Tidal Synchronization of Binaries in Pleiades

Draft version Februar y 27, 2026 Typeset using L A T E X tw o column style in AAST eX7.0.1 Tidal Sync hronization of Binaries in Pleiades Li W ang ( 王 莉 ) , 1, 2 Chenyu He ( 贺 辰 昱 ) , 1, 2 Chengyuan Li ( 李 程 远 ) , 1, 2 and Gang Li ( 李 刚 ) 3 1 Scho ol of Physics and Astr onomy, Sun Y at-sen University, Daxue R oad, Zhuhai, 519082, China 2 CSST Scienc e Center for the Guangdong-Hong Kong-Macau Gre ater Bay Ar e a, Zhuhai, 519082, China 3 Centr e for Astr ophysics, University of Southern Que ensland, T o owo omb a, QLD 4350, Austr alia ABSTRA CT Tidal interactions in close binaries play a key role in the long-term rotational and orbital evolution. The distributions of circularization across op en clusters (OCs ) place strong observ ational constraints on tidal dissipation in binaries. Ho wev er, direct observ ational constraints on synchronization among binaries in OCs remain limited. F or the 125 Myr OC Pleiades, this work com bines cluster mem b ership from Gaia Data Release 3 , rotation p erio ds from the K2 mission, and orbital solutions of the binary p opulation from a long-term sp ectroscopic surv ey , to inv estigate the degree of tidal s ync hronization in eac h binary by comparing the pseudo-synchronization p erio d to the rotation p erio d of the primary stars. Among 42 binaries with reliable orbital p erio ds P orb and rotation p erio ds, we identify seven tidally synchronized systems with P orb ≲ 8 . 6 da ys, including one early-t ype system and six late-type systems. F or binaries with longer P orb , primaries generally are sup er-synchronized, and most systems are eccen tric. W e find a sync hronization transition near P orb ≈ 8 . 6 − 14 da ys, comparable to the kno wn circularization p erio d ( P orb ≈ 7 . 2 da ys) in the Pleiades, whic h suggests similar critical p erio d scales for synchronization and circularization in this co ev al p opulation. Synchronization dep ends muc h more strongly on mass ratio than on primary mass. Most sync hronized systems in Pleiades hav e high mass ratios and are likely to ev olve into double white dwarf systems. Tides likely imp ose strong rotational braking on close early-t yp e binaries, while their influence on late-t yp e close binaries is weak er, and their spins largely follow the single-star sequence. Keywor ds: Op en star clusters (1160) — Close binary stars (254) — Sp ectroscopic binary stars (1557) — Eclipsing binary stars (444) — Stellar Rotation (1629) 1. INTRODUCTION More than half of all stars reside in binary or multiple systems, making binary evolution a central topic in mod- ern astroph ysics ( H. A. Abt & S. G. Levy 1976 ). In close binaries, tidal interactions dissipate angular momentum and energy and drive the system tow ard a minimum- energy , maxim um-entrop y equilibrium. The exp ected outcomes include spin-orbit alignmen t, sync hronization of stellar rotation with orbital motion, and orbital cir- cularization ( P . Hut 1980 ). This tide-driv en evolution links binary ph ysics to stellar p opulation statistics and is k ey to understanding internal stellar structure, angular- momen tum loss processes, and the long-term ev olution of binary systems. Accurate measurements of tidal ef- Corresponding author: Li W ang wangli79@mail2.sysu.edu.cn ficiencies and timescales are therefore essential for b oth theory and observ ation. The physics of tidal dissipation in binary stars has b een debated for several decades, resulting in tw o rep- resen tative theoretical framew orks: equilibrium tides and dynamical tides (see review of G. I. Ogilvie 2014 ). The equilibrium tides correspond to large-scale non- oscillatory tidal flows. In this picture, dissipation in late-t yp e stars arises from turbulent friction acting in con vectiv e en v elop es, with additional radiative damp- ing relev ant for early-t yp e stars (e.g., J. P . Zahn 1977 , 1989 ). This mec hanism is efficient in stars that p ossess outer conv ectiv e en v elop es ( J. P . Zahn 1989 ; J.-P . Zahn & L. Bouchet 1989 ). By contrast, dynamical tides treat the dissipation of small-scale tidally-forced oscillations: in ternal gra vity mo des excited in radiativ e regions and damp ed by radiative diffusion (e.g., J. P . Zahn 1975 , 1977 ; J. Go o dman & E. S. Dickson 1998 ; C. T erquem et al. 1998 ), and rotationally supp orted inertial wa ves 2 excited in conv ective en velopes and damp ed by viscos- it y (e.g., Y. W u 2005 ; G. I. Ogilvie & D. N. C. Lin 2007 ; J. Go o dman & C. Lackner 2009 ). Both mec hanisms re- mo ve orbital energy and angular momentum, leading to secular changes in orbital p erio d, eccentricit y , and stel- lar rotation. Tides generally circularize orbits and drive eac h star tow ard pseudo-synchronization b et ween spin and orbital motion ( P . Hut 1981 ). How ever, dynamical tides asso ciated with self-excited stellar oscillations do not necessarily lead to sync hronization; instead, they can drive in verse tides, in which angular momentum is transferred from the star to the orbit, pushing the system aw a y from spin–orbit sync hronization ( J. F uller 2021 ). Using a direct-solution treatmen t of dynamical tides implemented in GYRE-tides, M. Sun et al. ( 2023 ) and R. H. D. T ownsend & M. Sun ( 2023 ) sho w ed that tidal torques in radiative stars c hange sign o v er a range of stellar rotation rates, thereby blurring the concept of a unique pseudo-synchronized rotation state. Before the era of time-domain surveys for measuring stellar v ariabilit y , rotational velocities of stars w ere pri- marily obtained through sp ec troscop y , which measures pro jected rotation v elo cities ( v sin i ) inferred from broad- ening of sp ectral lines. v sin i is less suitable for con- straining models of tidal evolution, because of the am- biguities in tro duced by the unkno wn inclination of the rotation axis and non-rotational line broadening from the secondary ( S. Meib om et al. 2006 ). The adv en t of high-precision, long-term photometry , especially Kepler ( W. J. Boruc ki et al. 2010 ), K2 ( S. B. How ell et al. 2014 ), and TESS ( G. R. Rick er et al. 2015 ) surv eys, greatly expanded the n umber of rotation-p erio d measuremen ts ( P rot ). By trac king starsp ot modulation, these missions pro duced unprecedented homogeneous samples of eclips- ing binaries (EBs) for testing tidal theory (e.g. M. K. Zimmerman et al. 2017 ). J. C. Lurie et al. ( 2017 ) sho wed that most late-t yp e EBs in the Kepler field with orbital p erio d P orb < 10 days are tidally synchronized, with P rot ≈ P orb . A systematic searc h of TESS data re- v ealed a similar pattern among low-mass FGKM EBs with J. C. Lurie et al. ( 2017 ): a synchronous p opula- tion with P orb = P rot and a subsync hronous population with 8 P orb ≈ 7 P rot ( M. Hobson-Ritz et al. 2025 ). Plau- sible interpretations for such subsynchronous rotation include differen tial rotation ( J. C. Lurie et al. 2017 ), or a comp etition b etw een tidal dissipation and magnetic braking ( D. P . Fleming et al. 2019 ). Mean while, pul- sations in some EB systems provide a unique prob e of stellar rotation and tidal effects. Using gravit y mo des, G. Li et al. ( 2020 ) measured near-core rotation rates in early-t yp e stars in EBs and found a clear sync hroniza- tion trend, together with several systems exhibiting ex- tremely slow rotation. The presence of extremely slowly rotating stars in short-p erio d EBs blurs the b oundary of pseudo-sync hronization. Precise ages, together with the evolutionary state and history of binaries, are essential for measuring the pace of tidal sync hronization. Co ev al populations with well- determined ages are therefore esp ecially v aluable, and y oung op en clusters (OCs) pro vide natural lab oratories with a common age and metallicity but div erse binary prop erties. The observed distributions of circularization across OCs place strong constraints on tidal dissipation (e.g., K. M. P enev & J. A. Sc h ussler 2022 ; A. J. Bark er 2022 ; G. M. Mirouh et al. 2023 ). Beyond constraining the efficiency of tidal dissipation, tides also shap e the rotational evolution of cluster members, which in turn c hanges stellar surface temp eratures and evolutionary lifetimes ( F. D’An tona et al. 2015 ). F. D’Antona et al. ( 2015 ) prop osed that tidally-sync hronized binaries may accoun t for the bimodality of stellar rotation distribu- tion of the split main-sequence (MS) and extended MS turnoff stars in young massiv e clusters. N -b o dy simu- lations further suggest that, among the p opulation of MS binaries in clusters, tidal sync hronization is pre- dominan tly found in systems with near-equal masses ( L. W ang et al. 2023 ). Early observ ational w ork fo cused mainly on the relation b et ween the c haracteristic circu- larization p erio ds P circ and cluster ages ( S. Meib om & R. D. Mathieu 2005 ). In con trast, direct observ ational constrain ts on synchronization among MS binaries in OCs remain limited. S. Meib om et al. ( 2006 ) reported preliminary results for 13 solar-t ype detac hed binaries in M35 (NGC 2168; ∼ 150 Myr) and M34 (NGC 1039; ∼ 250 Myr), finding subsynchronous rotation in close systems. W e still lack a complete picture of how synchronization dep ends on p erio d and mass and of its ov erall impact in cluster en vironmen ts. The Pleiades is an excellent lab oratory for assessing tidal sync hronization in cluster binaries. It is nearby , ric h in members, and has a well-established age of ∼ 125 Myr with robust mem b ership catalogs (e.g., L. Long et al. 2023 ). Rotation p erio ds across a wide range of sp ectral types are av ailable from K2 and TESS (e.g., L. Long et al. 2023 ), pro viding a high signal-to-noise snapshot of how tides interact with stellar spin on the MS at the age of Pleiades. More imp ortan tly , more than 43 y ears of systematic sp ectroscopic monitoring ha ve greatly refined the census of spectroscopic bina- ries (SBs) with reliable orbital solutions in the Pleiades ( G. T orres et al. 2021 ). T aken together, these data com- bine high-quality sp ectroscop y with long-term photom- etry , making the Pleiades an ideal testb ed for empirical tests of tidal synchronization and circularization in a 3 co ev al p opulation. Here we aim to iden tify and charac- terize tidally synchronized binaries in the Pleiades and to compare them with results from N -b o dy simulations incorp orating stellar evolutionary models. This paper is organized as follows: Section 2 describ es the observ ations and the N -bo dy simulation. Section 3 presen ts the main results from the observ ations and the sim ulations in detail, follow ed by a discussion in Section 4 and a summary in Section 5 . 2. DA T A REDUCTION 2.1. Observations W e directly adopt the Pleiades stellar catalog of L. Long et al. ( 2023 ) based on Gaia Data Release 3 (DR3; Gaia Collab oration et al. 2023 ), which provides detailed information on 5D phase-space ( α , δ , µ α cos δ , µ δ , paral- lax) membership probabilities. T o ensure a reliable sam- ple, we retain the high-confidence subsample with mem- b ership probability P ≥ 0 . 9 as cluster members (1664 stars). The spatial distribution and the color-magnitude diagram (CMD) of cluster members are shown in Fig- ure 1 (a). L. Long et al. ( 2023 ) also measured rotation p erio ds for co ol MS stars using K2 photometry ( S. B. Ho well et al. 2014 ), spanning an effective-temperature range of 2800 − 6800 K (roughly corresp onding to dered- dened color ( G B P − G RP ) 0 ≈ 0 . 5 − 4), pro viding a ho- mogenized set of rotation p erio ds for cluster members. Additionally , we supplement this dataset with rotation p erio ds derived from TESS observ ations ( X. Gao et al. 2025 ; D. J. F ritzewski et al. 2025 ). T o inv estigate tidal evolution of Pleiades binaries, we require precise, homogeneous rotation p erio ds of pri- maries paired with reliable orbital parameters of binary systems (e.g., orbital p erio d, eccentricit y). W e therefore rely primarily on the long-baseline sp ectroscopic survey of G. T orres et al. ( 2021 ), whic h combines more than 43 y ears of sp ectroscopic observ ations o ver an area of ∼ 10 ◦ × 10 ◦ across the cluster. This program targets sp ectral types from mid-B to early-M and rep orts precise orbital solutions for 38 identified binaries and higher- order m ultiples, represen ting the most comprehensive SB dataset for the Pleiades to date. In addition, we also supplemen t another 9 SBs and 1 EB confirmed in the lit- erature ( G. Basri & E. L. Mart ´ ın 1999 ; T. J. Da vid et al. 2016 ; G. T orres 2020 ; M. Kounk el et al. 2021 ; Gaia Col- lab oration et al. 2023 ; A. F rasca et al. 2025 ; G. T orres et al. 2025 ). In total, w e collect 48 binaries with se- cure orbital solutions, of which 42 also ha ve trust worth y P rot measuremen ts. This binary sample is dominated b y F GK-type binaries. F o cusing on the short-p erio d regime where tides are strongest, we compile systems with P orb ≤ 10 days in T able 1 and highlight them in Figure 1 (a). The remaining systems with P orb > 10 days are listed in T able 2 in the App endix. Considering only four ob jects in our 48 binary sample are M-type, the M-type binary subsample is clearly in- complete, so w e analyze the completeness lev el only for binaries brighter than the M-t yp e regime (earlier than M-t yp e). Cross-matching the sp ectroscopic targets of G. T orres et al. ( 2021 ) with our membership catalog shows that, within 3 ◦ of the cluster center (46 of our 48 bi- naries lie in this region) and for G mag < 13 mag, the sp ectroscopic monitoring co vered ab out 78% of mem- b ers. W e adopt a simple rule on the CMD: stars bluer and fainter than the binary lo cus with a mass ratio of 0.6, derived from the best-fitting P ARSEC iso chrone for the Pleiades (125 Myr, Z = 0.0152, E ( B − V ) = 0.04; A. Bressan et al. 2012 ), are treated as singles, while the others are treated as binaries. With this classification, the surv ey of G. T orres et al. ( 2021 ) shows no strong mass ratio bias, with cov erage of singles and binaries of ab out 72% and 76%, resp ectively . W e take this sp ec- troscopic completeness of members as a pro xy for the completeness of our binary sample. F or the brightest close binary listed in T able 1 , HI I 1431 (EPIC 211082420), no rotation p erio d has b een rep orted in the literature. HI I 1431 is a double-lined SB (SB2) identified by G. T orres et al. ( 2021 ). In- sp ection of its K2 light curve sho ws an eclipsing bi- nary with mild ellipsoidal v ariations but no significan t out-of-eclipse starsp ot mo dulation, precluding a direct photometric determination of P rot . As an alternative, w e indirectly estimated its P rot from the pro jected ro- tation v elo city v sin i . Using the effective temp eratures and pro jected rotational v elo cities for the primary and secondary components ( T eff , p = 10450 ± 500K, T eff , s = 7700 ± 600K; v sin i p = 36 ± 3km / s, v sin i s = 30 ± 4km / s) deriv ed from spectroscopic fitting by G. T orres et al. ( 2021 ), we inferred the comp onent masses and radii ( M p = 2 . 36 ± 0 . 18 M ⊙ , R p = 2 . 14 ± 0 . 09 R ⊙ , M s = 1 . 66 ± 0 . 14 M ⊙ , R s = 1 . 71 ± 0 . 15 R ⊙ ) by in terp olating the b est-fitting 125 Myr P ARSEC isochrone of Pleiades ( A. Bressan et al. 2012 ). Com bining these with the sp ectroscopic orbital solution for this SB2 from G. T or- res et al. ( 2021 ) ( M p sin 3 i = 2 . 14 ± 0 . 01 M ⊙ , M s sin 3 i = 1 . 50 ± 0 . 01 M ⊙ ), and assuming the system has ac hiev ed spin-orbit alignment ( i rot ≃ i orb ), we deriv e rotation p e- rio ds of 2 . 90 ± 0 . 29 days and 2 . 78 ± 0 . 50 da ys for the primary and secondary , resp ectively . As a diagnostic for the degree of tidal synchronization of binary systems, we use the ratio betw een the pre- dicted pseudo-synchronization p erio d and the measured stellar rotation p erio d. The pseudo-synchronization p e- 4 0 1 2 3 4 G B P − G RP [mag] 0 . 0 2 . 5 5 . 0 7 . 5 10 . 0 12 . 5 15 . 0 17 . 5 20 . 0 G [mag] HII 1431 HII 1397 HII 761 DH 794 HII 2407 HCG 489 HCG 495 HII 1286 PPL 15 (a) T orres+2021 48 binaries in this work (pseudo-)synchronized binary P orb < 10 days 50 55 60 65 RA [deg] 17 . 5 20 . 0 22 . 5 25 . 0 27 . 5 30 . 0 32 . 5 DEC [deg] B5 A0 A5 F0 F5 G2 K0 K5 M0 M5 0 1 2 3 4 G B P − G RP [mag] 0 . 0 2 . 5 5 . 0 7 . 5 10 . 0 12 . 5 15 . 0 17 . 5 20 . 0 G [mag] (b) (pseudo-)synchronized binary 50 55 60 65 RA [deg] 17 . 5 20 . 0 22 . 5 25 . 0 27 . 5 30 . 0 32 . 5 DEC [deg] B5 A0 A5 F0 F5 G2 K0 K5 M0 M5 Figure 1. (a) CMD and spatial distribution of the Pleiades members. The Blac k dashed line is a 125 Myr P ARSEC iso chrone with Z = 0.0152 ( A. Bressan et al. 2012 ). In terstellar extinction has been applied to the iso chrone based on an assumed a v erage reddening of E ( B − V ) = 0.04. Blue pluses denote Pleiades members from the sp ectroscopic surv ey of G. T orres et al. ( 2021 ). Limegreen op en circles represen t 48 binaries with reliable orbital solutions from previous literature. Red stars corresp ond to the close binaries from T able 1 with orbital perio ds P orb ≤ 10 da ys, among whic h the (pseudo-)sync hronous systems are highlighted with black open squares. (b) As panel (a), but for the simulated Pleiades-lik e cluster. W e plot only tidally synchronized binaries with G mag < 15 mag as red star symbols to facilitate comparison with the observ ations. rio d P ps is giv en b y equation (42) of P . Hut ( 1981 ): P ps = (1 + 3 e 2 + 3 8 e 4 )(1 − e 2 ) 3 2 1 + 15 2 e 2 + 45 8 e 4 + 5 16 e 6 P orb , (1) where e is the eccentricit y and P orb is the orbital p e- rio d. Because the orbital angular velocity v aries o ver an eccentric orbit, exact spin-orbit synchronization is unattainable. Instead, P . Hut ( 1981 ) sho wed that the system ev olves tow ard a stable equilibrium in which the orbit-a veraged tidal torque v anishes. This o ccurs when the stellar spin angular velocity equals the in- stan taneous orbital angular velocity at p eriastron (the pseudo-sync hronous state), for which the rotation p e- rio d equals P ps . F or a binary with a circular orbit, P ps = P orb . P ps /P rot ≈ 1 thus indicates that the bi- nary approaches sync hronization in a circular orbit or pseudo-sync hronization in an eccentric orbit. 2.2. N -b o dy Simulation T o in vestigate the prop erties of tidal binary p opu- lations within the Pleiades, we run a realistic numeri- cal simulation using the high-p erformance N -b o dy co de PETAR . PETAR allows us to efficiently mimic the ev olution of a massiv e stellar system containing up to 10 5 particles with a large fraction of binaries, up to unit y ( L. W ang et al. 2020 , 2022 ). T o accurately follo w the dynami- cal and stellar ev olution of b oth single stars and binary systems, the recen tly up dated single and binary stellar ev olution co des, SSE and BSE ( C. A. T out et al. 1997 ; J. R. Hurley et al. 2000 , 2002 ; S. Banerjee et al. 2020 ), w ere incorp orated in PETAR to sim ulate wind mass loss, stellar type changes, mass transfer, and binary mergers. The BSE tide prescriptions are actually a simplification of the equilibrium tide from J. P . Zahn ( 1970 , 1975 , 1977 , 1989 ). W e use the newly up dated version of the star clus- ter initial mo del generator co de MCLUSTER ( A. H. W. K ¨ upp er et al. 2011 ; L. W ang et al. 2018 ) to gener- ate a Pleiades-like cluster. W e adopt empirically mo- tiv ated parameters c hosen to repro duce the present-da y n umber-density profile and total mass of the Pleiades. The initial total mass is 1300 M ⊙ , and the initial half- mass radius is 3 p c, including all stellar comp onents in 3D space. The cluster metallicit y is 0.0152. The initial particle masses were randomly sampled from a Kroupa- lik e initial mass function (IMF, P . Kroupa 2001 ) with the mass range of 0.08 − 150 M ⊙ . The 3D p ositions and v elo cities of stars were randomly sampled from the Plummer densit y profile ( S. J. Aarseth et al. 1974 ). The adopted sim ulation model in this w ork is initialized with a 100% primordial binary fraction. The initial orbital 5 T able 1. List of Nine Detac hed Binaries with P orb ≤ 10 days in the Pleiades N Name Gaia DR3 ID G mag P orb P rot e q P ps /P rot Lab el Ref (mag) (da y) (day) 1* HI I 1431 66729880383767168 6.816 2.461127 2.90318 0 0.7018 0.84774 SB2, EB 1 0.003 0.000015 0.29465 ... 0.0021 0.08604 2 HII 1397 65207709611941376 7.287 7.345274 ... 0 ... ... SB1 1 0.003 0.000018 ... ... ... ... 3* HI I 761 65276703968959488 10.393 3.307289 3.158 0 0.698 1.04727 SB2 1, 2, 3 0.003 0.000003 ... ... 0.061 0.03101 4* DH 794 63876402894075904 11.674 5.694369 6.338 0.0119 0.7103 0.89921 SB2 1, 3 0.003 0.000042 1.816 0.0021 ... 0.25765 5* HI I 2407 66747816167123712 11.900 7.050477 7.247 0 0.222 0.97288 SB2, EB 1, 3, 4 0.003 0.000009 1.778 ... 0.033 0.23869 6* HCG 489 65800930496992512 12.520 3.108737 3.054 0 0.9851 1.01792 SB2 1, 3 0.003 0.000012 0.478 ... 0.0034 0.15932 7* HCG 495 66612473157921024 12.604 8.57662 8.881 0.1368 0.982 1.07442 SB2 1, 3 0.003 0.00053 0.532 0.0071 0.012 0.06535 8* HI I 1286 65005262035285888 14.265 2.46234 2.461 0 0.98 1.00054 SB2 2, 3, 5 0.003 0.00008 0.977 ... 0.14 0.39721 9 PPL 15 65000073712701056 19.505 5.825 ... 0.42 0.85 ... SB2 6 0.005 0.3 ... 0.05 0.05 ... Note —The second line for eac h system lists the 1 σ uncertainties. References in the last column are: (1) G. T orres et al. ( 2021 ); (2) M. Kounkel et al. ( 2021 ); (3) L. Long et al. ( 2023 ); (4) T. J. Da vid et al. ( 2015 ); (5) A. F rasca et al. ( 2025 ); (6) G. Basri & E. L. Mart ´ ın ( 1999 ). Ob jects with asterisks after the num b ers are considered as synchronized or pseudo-synchronized binaries. parameter distributions of binary systems imp ose essen- tial constraints on forming tidal binaries. Our simula- tions follow the initial prop erties of binaries describ ed in D. Belloni et al. ( 2017 ). Their numerical sim ula- tions with these assumptions pro vide qualitativ ely sim- ilar go o d descriptions of observ ations of Galactic-field late-t yp e binaries, globular clusters, and OCs. See more details ab out initial setups of primordial binary p opula- tion in our previous work ( L. W ang et al. 2023 ). W e ev olve the syn thetic cluster for ∼ 125 Myr to match the current age of the Pleiades and confirm that the final snapshot reproduces the observ ed 3D num ber-density profile of the Pleiades. In this Pleiades-like cluster mo del, the fraction of solar-t ype stars with P orb < 10 4 da ys (the p erio d cutoff used in G. T orres et al. ( 2021 )) is 28%, consistent with the 25 ± 3% measured in the Pleiades by G. T orres et al. ( 2021 ). F or a direct com- parison with observ ations, the theoretical luminosities and temperatures of the simulated stars were then con- v erted into the Gaia DR3 photometric system using the P ARSEC Bolometric Correction ( YBC , Y. Chen et al. 2019 ). Figure 1 sho ws the CMDs of Pleiades and the syn thetic Pleiades-like cluster. Because our Pleiades bi- nary sample is severely incomplete for M-t ype systems, w e compare w ith the real observ ations only for binaries in the Pleiades-like cluster that hav e G mag < 13 mag (earlier than M) and P orb < 10 4 da ys. The compari- son actually fo cuses on the statistics of solar-type stars, whic h dominate the Pleiades binary sample. A detailed comparison betw een the sim ulated cluster and the ob- serv ations app ears in Section 3 , with further discussion in Section 4 . 3. RESUL TS 3.1. The P ps /P rot − P orb Diagr am The orbital p erio d is the dominan t parameter gov- erning tidal synchronization, as the sync hronization timescale scales approximately as P 6 orb according to P . Hut ( 1981 ). Figure 2 (a) displays the P ps /P rot − P orb dia- gram for 42 Pleiades binaries. Here, we fo cus on the de- gree of tidal sync hronization for the follo wing detached binary systems with orbital perio ds of less than 10 days. HII 1431, HII 761, DH 794, HCG 489, and HII 1286 : SB2 systems with sp ectral types A, G, G, M, and M, re- 6 10 0 10 1 10 2 10 3 10 4 P orb [days] 10 − 1 10 0 10 1 10 2 10 3 10 4 P ps /P rot (a) 2 . 5 5 . 0 7 . 5 10 . 0 0 . 5 1 . 0 1 . 5 HCG 495 0 . 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 0 . 8 Eccentricit y 10 0 10 1 10 2 10 3 10 4 P orb [days] 10 − 1 10 0 10 1 10 2 10 3 10 4 P ps /P rot (b) 1 2 3 0 . 5 1 . 0 1 . 5 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 Eccentricit y Figure 2. (a) Relation b etw een P ps /P rot and P orb for 42 binaries in Pleiades. The colors of the plotting sym bols indicate each system’s orbital eccentricit y . The horizontal black dashed line marks P ps = P rot , the synchronization line where systems are sync hronized or pseudo-synchronized. The inset highlights the (pseudo-)synchronized binaries. (b) As panel (a), but for the sim ulated Pleiades-like cluster. 10 0 10 1 10 2 10 3 10 4 P orb [da ys] 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 Eccen tricity HCG 495 (a) 10 0 10 1 10 2 10 3 10 4 P orb [da ys] 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 Eccen tricity (b) Figure 3. (a) Relation b etw een eccen tricity and P orb for 42 Pleiades binaries. The green curve represents the circularization function of S. Meib om & R. D. Mathieu ( 2005 ) fitted to FGK systems, resulting in a circularization p erio d P circ = 7 . 2 ± 1 . 0 da ys indicated b y the black dashed line. Red stars corresp ond to the (pseudo-)sync hronized binaries. (b) As panel (a), but for the simulated Pleiades-like cluster. The resulting circularization p erio d P circ = 1 . 8 ± 0 . 2 days. sp ectiv ely . The close agreement b etw een each system’s orbital p erio d and the primary’s rotation p erio d (T a- ble 1 ) indicates that the primaries are sync hronized with their circular orbits, with P ps /P rot ≈ 1 at the cluster age of 125 Myr. HII 1397 : An A-type single-lined SB (SB1) with a cir- cular orbit of P orb ∼ 7 . 35 da ys ( G. T orres et al. 2021 ). Its companion con tributes negligible flux to the system’s total ligh t, as indicated by the system’s lo cation on the single-star MS in the CMD. G. T orres et al. ( 2021 ) mea- sured pro jected rotational velocities for ob jects in the Pleiades field and rep orted v sin i = 7 ± 3 km/s for HI I 1397 , the slow est for early-type stars with a mean v sin i near 50 km/s. Even after accounting for incli- nation, HI I 1397 remains one of the slo w est rotators in early-t yp e stars, which likely suggests strong tidal brak- ing in such a close binary . Ho wev er, no indep endent rotation p erio d is av ailable, v sin i alone cannot estab- lish spin-orbit sync hronization. W e therefore classify HI I 1397 as a candidate near synchronization p ending a direct determination of P rot or tigh ter constraints on the stellar radius and inclination. T o further compli- cate matters, HI I 1397 is part of a complex quadruple system, accompanied by a fain ter equal-mass SB2 (HI I 1392 listed in T able 2 ) with a 767-da y orbital perio d and a large eccen tricity e = 0.82 ( G. T orres et al. 2021 ; D. Ch ulko v et al. 2025 ). Although the TESS mission col- lected high-precision photometry for this source ( G. R. Ric ker et al. 2015 ), the large pixel size of TESS (21 ′′ ) means the photometry is certainly con taminated b y flux from nearb y sources. The analysis of such a complex, 7 higher-order system is b eyond the scop e of this pap er, whic h fo cuses on individual detached binaries. HII 2407 : Previously identified as a G-type SB1 b y J.-C. Mermillio d et al. ( 1992 ), this system was reclassi- fied as an EB system from K2 photometry ( S. B. Ho well et al. 2014 ). A joint fit of radial velocity and photomet- ric data yielded a full orbital solution and constrain ts on the fundamental stellar parameters ( T. J. David et al. 2015 ). The primary has reached the MS, while the sec- ondary is still on the pre-main sequence (PMS) with a lo w mass ratio of q ∼ 0 . 22, which explains the system’s lo cation on the single-star MS in the CMD. The K2 light curv e exhibits starsp ot mo dulation, which manifests as roughly sin usoidal v ariations caused b y perio dic dips in brigh tness as sp ots rotate in to and out of view. Based on the phase and amplitude evolution of this mo dula- tion, L. Long et al. ( 2023 ) inferred a rotation p erio d of 7.25 days for the primary . Given the circular orbit with P orb ∼ 7 . 05 days, the primary is synchronized with the orbit, with P ps /P rot ≈ 1. HCG 495 : An M-type SB2 with a slightly eccentric or- bit ( e ≈ 0 . 14) and P orb = 8 . 58 da ys, which is the longest p erio d in T able 1 ( G. T orres et al. 2021 ). The primary rotates with P rot = 8 . 88 da ys ( L. Long et al. 2023 ), close to the pseudo-synchronous perio d exp ected for this ec- cen tricity , indicating the pseudo-synchronization state ( P ps /P rot ≈ 1). PPL 15 : This system is the first-kno wn brown dwarf SB rep orted by G. Basri & E. L. Mart ´ ın ( 1999 ). It has an eccentric orbit ( e ≈ 0 . 4) with a p erio d of approxi- mately 5.83 days. G. Basri & E. L. Mart ´ ın ( 1999 ) found that b oth comp onents app ear to hav e the same v sin i of appro ximately 10km/s. This is among the low est rota- tional v elo cities observ ed for v ery lo w mass stars in the Pleiades (even after correction for inclination), where rotational velocities of 25-50 km/s are typical. One p os- sibilit y is that the comp onents ha ve b ee n tidally pseudo- sync hronized, meaning their rotational angular v elocity matc hes the orbital angular velocity of the companion at periastron. Except for HI I 1397 and PPL 15, for whic h the pri- mary rotation p erio ds are unkno wn, the binaries in T a- ble 1 with P orb < 10 days follo w a clear and similar pat- tern. Primaries in circular orbits are synchronized with the orbital motion ( P ps /P rot ≈ 1). The only exception is HCG 495, which has the longest orbital p erio d in this subset (8.58 days), a sligh t eccen tricit y , and is consistent with pseudo-sync hronization. In con trast, systems with P orb > 10 days deviate mark edly from the sync hroniza- tion line in Figure 2 (a), indicating that their primaries are sup er-synchronized. Moreov er, P ps /P rot increases appro ximately linearly with P orb , which suggests that this ratio is set mainly b y the orbital p erio d and tidal torques ha v e a diminishing influence on P rot . In Figure 3 (a), the transition perio d b etw een circular and eccentric orbits for F GK binaries o ccurs at P orb ≈ 7 . 2 ± 1 . 0 days ( G. T orres et al. 2021 ). Notably , Fig- ures 2 (a) and 3 (a) sho w that the transition from pseudo- sync hronized to non-pseudo-synchronized systems falls at nearly the same orbital p erio d. The existence of HCG 495 suggests that the pseudo-synchronization b oundary ( P syn ) lies at a slightly longer orbital p e- rio d than the circularization transition ( P circ ), since it app ears pseudo-sync hronized while its orbit remains sligh tly eccen tric. It is difficult to confirm precisely due to the lack of observ ational samples with orbital p eri- o ds b etw een 8.6 and 14 days. W e further find that the P ps /P rot − P orb distribution is tighter and clearer than the e − P orb distribution, and therefore provides a more effectiv e diagnostic of tidal evolution. As in the observ ations, four binaries with 0 . 9 < P ps /P rot < 1 . 1 in the simulated Pleiades-like cluster are considered as (pseudo-)synchronous systems and are highlighted with red star symbols sho wn in Fig- ure 1 (b). All these (pseudo-)sync hronous binaries are primordial binaries evolving from t ypical stellar evolu- tionary pro cesses. These systems are plotted in Fig- ures 2 (b) and 3 (b) for comparison with the Pleiades ob- serv ations. Overall, the sim ulated trends in b oth Fig- ures agree qualitatively with the observ ation, but the quan titative differences are significan t. Tidally synchro- nized binaries in the simulation concentrate at P orb ≈ 1 to 3 da ys. Defining the transition orbital perio d ( P syn ) b et ween sync hronization and pseudo-synchronization as the maximum orbital p erio d among synchronized sys- tems giv es P syn ≈ 2 . 5 days. F or the circularization tran- sition, we apply the same pro cedure as in the observ a- tions by fitting the circularization function of S. Meibom & R. D. Mathieu ( 2005 ) to the F GK binaries and obtain P circ = 1 . 8 ± 0 . 2 da ys. In this simulation, P circ is close to P syn . Both characteristic p erio ds in the Pleiades are clearly larger than these simulated v alues. W e note that the spectroscopic monitoring complete- ness of the curren t binary sample in Pleiades is ∼ 78%. The impact of undetected systems on the distributions in Figures 2 (a) and 3 (a) remains uncertain. A more complete binary census of the Pleiades is therefore still needed. 3.2. Dep endenc e on Stel lar Mass and Mass R atio W e now in v estigate the dependence of tidal sync hro- nization on stellar mass. F or a fixed semi-ma jor axis, the sync hronization timescale scales as R − 6 , decreasing with the sixth p ow er of stellar radius ( P . Hut 1981 ). 8 − 0 . 5 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 ( G B P − G RP ) 0 [mag] 10 0 10 1 10 2 10 3 10 4 10 5 P ps /P rot (a) 10 1 10 2 10 3 P orb [days] B5 A0 A5 F0 F5 G2 K0 K5 M0 M5 − 0 . 5 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 ( G B P − G RP ) 0 [mag] 10 0 10 1 10 2 10 3 10 4 10 5 P ps /P rot (b) 10 1 10 2 10 3 P orb [days] B5 A0 A5 F0 F5 G2 K0 K5 M0 M5 Figure 4. (a) Diagram of P ps /P rot v ersus ( G B P − G RP ) 0 . Poin ts are color-co ded by their P orb . Red star symbols mark (pseudo-)sync hronized systems. (b) As panel (a), but for the simulated Pleiades-lik e cluster. Larger radii, therefore, lead to faster synchronization and, at a given cluster age, allow synchronization at longer orbital p erio ds. W e adopt photometric color as a proxy for binary mass and correct for in terstellar red- dening using the E ( B − V ) v alues for eac h star from Gaia DR3 ( Gaia Collab oration et al. 2023 ). In Figure 4 , w e compare the distribution of P ps /P rot against dered- dened color among the MS binaries. The distributions are similar across these sp ectral types, and w e find no significan t dep endence of the synchronization state on primary mass within the AF GK regime. The sim ulated Pleiades-lik e cluster displa ys a similar trend. The synchronization timescale also dep ends on the mass ratios of binary systems. The tidal sync hroniza- tion timescale is predicted to decrease with an increas- ing mass ratio ( P . Hut 1981 ). Binaries with equal mass ratios ( q = 1) should hav e m uch shorter synchroniza- tion timescales than those with low mass ratios, all else b eing equal. Indeed, we find that synchronization has a stronger dep endence on the mass ratio than on the primary mass. In Figure 1 (a), of the seven tidally syn- c hronized binaries in the Pleiades, all except HI I 2407 lie noticeably ab ov e the observed single-star MS, con- sisten t with their high mass ratios. 86% (6/7) of these tidally synchronized binaries hav e q > 0 . 6. W e iden tify high mass-ratio binary candidates directly in the CMD b y selecting sources that are brighter and redder than the q = 0 . 6 binary lo cus, and we find that sync hronized systems mak e up about 8% of this high mass ratio group in Pleiades. This fraction rises to 17% when restricted to the sp ectroscopically observed sample from G. T or- res et al. ( 2021 ), accoun ting for the incompleteness of sp ectroscopic cov erage. The FGK SB sample of G. T or- res et al. ( 2021 ) is relatively complete and reco vers many systems that blend in to the single-star MS. As discussed in Section 2.1 , their sp ectroscopic monitoring sho ws no strong bias with resp ect to mass ratio. W e therefore infer that the higher incidence of sync hronized binaries at high q is unlikely to b e an artifact of incomplete- ness. Moreov er, in the sim ulated Pleiades-like cluster, the four tidally synchronized systems we identify are all equal-mass binaries, consistent with the observ ations as illustrated in Figure 1 . 3.3. Imp act of tides on stel lar r otation − 0 . 5 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 ( G B P − G RP ) 0 [mag] 0 2 4 6 8 10 12 14 P rot [da ys] HII 1431 slow rotators median rotators fast rotators HCG 495 10 1 10 2 10 3 P orb [da ys] B5 A0 A5 F0 F5 G2 K0 K5 M0 M5 Figure 5. Diagram of P rot v ersus ( G B P − G RP ) 0 of Pleiades, highligh ting 42 binaries with colors representing their P orb . The three curves represent the stellar rotation models of L. Amard et al. ( 2019 ) for lo w-mass single stars at an age of 125 Myr, with three different initial rotation rates (slo w, median, and fast) lab eled. Red star symbols and background gray p oin ts mark (pseudo-)synchronized binaries and single-star candidates of the Pleiades, resp ectively . T o explore how tides shape the rotation of close bina- ries, we examine their distribution in the P rot − ( G B P − G RP ) 0 diagram of the Pleiades (Figure 5 ). W e com- 9 pare P rot for 42 binaries with that of single stars, where singles are defined by the renormalized unit w eight er- ror (RUWE) parameter in Gaia < 1.4 ( L. Lindegren et al. 2018 ), b eing photometrically singles on the CMD as described in Section 2.1 , and not b eing detected as SB/EBs, or higher-order systems. W e note that this sample of single candidates inevitably con tains some unresolv ed binaries. Figure 5 also ov erla ys the low- mass F GK-t yp e single-star gyro chronology tracks of L. Amard et al. ( 2019 ) for slow, median, and fast initial rotation rates 4 at the cluster age and metallicity . These mo dels implemen t the wind-braking la w proposed by S. P . Matt et al. ( 2015 ). F or F GK-type single stars, rotation p erio ds primarily fall on to t wo distinct sequences: a slow-rotator sequence (where Prot increases monotonically with color) and a fast-rotator sequence, with some stars residing in the in termediate gap, as originally demonstrated b y S. A. Barnes ( 2003 ). This phenomenon has motiv ated ex- tensiv e research in to the underlying ph ysics (e.g., S. A. Barnes 2010 ; S. P . Matt et al. 2012 ; S. P . Matt et al. 2015 ; F. Gallet & J. Bouvier 2015 ; F. Gallet et al. 2018 ; L. Amard et al. 2019 ; F. Spada & A. C. Lanzafame 2020 ). Late-type tidally synchronized binaries, together with most late-t yp e binaries, fall on the predicted slow- rotator sequence of single stars. W e also identify four long-p erio d binaries ( P orb > 500 days) that reside on the fast-rotator sequence ( P rot < 1 . 2 days). They com- prise one young stellar ob ject, tw o RS Canum V enati- corum v ariables, and one BY Draconis v ariable. Early- t yp e singles with ( G B P − G RP ) 0 < 0.4 generally rotate rapidly , barring a single exception. Notably , four of six early-t yp e binaries rotate more slo wly than the bulk of single stars at similar colors. In this early-type subset, their orbital p erio ds range from 2.46 da ys (HI I 1431, the sync hronized and slow est rotator in this subset) to 4017 da ys. The primaries app ear to rotate more slowly in shorter-p erio d systems, but the small sample does not allo w a firm conclusion. Ab o ve the slow-rotation curve in the F G-t yp e regime and the bulk single-star locus in the early-t ype regime, fiv e binaries with relativ ely short orbital perio ds, includ- ing the synchronized system, HI I 1431, hav e primaries that rotate anomalously slowly , with rotation p erio ds ab out 1.3 to 2.5 times longer than those of single stars of similar color. F or HI I 1431, this slo w rotation further supp orts a tidal origin for its rotational ev olution. 4 The initial rotation rates of slow, median, and fast rotators are 9.0, 4.5, and 1.6(2.3 for the 1 . 2 − 1 . 5 M ⊙ models) days ( L. Amard et al. 2019 ). 4. DISCUSSION By directly comparing the pseudo-sync hronization p erio d P ps with the primary’s rotation p erio d P rot , w e identify seven tidally synchronized binaries in the Pleiades, comprising one early-type system and six late- t yp e systems. F or P orb < 8 . 6 days, primaries in de- tac hed binaries either reach synchronization on circular orbits or ac hieve pseudo-synchronization on mildly ec- cen tric orbits ( P ps /P rot ≈ 1). At longer orbital p erio ds, systems are generally super-synchronized and mostly ec- cen tric. Using a co ev al, single-metallicity Pleiades sam- ple, we place the synchronization b oundary and the cir- cularization b oundary in a unified empirical framew ork and find a key result: the pseudo-synchronization tran- sition ( P syn ≈ 8 . 6 days) is of the same order as the FGK circularization p erio d ( P circ ≈ 7 . 2 ± 1 . 0 days), similar to observ ations in M35 ( ∼ 150 Myr) reported b y S. Meib om et al. ( 2006 ) and our synthetic Pleiades-lik e cluster un- der the classical picture of equilibrium tides ( J. P . Zahn 1989 ). Observ ations of non-cluster binaries from Kepler and TESS data similarly yield synchronization and cir- cularization p erio ds of approximately ten days ( G. Li et al. 2020 ; L. W. IJsp eert et al. 2024 ). Although the binary sample is sparse for P orb b et ween 8.6 and 14 da ys, and we do not claim exact equalit y of the tw o b ound- aries, their pro ximit y is unlik ely to b e a sampling arti- fact and more plausibly reflects the characteristic scale of tidal dissipation in young co ev al p opulations (125-150 Myr). In simulation, the Pleiades-lik e cluster mo del yields a circularization p erio d for solar-mass binaries of only P circ = 1 . 8 ± 0 . 2 da ys, far shorter than the observed 7 . 2 ± 1 . 0 days in cluster Pleiades (Figure 3 ). The sim ulated transition from sync hronization to pseudo- sync hronization also o ccurs at m uc h shorter orbital p e- rio ds than observ ed as shown in Figure 2 , which shows that the standard BSE tidal prescription (a simplified form of the equilibrium-tide theory ( J. P . Zahn 1970 , 1975 , 1977 , 1989 )) is evidently to o inefficient to cir- cularize and synchronize binaries with P orb > 2 days. A. J. Bark er ( 2020 ) argued that classical equilibrium- tide models in MS conv ective en v elop es t ypically o veres- timate tidal dissipation by a factor of 2-3, implying that relying on equilibrium tides alone can mislead the in ter- pretation of observ ations. This motiv ates revised tidal dissipation physics in which dynamical tides, driven by inertial wa ves in conv ective zones and internal gravit y w av es in radiative zones, should pla y a key role. J. J. Zanazzi & Y. W u ( 2021 ) demonstrated that resonant lo c king of gravit y mo des during the PMS can drive sub- stan tial circularization, with relativ ely little additional c hange on the MS. A. J. Bark er ( 2022 ) sho w ed that the 10 observ ed circularization p erio ds can b e explained by dis- sipation of inertial wa ves in con vectiv e env elop es, which is especially effective during the PMS and can also oper- ate on the MS when the stellar spin approac hes sync hro- nization. In this framework, the predicted P circ increases with MS age, consisten t with observ ations. A. J. Barker ( 2022 ) further demonstrated that neither equilibrium- tide dissipation nor gravit y-w av e dissipation alone is lik ely to match the observed P circ . Using the large Gaia DR3 sample of MS SBs, D. Bashi et al. ( 2023 ) found that circularization is unlikely to o ccur primarily on the MS and is more consistent with o ccurring during the PMS, as originally prop osed by J. P . Zahn ( 1989 ). HI I 2407 is particularly noteworth y . It is an MS+PMS tidally circularized and sync hronized system with a low mass ratio ( q ≈ 0 . 22). Bey ond any p ossible p erturba- tions from an unseen distant tertiary , the primordial ec- cen tricity can mo dify the time required to reac h circular- ization and synchronization. The circular, synchronized state of this low mass ratio system may therefore reflect an in trinsically small initial eccentricit y rather than un- usually efficient dissipation. In addition, the secondary remains on the PMS, with a relativ ely large radius and a deep conv ective en velope, which w ould significantly en- hance tidal dissipation. HII 2407 th us p oten tially sup- p orts the view that tidal evolution acts primarily during the PMS and provides useful constrain ts on the relev an t timescales. T o refine our understanding of tidal interac- tions and prob e the full p otential of tides, w e will extend the analysis to clusters spanning a wider range of ages in future work. The dep endence of tidal sync hronization on mass ra- tio is mark edly stronger than its dep endence on primary mass. Among short-p erio d systems that are (pseudo- )sync hronized, high mass-ratio binaries ( q > 0 . 6) mak e up ab out 86%, consistent with predictions from our Pleiades-lik e cluster sim ulation. The simulation sug- gests that tidally synchronized binaries in clusters are predominan tly “t win” binaries, whose fundamental pa- rameters are essentially identical (mass, radius, metallic- it y , etc.). In the Pleiades, we indeed iden tify HCG 489, HCG 495, and HI I 1286 as equal-mass, tidally synchro- nized twins. T ak en together, the observ ations and sim- ulations p oin t to a very intriguing conclusion that the tidal-sync hronized detac hed MS binaries in suc h y oung clusters ( ∼ 125 Myr) are expected to b e mostly twin binaries. F or the sev en synchronized systems in the Pleiades, preliminary BSE exp eriments based on the measured parameters listed in T able 1 indicate that HI I 1431 and HCG 495 are likely to undergo common- en velope evolution and become a double carb on-o xygen white dwarf (WD) system and a double helium WD system, resp ectively , whic h are prime targets for next- generation space-based gravitational-w a v e observ atories in the millihertz band ( L. Ren et al. 2023 ). These re- sults p oint to star clusters as p otential birthplaces of this important class of gravitational-w a v e sources. Evidence that tides shape the rotational ev olution of early-t yp e binaries is clear in the Pleiades color-p erio d diagram (Figure 5 ), albeit limited by the small sample size. Most early-t yp e binaries with ( G B P − G RP ) 0 < 0 . 4 ha ve primaries that rotate anomalously slowly , consis- ten t with extra tidal braking. The effect strengthens to ward shorter orbital p erio ds. The tidally sync hro- nized system HI I 1431 with P orb ≈ 2 . 46 days hosts the slo west-rotating primary in this subset, whereas systems with P orb ≈ 4017 days rotate relatively faster. This trend, in which shorter P orb corresp onds to slo wer pri- mary rotation, indicates efficient extraction of spin an- gular momentum b y tides in close binaries. Regarding the sole early-type single candidate with anomalously slo w rotation ( P rot ≈ 3 . 6 days), further in v estigation is required to determine whether it is an unresolv ed binary sub ject to similar tidal interactions. In contrast, the ro- tational distribution of co ol single stars in the Pleiades in trinsically exhibits significant scatter. F or low-mass binaries, the interpla y b etw een magnetic braking and tidal torques mak es it even more difficult to isolate the impact of tides on binary rotation. Most late-type bi- naries lie on the single-star slow-rotator sequence, and their spins do not differ markedly from those of sin- gle stars at the same color. This implies that magnetic braking go v erns the rotational ev olution at the presen t cluster age, with tides con tributing negligible additional spin-do wn or spin-up effects. 5. SUMMAR Y This work aims to examine the detac hed tidally syn- c hronized binaries in the cluster Pleiades. Our main results and conclusions are summarized b elow: 1. By directly comparing P ps with the primary’s ro- tation p erio d P rot , we iden tify seven tidally syn- c hronized binaries ( P orb ≲ 8 . 6 days) within 42 bi- naries in the Pleiades, comprising one early-type system and six late-type systems. At longer orbital p erio ds, primaries generally rotate faster than the pseudo-sync hronous rate, and most systems are ec- cen tric. 2. The synchronized system HCG 495 with the longest orbital p erio d shows a mild eccentric or- bit, while the others are circular. W e find a syn- c hronization transition near P orb ≈ 8 . 6 − 14 da ys, comparable to the known circularization p erio d 11 ( P orb ≈ 7 . 2 da ys) in the Pleiades, which suggests similar critical p erio d scales for sync hronization and circularization in this co ev al p opulation. 3. Synchronization dep ends m uch more strongly on mass ratio than on primary mass. Most syn- c hronized systems in Pleiades ha ve high mass ra- tios. Tidally synchronized detached MS binaries in y oung clusters ( ∼ 125 Myr) are exp ected to b e dominated b y twin systems. These high-mass- ratio synchronized binaries are likely to ev olve in to double WD systems, which are prime tar- gets for next-generation space-based gravitational- w av e observ atories in the millihertz band ( L. Ren et al. 2023 ). 4. Sub ject to small-num b er statistics, our results suggest that tides likely imp ose strong rotational braking on close early-type binaries, mo ving them a wa y from the single-star rotation track. In con- trast, the tidal influence on late-t yp e close binaries is weak er, and their spins largely follow the single- star slo w-rotator sequence. A CKNOWLEDGMENTS W e thank the anon ymous referee for the v aluable com- men ts and suggestions for improving our man uscript. L.W. and C.L. are supported by the National Natural Science F oundation of China (NSFC) through gran t 12233013. C.H. is supp orted by the NSFC through gran t 12503045. G.L. ac knowledges the supp ort of the Australian Research Council through the DECRA pro ject DE250100773. This w ork has made use of data from the Europ ean Space Agency (ESA) mission Gaia (h ttps://www.cosmos.esa.int/gaia), pro cessed by the Gaia Data Pro cessing and Analysis Consortium (DP AC, h ttps://www.cosmos.esa.int/w eb/gaia/dpac/consortium). F unding for the DP AC has b een pro vided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. APPENDIX T able 2 . List of Detac hed Binaries with P orb > 10 days in the Pleiades N Name Gaia DR3 ID G mag P orb P rot e q P ps /P rot Lab el Ref (mag) (da y) (da y) 10 HI I 2168 66526127137440128 3.616 290.992 ... 0.2357 0.721 ... SB2 7 0.004 0.003 ... 0.0001 0.014 ... 11 HI I 563 65296907494549120 4.261 3172.4 ... 0.391 ... ... SB1 8 0.003 9.9 ... 0.079 ... ... 12 HI I 2507 66507469798631808 6.806 16.72623 1.645 0 0.5493 10.1679 SB2 1, 3 0.003 0.00004 0.309 ... 0.0007 1.90996 13 TRU S194 65776736943479808 7.197 3635 0.521 0.528 ... 21592.8 SB1 8, 9 0.003 19 0.008 0.035 ... 2727.4 14 TRU S26 64313222543810560 7.271 71.820 0.671 0.284 ... 160.188 SB1 8, 9 0.003 0.008 ... 0.048 ... 29.1624 15 HI I 1407 64898368889386624 8.124 953.08 1.454 0.322 ... 1079.87 SB1 1, 3 0.003 0.90 0.351 0.011 ... 262.52 16 HI I 1762 66724451545088128 8.214 4017 1.687 0.468 ... 6005.62 SB1 1, 3 0.003 155 0.335 0.039 ... 1432.74 17 TRU S93 70024154658949120 8.646 1059.7 ... 0.392 ... ... SB1 1 T able 2 c ontinue d 12 T able 2 (c ontinue d) N Name Gaia DR3 ID G mag P orb P rot e q P ps /P rot Lab el Ref (mag) (da y) (da y) 0.003 3.8 ... 0.037 ... ... 18 HI I 605 69819404977607168 8.901 20.7976 7.399 0.4318 0.6207 6.32416 SB2 1, 3 0.003 0.0001 2.373 0.0026 0.0068 2.02891 19 AK I I-346 64575730944903552 9.081 6123 3.376 0.030 ... 1823.48 SB1 1, 3 0.003 123 0.379 0.083 ... 214.91 20 AK I I I-419 71296667568555904 9.215 34.32148 4.182 0.6493 0.9003 41.4922 SB2 1, 9 0.003 0.00007 ... 0.0022 0.0031 1.0194 21 HI I 745 65277975278721152 9.320 1541.4 0.838 0.120 ... 1998.56 SB1 1, 3 0.003 9.8 0.171 0.025 ... 413.40 22 HI I 164 65090680344356992 9.430 268.704 0.815 0.2505 0.318 455.893 SB2 1, 10 0.003 0.056 0.001 0.0072 0.017 7.485 23 HI I 233 65242069352190976 9.540 1241.5 2.397 0.242 ... 702.677 SB1 1, 3 0.003 1.4 ... 0.025 ... 47.787 24 HI I 727 66802654309459712 9.593 7271 1.139 0.832 ... 103223.5 SB1 1, 3 0.003 153 0.314 0.039 ... 46833.7 25 HI I 1392 65207709613871744 9.740 767.04 ... 0.8206 0.930 ... SB2 1 0.003 0.25 ... 0.0075 0.015 ... 26 HI I 1117 65199978672758272 10.045 26.0271 6.228 0.5745 0.980 15.3823 SB2 1, 3 0.003 0.0001 0.897 0.0062 0.014 2.2466 27 HI I 2500 66507469798632320 10.235 2391 1.638 0 ... 1459.71 SB1 1, 3 0.003 17 0.12 ... ... 107.44 28 HI I 2172 66771146429454592 10.305 30.2130 4.337 0.3294 ... 11.7012 SB1 1, 3 0.003 0.0001 1.166 0.0037 ... 3.1479 29 HI I 2147 66503449709270400 10.431 6641 0.777 0.105 0.9168 9112.99 SB2 3, 11 0.003 42 0.046 0.011 0.0039 555.43 30 HI I 250 69829609819915648 10.561 971.59 4.232 0.6613 ... 1228.07 SB1 1, 3 0.003 0.67 ... 0.0099 ... 64.09 31 HI I 173 69883417170175488 10.629 481.25 5.709 0.0986 0.953 89.2185 SB2 1, 3 0.003 0.10 ... 0.0067 0.011 1.6075 32 HI I 120 65232105028172160 10.641 2940 3.985 0.303 ... 1157.52 SB1 1, 3 0.003 12 0.385 0.026 ... 135.33 33 HI I 3097 66863058730966528 10.706 780.38 3.376 0.777 ... 2403.79 SB1 1, 3 0.003 0.14 0.393 0.010 ... 326.97 34 HI I 320 68310359628169088 10.848 757.01 1.187 0.3064 0.857 1009.26 SB2 1, 3 0.003 0.22 ... 0.0049 0.023 80.49 35 PELS 38 64743646986745600 10.885 17.28588 6.011 0.0811 0.573 2.98904 SB2 1, 10 0.003 0.00007 0.002 0.0018 0.012 0.00514 36 HI I 2406 64928605459180416 10.938 33.00630 4.986 0.5109 0.544 19.2812 SB2 1, 3 0.003 0.00005 0.572 0.0011 0.017 2.2132 37 HI I 571 69864313155605120 11.010 15.87225 5.202 0.3281 ... 5.10767 SB1 1, 3 T able 2 c ontinue d 13 T able 2 (c ontinue d) N Name Gaia DR3 ID G mag P orb P rot e q P ps /P rot Lab el Ref (mag) (da y) (da y) 0.003 0.00003 ... 0.0031 ... 0.10085 38 AK I I I-664 70941383577307392 11.023 14.073 6.531 0.313 ... 3.46804 SB1 1, 3 0.003 0.001 ... 0.011 ... 0.10994 39 HI I 2284 66502281478384000 11.115 807.39 5.545 0.4111 ... 307.711 SB1 1, 3 0.003 0.45 2.152 0.0061 ... 119.553 40 TYC 1254-41-1 50176423589328512 11.274 98.356 14.238 0.1304 ... 7.61414 SB1 3, 12 0.003 0.427 0.118 0.0478 ... 0.52380 41 AK I-2-288 70049477786093056 11.516 17.4667 8.069 0.2010 0.8700 2.69363 SB2 1, 3 0.003 0.0001 ... 0.0030 0.0053 0.03507 42 AK IV-287 67411989910269696 11.553 1808 0.396 0.229 ... 6020.17 SB1 1, 3 0.003 26 ... 0.064 ... 1648.27 43 HI I 522 65194584193758336 11.684 23.8375 7.10 0.109 ... 3.59702 SB1 1, 3 0.003 0.0005 1.05 0.023 ... 0.54151 44 HI I 1348 66734720809017856 12.229 94.805 9.874 0.5543 0.7799 32.7101 SB2 1, 3 0.003 0.012 0.118 0.0017 0.0098 0.4432 45 HCG 384 66937859881182848 12.782 542.11 2.352 0.6624 0.820 1239.42 SB2 1, 3 0.003 0.27 ... 0.0080 0.035 68.44 46 HI I 3104 65660158649542784 12.847 1312.5 3.941 0.207 ... 419.421 SB1 1, 3 0.003 4.5 0.926 0.051 ... 107.608 47 HI I 1653 66816187748666624 12.904 548.2 0.737 0 ... 743.826 SB1 1, 3 0.003 1.1 0.227 ... .... 229.107 48 HCG 76 64456262134709376 15.732 32.747 1.979 0.1328 0.917 18.3020 EB 3, 13 0.003 0.002 0.263 0.0043 0.013 2.4349 Note —The second line for each system lists the 1 σ uncertain ties. 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