EDP Sciences
Free access
Volume 499, Number 3, June I 2009
Page(s) 935 - 942
Section Planets and planetary systems
DOI http://dx.doi.org/10.1051/0004-6361/200810837
Published online 19 March 2009
A&A 499, 935-942 (2009)
DOI: 10.1051/0004-6361/200810837

Planetary companion candidates around the K giant stars 42 Draconis and HD 139 357

M. P. Döllinger1, A. P. Hatzes2, L. Pasquini1, E. W. Guenther2, M. Hartmann2, and L. Girardi3

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
3  INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy

Received 20 August 2008 / Accepted 11 March 2009

Context. For the past 3 years we have been monitoring 62 K giant stars using precise stellar radial velocity (RV) measurements with the 2 m Alfred Jensch Telescope of the Thüringer Landessternwarte Tautenburg (TLS).
Aims. To probe the dependence of planet formation on stellar mass by finding planets around intermediate-mass giant stars.
Methods. We present high accuracy RV measurements of the K1.5 III star 42 Dra and the K4 III star HD 139357. The wavelength reference for the RV measurements was provided by an iodine absorption cell placed in the optical path of the spectrograph.
Results. Our measurements reveal that the time series of the radial velocity of 42 Dra shows a periodic variation of 479.1 days with a semiamplitude of K = 112.5 m s-1. An orbital solution yields a mass function of f(m) = (5.29 $\pm$ 0.62) $\times$ 10-8 solar masses ($M_{\odot}$) and an eccentricity of e = 0.38 $\pm$ 0.06. From our template spectra, taken without the iodine cell, we determine a metallicity of -0.46 $\pm$ 0.04 dex and a stellar mass of 0.98 $\pm$ 0.06 $M_{\odot}$ for this star. HD 139 357 shows periodic RV variations of 1125.7 days with a semiamplitude K = 159.9 m s-1. An orbital solution yields an eccentricity, e = 0.10 $\pm$ 0.02 and mass function, f(m) = (4.79 $\pm$ 0.57) $\times$ 10-7 $M_{\odot}$. An iron abundance of -0.13 $\pm$ 0.04 dex is obtained, and a stellar mass of 1.31 $\pm$ 0.24 $M_{\odot}$ for the parent star is derived. An analysis of the HIPPARCOS photometry as well as our H$\alpha$ core flux measurements reveal no variability with the radial velocity period. Keplerian motion is the most likely explanation for the observed radial velocity variations for these stars.
Conclusions. The K giant stars 42 Dra and HD 139 357 host extrasolar planets with “minimum masses” of 3.88 $\pm$ 0.85 Jupiter masses MJup and 9.76$\pm$2.15 MJup, respectively.

Key words: stars: general -- stars: variables: general -- stars: individual: 42 Dra -- stars: individual: HD 139 357 -- stars: planetary systems -- techniques: radial velocities -- stars: late-type

© ESO 2009