Low-mass companions to Hyades stars

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany e-mail: guenther@tls-tautenburg.de
² Planetary Systems Branch, Code 693, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
³ McDonald Observatory, University of Texas at Austin, Austin, TX 78712, USA
⁴ Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA, USA
⁵ Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, 7000 East Avenue L-413, Livermore, CA 94551, USA


Received: 9 February 2005
Accepted: 20 July 2005

Abstract

It is now well established that a large fraction of the low-mass stars are binaries or higher order multiples. Similarly a sizable fraction have giant planets. In contrast to these, the situation for brown dwarf companions is complicated: While close systems seem to be extremely rare, wide systems are possibly more common. In this paper, we present new results on a survey for low-mass companions in the Hyades. After measuring precisely the radial velocity of 98 Hyades dwarf stars for 5 years, we have selected all stars that show low-amplitude long-period trends. With AO-observations of these 14 stars we found companion candidates around nine of them, where one star has two companions. The two companions of HIP 16548 have masses between 0.07 to 0.08 , and are thus either brown dwarfs or very low mass stars. In the case of HAN 172 we found a companion with a mass between 0.08 to 0.10 , which is again between a star and a brown dwarf. The other seven stars all have stellar companions. In two additional cases, the RV-variations are presumably caused by stellar activity, and in another case the companion could be a short-period binary. The images of the remaining two stars are slightly elongated, which might imply that even these are binaries. Because at least 12 of the 14 stars showing low-amplitude RV trends turn out to have companions with a mass M_Jupiter, or are just active, we finally estimate the number of companions with masses between 10 M_Jupiter and 70 M_Jupiter within 8 AU of the host stars in the Hyades as ≤2%.