Discovery of a Substellar Companion to the K2 III Giant ɩ Draconis

Sabine Frink, University of California - San Diego
David Mitchell, University of California - San Diego
Andreas Quirrenbach, University of California - San Diego
Debra A. Fischer, University of California - Berkeley
Geoffrey W. Marcy, University of California - Berkeley
R. Paul Butler, Carnegie Institute of Washington

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Copyright © 2002 IOS Publishing. The definitive version is available at http://dx.doi.org/10.1086/341629.

NOTE: At the time of publication, the author David Mitchell was not yet affiliated with Cal Poly.

Abstract

We report precise radial velocity measurements of the K giant ɩ Dra (HD 137759, HR 5744, HIP 75458), carried out at Lick Observatory, which reveal the presence of a substellar companion orbiting the primary star. A Keplerian fit to the data yields an orbital period of about 536 days and an eccentricity of 0.70. Assuming a mass of 1.05 M8 for ɩ Dra, the mass function implies a minimum companion mass m₂ sin i of 8.9 M_J, making it a planet candidate. The corresponding semimajor axis is 1.3 AU. The nondetection of the orbital motion by Hipparcos allows us to place an upper limit of 45 M_J on the companion mass, establishing the sub-stellar nature of the object. We estimate that transits in this system could occur already for inclinations as low as 81.5^o, as a result of the large diameter of the giant star. The companion to ɩ Dra is the first brown dwarf or planet found to orbit a giant rather than a main-sequence star.