Discovery of a planet around the K giant star 4 Ursae Majoris^*

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: mdoellin@eso.org.de
² Max-Planck-Institut für Astrophysik, Garching bei München, Germany
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ INAF-Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁵ Dipartimento di Fisica “E.R. Caianiello”, Universita di Salerno, via S. Allende, 84081 Baronissi (Salerno), Italy

Received: 20 December 2006
Accepted: 2 March 2007

Abstract

For the past 3 years we have been monitoring a sample of 62 K giant stars using precise stellar radial velocity measurements with high accuracy taken at the Thüringer Landessternwarte Tautenburg. We search for sub-stellar companions to giant stars and try to understand the nature of the diverse radial velocity variations exhibited by K giant stars. We present precise stellar radial velocity measurements of the K1III giant star 4 UMa (HD 73108). These were obtained using the coudé echelle spectrograph of 2-m Alfred Jensch Telescope. The wavelength reference for the radial velocity measurements was provided by an iodine absorption cell. Our measurements reveal that the radial velocity of 4 UMa exhibits a periodic variation of 269.3 days with a semiamplitude K = 216.8 m s^-1. A Keplerian orbit with an eccentricity, e = 0.43 ± 0.02 is the most reasonable explanation for the radial velocity variations. The orbit yields a mass function, . From our high resolution spectra we calculate a metallicity of -0.25 ± 0.05 and derive a stellar mass of 1.23 for the host star. The K giant star 4 UMa hosts a substellar companion with minimum mass .