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

Guillermo Torres, Robert P. Stefanik, David W. Latham

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.

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

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 , 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 days is determined to be %, 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 value of 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 km s 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.
Paper Structure (20 sections, 4 equations, 36 figures)

This paper contains 20 sections, 4 equations, 36 figures.

Table of Contents

  1. Introduction
  2. Sample
  3. Observations
  4. Radial Velocities
  5. Orbital Solutions
  6. The Griffin velocity zero-point
  7. Comparison with Gaia DR3 orbits
  8. RV variability
  9. Membership
  10. Binarity
  11. Completeness
  12. The Multiplicity Fraction in the Hyades
  13. Distribution of binary properties
  14. The Period Distribution
  15. The Eccentricity Distribution
  16. ...and 5 more sections

Figures (36)

  • Figure 1: Mass-luminosity relation for the Hyades in the visual band. Empirical determinations are shown for vB 96, vB 304, and vB 185, as labeled. Circles represent other previously published dynamical mass determinations in the cluster. The lines show model isochrones from the PARSEC v1.2S series Chen:2014 and the MIST series Choi:2016.
  • Figure 2: Top: Measured positions of the secondary of vB 149 relative to the primary (plus sign) on the plane of the sky. Periastron is indicated with a square labeled "P", and the dotted line represents the line of nodes. The letter "$\Omega$" marks the ascending node, at which the secondary is receding from the observer. Middle and bottom: Position angles and separations as a function of time, shown with our model.
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  • ...and 31 more figures