Planetary companions around the K giant stars 11 Ursae Minoris and HD 32518^*

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: mdoellin@eso.org.de
² Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany

Received: 22 January 2009
Accepted: 10 August 2009

Abstract

Context. 11 UMi and HD 32518 belong to a sample of 62 K giant stars that has been observed since February 2004 using the 2m Alfred Jensch telescope of the Thüringer Landessternwarte (TLS) to measure precise radial velocities (RVs).

Aims. The aim of this survey is to investigate the dependence of planet formation on the mass of the host star by searching for planetary companions around intermediate-mass giants.

Methods. An iodine absorption cell was used to obtain accurate RVs for this study.

Results. Our measurements reveal that the RVs of 11 UMi show a periodic variation of 516.22 days with a semiamplitude of K = 189.70 m s^-1. An orbital solution yields a mass function of  = (3.608 ± 0.441)  10^-7 solar masses () and an eccentricity of e = 0.083 ± 0.03. The RV curve of HD 32518 shows sinusoidal variations with a period of 157.54 days and a semiamplitude of K = 115.83 m s^-1. An orbital solution yields an eccentricity, e = 0.008 ± 0.03 and a mass function,  = (2.199 ± 0.235)  10^-8 . The HIPPARCOS photometry as well as our Hα core flux measurements reveal no variability with the RV period. Thus, Keplerian motion is the most likely explanation for the observed RV variations for both giant stars.

Conclusions. An exoplanet with a “minimum mass” of m sin i = 10.50 ± 2.47 Jupiter masses () orbits the K giant 11 UMi. The K1 III giant HD 32518 hosts a planetary companion with a “minimum mass” of m sin i = 3.04 ± 0.68  in a nearly circular orbit. These are the 4th and 5th planets published from this TLS survey.