Low-mass spectroscopic binaries in the Hyades: a candidate brown dwarf companion

We have used the HIRES echelle spectrograph on the Keck I telescope to obta in high-resolution spectroscopy of 51 late-type M dwarfs in the Hyades clus ter. Cross-correlating the calibrated data against spectra of white dwarfs allows us to determine heliocentric velocities with an accuracy of +/- 0.3 km s(-1). 27 stars were observed at two epochs in 1997; two stars, RHy 42 a nd RHy 403, are confirmed spectroscopic binaries. RHy 42 is a double-lined, equal-mass system; RHy 403 is a single-lined, short-period binary, P simil ar to 1.275 d. RHy 403A has an absolute magnitude of M-I=10.85, consistent with a mass of 0.15 M.. The systemic mass function has a value [M-2 sin(i)] (3)/(M-1+M-2)(2)=0.0085, which, combined with the non-detection of a second ary peak in the cross-correlation function, implies 0.095 > M-2> 0.07 M., a nd the strong possibility that the companion is the first Hyades brown dwar f to be identified. Unfortunately, the maximum expected angular separation in the system is only similar to 0.25 mas. Five other low-mass Hyads are id entified as possible spectroscopic binaries, based either on repeat observa tions or on a comparison between the observed radial velocity and the value expected for Hyades cluster members. Combined with HST imaging data, we in fer a binary fraction between 23 and 30 per cent. All of the stars are chro mospherically active. RHy 281 was caught in mid-flare and, based on that de tection, we estimate a flaring frequency of similar to 2.5 per cent for low -mass Hyades stars. Nine stars have rotational velocities, v sin(i), exceed ing 20 km s(-1), and most of the sample have detectable rotation. We examin e the H alpha emission characteristics of low-mass cluster members, and sho w that there is no evidence for a correlation with rotation.