Tidal Synchronization of Binaries in Pleiades

Abstract

Tidal interactions in close binaries play a key role in the long-term rotational and orbital evolution. The distributions of circularization across open clusters (OCs) place strong observational constraints on tidal dissipation in binaries. However, direct observational constraints on synchronization among binaries in OCs remain limited. For the 125 Myr OC Pleiades, this work combines cluster membership from Gaia Data Release 3, rotation periods from the K2 mission, and orbital solutions of the binary population from a long-term spectroscopic survey, to investigate the degree of tidal synchronization in each binary by comparing the pseudo-synchronization period to the rotation period of the primary stars. Among 42 binaries with reliable orbital periods Porb and rotation periods, we identify seven tidally synchronized systems with Porb < 8.6 days, including one early-type system and six late-type systems. For binaries with longer Porb, primaries generally are super-synchronized, and most systems are eccentric. We find a synchronization transition near Porb ~ 8.6-14 days, comparable to the known circularization period (Porb ~ 7.2 days) in the Pleiades, which suggests similar critical period scales for synchronization and circularization in this coeval population. Synchronization depends much more strongly on mass ratio than on primary mass. Most synchronized systems in Pleiades have high mass ratios and are likely to evolve into double white dwarf systems. Tides likely impose strong rotational braking on close early-type binaries, while their influence on late-type close binaries is weaker, and their spins largely follow the single-star sequence.

Figures (5)

• Figure 1: (a) CMD and spatial distribution of the Pleiades members. The Black dashed line is a 125 Myr PARSEC isochrone with $Z$ = 0.0152 Bressan2012. Interstellar extinction has been applied to the isochrone based on an assumed average reddening of $E(B-V)$ = 0.04. Blue pluses denote Pleiades members from the spectroscopic survey of Torres2021. Limegreen open circles represent 48 binaries with reliable orbital solutions from previous literature. Red stars correspond to the close binaries from Table \ref{['t1']} with orbital periods $P_{\rm orb}\leq 10$ days, among which the (pseudo-)synchronous systems are highlighted with black open squares. (b) As panel (a), but for the simulated Pleiades-like cluster. We plot only tidally synchronized binaries with $G_{\rm mag}<15$ mag as red star symbols to facilitate comparison with the observations.
• Figure 2: (a) Relation between $P_{\rm ps}/P_{\rm rot}$ and $P_{\rm orb}$ for 42 binaries in Pleiades. The colors of the plotting symbols indicate each system's orbital eccentricity. The horizontal black dashed line marks $P_{\rm ps} = P_{\rm rot}$, the synchronization line where systems are synchronized or pseudo-synchronized. The inset highlights the (pseudo-)synchronized binaries. (b) As panel (a), but for the simulated Pleiades-like cluster.
• Figure 3: (a) Relation between eccentricity and $P_{\rm orb}$ for 42 Pleiades binaries. The green curve represents the circularization function of Meibom2005 fitted to FGK systems, resulting in a circularization period $P_{\rm circ} = 7.2 \pm 1.0$ days indicated by the black dashed line. Red stars correspond to the (pseudo-)synchronized binaries. (b) As panel (a), but for the simulated Pleiades-like cluster. The resulting circularization period $P_{\rm circ} = 1.8 \pm 0.2$ days.
• Figure 4: (a) Diagram of $P_{\rm ps}/P_{\rm rot}$ versus $(G_{BP}-G_{RP})_0$. Points are color-coded by their $P_{\rm orb}$. Red star symbols mark (pseudo-)synchronized systems. (b) As panel (a), but for the simulated Pleiades-like cluster.
• Figure 5: Diagram of $P_{\rm rot}$ versus $(G_{BP}-G_{RP})_0$ of Pleiades, highlighting 42 binaries with colors representing their $P_{\rm orb}$. The three curves represent the stellar rotation models of Amard2019 for low-mass single stars at an age of 125 Myr, with three different initial rotation rates (slow, median, and fast) labeled. Red star symbols and background gray points mark (pseudo-)synchronized binaries and single-star candidates of the Pleiades, respectively.