Precise radial velocities of giant stars

V. A brown dwarf and a planet orbiting the K giant stars τ Geminorum and 91 Aquarii^⋆,⋆⋆

¹ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
² Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407, USA
e-mail: dsmitche@calpoly.edu
³ Department of Astronomy, Yale University, New Haven, CT 06511, USA

Received: 16 April 2013
Accepted: 22 May 2013

Abstract

Aims. 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.

Methods. 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 were taken at Lick Observatory. Our survey includes 373 G and K giants. Radial velocity data showing periodic variations were fitted with Keplerian orbits using a χ² minimization technique.

Results. We report the presence of two substellar companions to the K giant stars τ Gem and 91 Aqr. The brown dwarf orbiting τ Gem has an orbital period of 305.5 ± 0.1 days, a minimum mass of 20.6 M_J, 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_J, 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.