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

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 Thuringer 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 coude 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 ms(-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, f ( m) = ( 2.05 +/- 0.24) x 10(-7) M circle dot. From our high resolution spectra we calculate a metallicity of - 0.25 +/- 0.05 and derive a stellar mass of 1.23 M circle dot +/- 0.15 for the host star. The K giant star 4 UMa hosts a substellar companion with minimum mass M sin i = 7.1 +/- 1.6 M-Jupiter.