Regular Radial Velocity Variations in Nine G- and K-type Giant Stars: Eight Planets and One Planet Candidate

Abstract

We report the detection of radial velocity variations in nine evolved G- and K-type giant stars. The observations were conducted at Okayama Astrophysical Observatory. Planets or planet candidates can best explain these regular variations. However, a coincidence of near 280-day variability among five of them prevents us from fully ruling out stellar origins for some of the variations, since all nine stars behave similarly in stellar properties. In the planet hypotheses to the RV variations, the planets (including one candidate) may survive close to the boundary of the so-called "planet desert" around evolved stars, having orbital periods between 255 and 555 days. Besides, they are the least-massive giant planets detected around G- and K-type giant stars, with minimum masses between 0.45$M_{\mathrm{J}}$ and 1.34$M_{\mathrm{J}}$. We further investigated other hypotheses for our detection, yet none of them can better explain regular RV variation. With our detection, it is convinced that year-long regular variation with amplitude down to 15 $\mathrm{m}{\mathrm{s}}^{-1}$ for G- and K-type giant stars is detectable. Moreover, we performed simulations to further confirm the detectability of planets around these stars. Finally, we explored giant planets around intermediate-mass stars, and likewise found a 4 Jupiter mass gap (e.g. Santo et al. 2017), which is probably a boundary of the giant planet population.

59 pages, 32 figures, 12 tables, accepted for publication in PASJ, all RVs will be available online as supplementary after the publication

References in corpus (31)

• The Gaia mission
• Modules for Experiments in Stellar Astrophysics (MESA)
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• Accurate, Empirical Radii and Masses of Planets and their Host Stars with Gaia Parallaxes
• Assessing statistical significance of periodogram peaks
• Asteroseismology and Gaia: Testing Scaling Relations Using 2200 Kepler Stars with TGAS Parallaxes
• Retired A Stars and Their Companions: Exoplanets Orbiting Three Intermediate-Mass Subgiants
• Precise Radial Velocities of Giant Stars VII. Occurrence Rate of Giant Extrasolar Planets as a Function of Mass and Metallicity
• BGLS: A Bayesian formalism for the generalised Lomb-Scargle periodogram
• Stellar parameters and chemical abundances of 223 evolved stars with and without planets
• An Eccentric Hot Jupiter Orbiting the Subgiant HD 185269
• Extrasolar planets and brown dwarfs around A-F type stars VI. High precision RV survey of early type dwarfs with HARPS
• A Planetary Mass Companion to the K0 Giant HD 17092
• Revisiting Proxima with ESPRESSO
• High eccentricity planets from the Anglo-Australian Planet Search
• The properties of planets around giant stars
• Observational evidence for two distinct giant planet populations
• Seeing double with K2: Testing re-inflation with two remarkably similar planets around red giant branch stars
• Precise radial velocities of giant stars: I. Stable stars
• Discovery of a planet around the K giant star 4 UMa
• The lithium abundances for a large sample of red giants
• Bisectors of the cross-correlation function applied to stellar spectra. Discriminating stellar activity, oscillations and planets
• Binarity as the Origin of Long Secondary Periods in Red Giant Stars
• The Pan-Pacific Planet Search VII: The most eccentric planet orbiting a giant star
• Precise Radial Velocity Measurements for Kepler Giants Hosting Planetary Candidates: Kepler-91 and KOI-1894
• Dynamical and Observational Constraints on Additional Planets in Highly Eccentric Planetary Systems
• The Pan-Pacific Planet Search VI: Giant planets orbiting HD 86950 and HD 222076
• Tracking Advanced Planetary Systems with HARPS-N (TAPAS). I. A multiple planetary system around the red giant star TYC 1422-614-1
• Precise radial velocities of giant stars. XII. Evidence against the proposed planet Aldebaran b
• Spectroscopic study of red giants in the Kepler field with asteroseismologically established evolutionary status and stellar parameters
• Precise radial velocities of giant stars XV. Mysterious nearly periodic radial velocity variations in the eccentric binary $\epsilon$ Cygni

Cited by in corpus (1)

• A Trio of Giant Planets Orbiting Evolved Star HD 184010