The discovery of a planetary candidate around the evolved low-mass Kepler giant star HD 175370

TL;DR

This paper reports the discovery of a planetary candidate orbiting the evolved giant star HD 175370, based on five years of radial velocity measurements, with additional evidence suggesting a possible stellar companion.

Contribution

First detection of a planetary candidate around an evolved low-mass giant star using long-term radial velocity monitoring.

Findings

Planetary candidate with Msini = 4.6 M_J and 349.5-day orbit.

Detection of a long-term trend indicating a possible stellar companion.

Radial velocity variations could also be caused by stellar pulsations, requiring further study.

Abstract

We report on the discovery of a planetary companion candidate with a minimum mass Msini = 4.6 M_J orbiting the K2 III giant star HD 175370 (KIC 007940959). This star was a target in our program to search for planets around a sample of 95 giant stars observed with Kepler. This detection was made possible using precise stellar radial velocity measurements of HD 175370 taken over five years and four months using the coude echelle spectrograph of the 2-m Alfred Jensch Telescope and the fibre-fed echelle spectrograph HERMES of the 1.2-m Mercator Telescope. Our radial velocity measurements reveal a periodic (349.5 days) variation with a semi-amplitude K = 133 m/s, superimposed on a long-term trend. A low-mass stellar companion with an orbital period of ~88 years in a highly eccentric orbit and a planet in a Keplerian orbit with an eccentricity e = 0.22 are the most plausible explanation of the radial velocity variations. However, we cannot exclude the existence of stellar envelope pulsations as a cause for the low-amplitude radial velocity variations and only future continued monitoring of this system may answer this uncertainty. From Kepler photometry we find that HD 175370 is most likely a low-mass red-giant branch or asymptotic-giant branch star.