Precise radial velocities of giant stars V. A brown dwarf and a planet orbiting the K giant stars tau Gem and 91 Aqr

TL;DR

This study reports the discovery of a brown dwarf and a planet orbiting K giant stars tau Gem and 91 Aqr, using high-precision radial velocity measurements over a decade, revealing companions with unusually circular orbits.

Contribution

It presents the detection and characterization of two substellar companions around K giant stars, highlighting their circular orbits which are atypical for such systems.

Findings

Brown dwarf orbiting tau Gem with 305.5-day period and 20.6 M_Jup

Planet orbiting 91 Aqr with 181.4-day period and 3.2 M_Jup

Both companions have exceptionally circular orbits

Abstract

We aim to detect and characterize substellar companions to K giant stars, to further our knowledge of planet formation and stellar evolution of intermediate-mass stars. For more than a decade we have used Doppler spectroscopy to acquire high precision radial velocity measurements of K giant stars. All data for this survey have been taken at Lick Observatory. Our survey includes 373 G and K giants. Radial velocity data showing periodic variations are fitted with Keplerian orbits using a chi^2 minimization technique. We report the presence of two substellar companions to the K giant stars tau Gem and 91 Aqr. The brown dwarf orbiting tau Gem has an orbital period of 305.5 +/- 0.1 days, a minimum mass of 20.6 M_Jup, and an eccentricity of 0.031 +/- 0.009. The planet orbiting 91 Aqr has an orbital period of 181.4 +/- 0.1 days, a minimum mass of 3.2 M_Jup, and an eccentricity of 0.027 +/- 0.026. Both companions have exceptionally circular orbits for their orbital distance, as compared to all previously discovered planetary companions.