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Pleiades Binary Fraction Revisited

Dmitry Chulkov

Abstract

One of the nearest and best studied open clusters, Pleiades is an important cornerstone of stellar astrophysics. Despite its role as reference coeval stellar population, its multiplicity properties remain vaguely determined. The combined use of Gaia DR3 multiband photometry, astrometric parameter RUWE, non-single star solutions along with available ground-based spectroscopic, high angular resolution, and polarimetric observations enable more robust constraints on the binary star population in the cluster. Several conclusions may have broader implications for other stellar populations. Twin binaries, with mass ratio close to $q\sim 1$, tend to have lower RUWE, increasing their membership selection probability, relative to $q\sim 0.5$ systems that are disfavored. The frequently observed peak in mass ratio distribution for $q\sim 1$ binaries may be partially attributed to this bias. Photometrically fitted mass ratio is underestimated for double-lined spectroscopic binaries in agreement with other authors. Differential extinction photometrically mimics stellar binarity. An area of enlarged absorption is traced by increased polarization south of the Merope star and excluded from the analysis to avoid this bias. The fraction of systems with $q>0.6$ companions is measured to be $f=16.4\%^{+2.6}_{-0.6}$ for $m>0.5~M_\odot$ stars, which is larger than recent Gaia-based estimates, but compatible with the pre-Gaia values for Pleiades and the field population. Binary fraction shows no steady increase with stellar mass in the 0.5 $-$ 1.2 $M_\odot$ range, while mass ratio has a bimodal distribution with a minimum near $q\sim 0.7$.

Pleiades Binary Fraction Revisited

Abstract

One of the nearest and best studied open clusters, Pleiades is an important cornerstone of stellar astrophysics. Despite its role as reference coeval stellar population, its multiplicity properties remain vaguely determined. The combined use of Gaia DR3 multiband photometry, astrometric parameter RUWE, non-single star solutions along with available ground-based spectroscopic, high angular resolution, and polarimetric observations enable more robust constraints on the binary star population in the cluster. Several conclusions may have broader implications for other stellar populations. Twin binaries, with mass ratio close to , tend to have lower RUWE, increasing their membership selection probability, relative to systems that are disfavored. The frequently observed peak in mass ratio distribution for binaries may be partially attributed to this bias. Photometrically fitted mass ratio is underestimated for double-lined spectroscopic binaries in agreement with other authors. Differential extinction photometrically mimics stellar binarity. An area of enlarged absorption is traced by increased polarization south of the Merope star and excluded from the analysis to avoid this bias. The fraction of systems with companions is measured to be for stars, which is larger than recent Gaia-based estimates, but compatible with the pre-Gaia values for Pleiades and the field population. Binary fraction shows no steady increase with stellar mass in the 0.5 1.2 range, while mass ratio has a bimodal distribution with a minimum near .

Paper Structure

This paper contains 27 sections, 5 equations, 11 figures, 1 table.

Table of Contents

  1. Introduction
  2. Cluster members
  3. Spectroscopic binaries
  4. RUWE
  5. Binarity indicator
  6. Forward modeling of RUWE
  7. RUWE behavior
  8. Impact on cluster member selection
  9. Gaia DR3 NSS solutions
  10. Color-magnitude diagram analysis
  11. Subsample with reliable Gaia data
  12. Binary stars in the CMD
  13. Resolved systems
  14. Validation for SB2 systems
  15. Sources with large photometric residuals
  16. ...and 12 more sections

Figures (11)

  • Figure 1: Reported orbital periods and RUWE values for spectroscopic systems. RUWE $<1.2$ cutoff is used in Section \ref{['subsample']} to select sources with reliable astrometric solutions.
  • Figure 2: Left: RUWE for resolved sources depending on separation and mass ratio. When both components appear in Gaia DR3, RUWE of the brighter star is shown. Middle: RUWE for resolved sources with $\rho<1\arcsec$; observational data are from 2025AJ....169..145C. Right: RUWE for sources without any resolved companions within $4\arcsec$ depending on $G$ magnitude.
  • Figure 3: Prediction of RUWE values for Pleiades stars with the GaiaUnlimited package (2024AA...688A...1C, Section \ref{['Unlimited']}). The RUWE increase for marginally resolved sources that affects binaries with $P\gtrsim 20$ years is not considered.
  • Figure 4: Left: orbital periods according to Gaia DR3 nss solutions (Section \ref{['nss']}) and ground-based estimates from 2021ApJ...921..117T. The error bars are smaller than symbol sizes. Right: parallaxes from the main source catalog and nss solutions.
  • Figure 5: Pleiades stars in the CMD using Gaia (top) and 2MASS (bottom) photometric data. Apparent and absolute magnitudes are shown in the left and right panels, respectively. Extinction correction is applied to the model curves when deriving absolute magnitudes. Isolated sources are defined as those with a single Gaia counterpart within $4\arcsec$. A separate threshold of RUWE $<1.2$ is adopted (Section \ref{['subsample']}). For Atlas, a distance estimate of 136.2 pc is used 2025ApJ...990..107T.
  • ...and 6 more figures