The Pleiades Binary Fraction Revisited

Abstract One of the nearest and best-studied open clusters, the Pleiades is an important cornerstone of stellar astrophysics. Despite its role as a reference coeval stellar population, its multiplicity properties are still not well characterized. The combined use of Gaia Data Release 3 multiband photometry, the renormalized unit weight error (RUWE) astrometric parameter, and 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 ratios close to q ∼ 1, tend to have lower RUWE values, increasing their membership selection probability relative to q ∼ 0.5 systems that are disfavored. The frequently observed peak in mass ratio distribution for q ∼ 1 binaries may be partially attributed to this bias. Photometrically fitted mass ratios are 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 &gt; 0.6 companions is measured to be <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>f</mml:mi> <mml:mo>=</mml:mo> <mml:mn>16.4</mml:mn> <mml:msubsup> <mml:mo>%</mml:mo> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2.6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> for m &gt; 0.5 M ⊙ stars, which is larger than recent Gaia-based estimates, but compatible with the pre-Gaia values for Pleiades and the field population. The binary fraction shows no steady increase with stellar mass in the 0.5–1.2 M ⊙ range, while the mass ratio has a bimodal distribution with a minimum near q ∼ 0.7.