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

• Published in: Astronomy and astrophysics (Berlin) 2009-06, Vol.499 (3), p.935-942
• Main Authors: Döllinger, M. P., Hatzes, A. P., Pasquini, L., Guenther, E. W., Hartmann, M., Girardi, L.
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; stars: general; stars: individual: 42 Dra; stars: individual: HD 139 357; stars: late-type; stars: planetary systems; stars: variables: general; techniques: radial velocities
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/200810837

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 ± 0.62) $\times$ 10-8 solar masses ($M_{\odot}$) and an eccentricity of e = 0.38 ± 0.06. From our template spectra, taken without the iodine cell, we determine a metallicity of -0.46 ± 0.04 dex and a stellar mass of 0.98 ± 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 ± 0.02 and mass function, $f(m)$ = (4.79 ± 0.57) $\times$ 10-7 $M_{\odot}$. An iron abundance of -0.13 ± 0.04 dex is obtained, and a stellar mass of 1.31 ± 0.24 $M_{\odot}$ for the parent star is derived. An analysis of the HIPPARCOS photometry as well as our Hα 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 ± 0.85 Jupiter masses $M_{\mathrm{Jup}}$ and 9.76±2.15 $M_{\mathrm{Jup}}$, respectively.