Substellar Companions to Seven Evolved Intermediate-Mass Stars

Abstract

We report on the detections of substellar companions orbiting around seven evolved intermediate-mass stars from precise Doppler measurements at Okayama Astrophysical Observatory. $o$ UMa (G4 II-III) is a giant with a mass of 3.1 $M_{\odot}$ , and hosts a planet with a minimum mass of $m_2$ sin $i$$=$ 4.1 $M_{\rm J}$ in an orbit with a period $P$$=$ 1630 d and an eccentricity $e$$=$ 0.13. This is the first planet candidate ($\lt $ 13 $M_{\rm J}$ ) ever discovered around a star more massive than 3 $M_{\odot}$ . $o$ CrB (K0 III) is a 2.1 $M_{\odot}$ giant, and has a planet of $m_2$ sin $i$$=$ 1.5 $M_{\rm J}$ in a 187.8 d orbit with $e$$=$ 0.19. This is one of the least-massive planets ever discovered around an $\sim$ 2 $M_{\odot}$ star. HD 5608 (K0 IV) is an 1.6 $M_{\odot}$ subgiant hosting a planet of $m_2$ sin $i$$=$ 1.4 $M_{\rm J}$ in a 793 d orbit with $e$$=$ 0.19. The star also exhibits a linear velocity trend, suggesting the existence of an outer, more massive companion. 75 Cet (G3 III:) is a 2.5 $M_{\odot}$ giant hosting a planet of $m_2$ sin $i$$=$ 3.0 $M_{\rm J}$ in a 692 d orbit with $e$$=$ 0.12. The star also shows a possible additional periodicity of about 200 d and 1880 d with a velocity amplitude of $\sim$ 7–10 m s$^{-1}$ , although these are not significant at this stage. $\nu $ Oph (K0 III) is a 3.0 $M_{\odot}$ giant, and has two brown-dwarf companions of $m_2$ sin $i$$=$ 24 $M_{\rm J}$ and 27 $M_{\rm J}$ , in orbits with $P$$=$ 530.3 d and 3190 d, and $e$$=$ 0.126 and 0.17, respectively, which were independently announced by Quirrenbach, Reffert, and Bergmann (2011, AIP Conf. Proc. 1331, 102). The ratio of the periods is close to 1:6, suggesting that the companions are in mean motion resonance. We also independently confirmed planets around $\kappa $ CrB (K0 III-IV) and HD 210702 (K1 IV), which were announced by Johnson et al. (2008, ApJ, 675, 784) and Johnson et al. (2007a, ApJ, 665, 785), respectively. All of the orbital parameters we obtained are consistent with the previous results.

Precise radial velocities of giant stars

“…Orbital values for the companions to Pollux and ν Oph are taken from our data, since it is the most extensive RV data set for these stars, with the best available orbital fits. There are, however, other published values (Hatzes et al 2006;Sato et al 2012). In particular, the alternate published value for the companion to Pollux lists an eccentricity of 0.02 ± 0.03, as opposed to our current best fit value of 0.05 ± 0.01.…”

Precise radial velocities of giant stars

“…Based on our Lick data, two brown dwarf companions were first discovered by Quirrenbach et al (2011) and later confirmed by Sato et al (2012). The Keplerian nature of the optical velocity data was generally accepted, since other phenomena intrinsic to the star are very unlikely to be responsible for the Doppler signal.…”

Homogeneous spectroscopic parameters for bright planet host stars from the northern hemisphere

“…omi CrB: Sato et al (2012) reported a 1.5 M jup giant planet orbiting around omi CrB, assuming a stellar mass of 2.13 M . With our mass determination of 1.07 M we put the planet at a subJupiter mass class with only 0.65 M jup .…”