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 obtain high‐resolution spectroscopy of 51 late‐type M dwarfs in the Hyades cluster. 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 and 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 ∼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 secondary peak in the cross‐correlation function, implies 0.095> M 2 >0.07 M ⊙ , and the strong possibility that the companion is the first Hyades brown dwarf to be identified. Unfortunately, the maximum expected angular separation in the system is only ∼0.25 mas. Five other low‐mass Hyads are identified as possible spectroscopic binaries, based either on repeat observations or on a comparison between the observed radial velocity and the value expected for Hyades cluster members. Combined with HST imaging data, we infer a binary fraction between 23 and 30 per cent. All of the stars are chromospherically active. RHy 281 was caught in mid‐flare and, based on that detection, we estimate a flaring frequency of ∼2.5 per cent for low‐mass Hyades stars. Nine stars have rotational velocities, v  sin( i ), exceeding 20 km s −1 , and most of the sample have detectable rotation. We examine the H α emission characteristics of low‐mass cluster members, and show that there is no evidence for a correlation with rotation.