A planetary companion around the K giant ϵ Corona Borealis^⋆

¹ Korea Astronomy and Space Science Institute, 776, Daedeokdae-Ro, Youseong-Gu, 305-348 Daejeon, Korea
e-mail: bclee@kasi.re.kr; iwhan@kasi.re.kr; kmkim@kasi.re.kr
² Department of Astronomy and Atmospheric Sciences, Kyungpook National University, 702-701 Daegu, Korea
e-mail: mgp@knu.ac.kr
³ National Astronomical Research Institute of Thailand, 50200 Chiang Mai, Thailand
⁴ Crimean Astrophysical Observatory, Nauchny, 98409 Crimea, Ukraine
e-mail: davidmkrt@gmail.com

Received: 5 April 2012
Accepted: 31 August 2012

Abstract

Aims. Our aim is to search for and study the origin of the low-amplitude and long-periodic radial velocity (RV) variations in K giants.

Methods. We present high-resolution RV measurements of K2 giant ϵ CrB from February 2005 to January 2012 using the fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO).

Results. We find that the RV measurements for ϵ CrB exhibit a periodic variation of 417.9 ±  0.5 days with a semi-amplitude of 129.4 ± 2.0 m s^-1. There is no correlation between RV measurements and chromospheric activity in the Ca II H region, the Hipparcos photometry, or bisector velocity span.

Conclusions. Keplerian motion is the most likely explanation, with the RV variations arising from an orbital motion. Assuming a possible stellar mass of 1.7 ± 0.1 M_⊙ for ϵ CrB, we obtain a minimum mass for the planetary companion of 6.7 ± 0.3 M_Jup with an orbital semi-major axis of 1.3 AU and eccentricity of 0.11. We also discuss the implications of our observations for stellar metallicity versus planet occurrence rate and stellar mass versus planetary mass relations.