Long-period radial velocity variations of nine M red giants

Abstract

Context. Certain periodic variations of radial velocities (RVs) of wobbling giants originate from exoplanets. A number of exoplanets have been discovered around giant stars. Aims. The purpose of our study is to find low-amplitude and long-period RV variations around bright M (super) giants in the red giant branch (or asymptotic giant branch) stage, which are long-period variables (LPVs) or high proper motion stars. Methods. The high-resolution fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO) was used to record numerous spectra of nine giants. The observation period for the targets spanned 16 yr, from 2005 to 2022. Results. We found two substellar companions from the precise RV observations of nine M giants. One companion has a 28.26−2.17+2.05 MJ orbiting period of 663.87−4.31+4.61 days at a distance of 2.03−0.01+0.01 AU (HD 6860), and the other has a 15.83−2.74+2.33 MJ orbiting period of 466.63−1.28+1.47 days at a distance of 1.33−0.11+0.08 AU (HD 112300). Our estimate of the stellar parameters for HD 6860 makes it the currently largest star with a substellar companion. We also found RV variations mimicking a planetary companion in HD 18884 and confirmed LPVs in two stars, HD 39801 and HD 42995. The RV variations of some stars seem to be associated with stellar activities rather than reflex orbital motion due to their companions. Variations like this are also detected for HD 6860 and HD 112300, which host substellar companions.