Long-term Spectroscopic Survey of the Hyades Cluster: The Binary Population

TL;DR

This long-term spectroscopic survey of the Hyades cluster provides detailed data on its binary star population, orbital characteristics, and internal kinematics, revealing a higher binary fraction and insights into tidal circularization and stellar motions.

Contribution

The study offers the first comprehensive, multi-decade spectroscopic analysis of Hyades binaries, updating orbital solutions and analyzing the cluster's binary fraction, period distribution, and internal velocity dispersion.

Findings

Binary fraction in Hyades is 40% for periods up to 10^4 days.

Orbital period and eccentricity distributions are similar to field binaries.

Tidal circularization period is approximately 5.9 days.

Abstract

We report the results of a radial velocity monitoring program in the Hyades region, carried out at the Center for Astrophysics over a period of more than 45 yr. Nearly 12,000 spectra were gathered for 625 stars brighter than $V \approx 14.5$, of which 55% are members or possible members of the cluster. New or updated spectroscopic orbital solutions are presented for more than 100 members and non-members, including several triple systems. In a few cases we incorporate available astrometry. The frequency of binaries in the Hyades with periods up to $10^4$ days is determined to be $40 \pm 5$%, after corrections for incompleteness. This is marginally higher than in other open clusters. The orbital period and eccentricity distributions are found to be similar to those of solar-type binaries in the field. The mass ratio distribution is essentially flat, or slightly rising toward mass ratios of unity. We revisit the determination of the tidal circularization period, obtaining a longer $P_{\rm circ}$ value of $5.9 \pm 1.1$ days compared to the previous estimate of 3.2 days, still somewhat short of the value expected if most or all of the action of tides happens during the pre-main-sequence phase. We estimate a line-of-sight velocity dispersion of $0.21 \pm 0.05$ km s$^{-1}$ within 5.5 pc of the cluster center (approximately the half-mass radius) and a larger dispersion beyond that distance. Our velocity measurements are accurate enough to clearly reveal the signatures of gravitational redshift and convective blueshift among the dwarfs and giants in the Hyades.