Smith, A M S
• Format: Journal Article
• Language: English
• Published: Oxford University Press 21.08.2018
• Subjects: Asteroseismology; Planets and satellites: composition; Planets and satellites: formation; Planets and satellites: fundamental parameters; asteroseismology; planets and satellites: formation; planets and satellites: composition; planets and satellites: fundamental parameters
• ISSN: 0035-8711; 1365-2966; 1365-2966
• DOI: 10.1093/mnras/sty1390

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 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 $1.12^{+0.04}_{-0.01} \mathrm{ M}_\odot$ and $1.657^{+0.020}_{-0.004} \mathrm{ R}_\odot$, 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 d, Rp = 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.