Precise Radial Velocity Measurements for Kepler Giants Hosting Planetary Candidates: Kepler-91 and KOI-1894

TL;DR

This study uses high-precision radial-velocity measurements to confirm a planet around Kepler-91 and suggests an additional companion, while also indicating a potential sub-saturn-mass planet around KOI-1894.

Contribution

It provides the first independent confirmation of Kepler-91b and reports a possible new planet around KOI-1894 using Subaru/HDS data.

Findings

Confirmed Kepler-91 hosts a 0.66 M_Jup planet.

Detected a ~20 m/s velocity offset indicating an additional companion.

Possible detection of a sub-saturn-mass planet around KOI-1894.

Abstract

We present results of radial-velocity follow-up observations for the two Kepler evolved stars Kepler-91 (KOI-2133) and KOI-1894, which had been announced as candidates to host transiting giant planets, with the Subaru 8.2m telescope and the High Dispersion Spectrograph (HDS). By global modeling of the high-precision radial-velocity data taken with Subaru/HDS and photometric ones taken by Kepler mission taking account of orbital brightness modulations (ellipsoidal variations, reflected/emitted light, etc.) of the host stars, we independently confirmed that Kepler-91 hosts a transiting planet with a mass of 0.66 M_Jup (Kepler-91b), and newly detected an offset of ~20 m s$^{-1}$ between the radial velocities taken at ~1-yr interval, suggesting the existence of additional companion in the system. As for KOI-1894, we detected possible phased variations in the radial velocities and light curves with 2--3 sigma confidence level which could be explained as a reflex motion and ellipsoidal variation of the star caused by the transiting sub-saturn-mass (~0.18 M_Jup) planet.