Lost Sisters Found: TESS and Gaia Reveal a Dissolving Pleiades Complex

TL;DR

This work tackles how young star clusters dissolve and disperse by introducing a rotation-informed Bayesian framework that fuses TESS-derived stellar rotation periods with Gaia kinematics to identify diffuse, coeval cluster remnants. By applying gyro-tagging to the Pleiades, the authors uncover a large, coherent structure—the Greater Pleiades Complex—spanning hundreds of parsecs and comprising multiple subgroups with uniform ages and chemical signatures. They validate the complex through CMD and rotation-age sequences, detailed abundances, and backward orbit traceback, finding strong evidence for a common origin in several subgroups (e.g., UPK 303, HSC 1964, UPK 545) while others (AB Dor, GPC-1, GPC-2) remain more ambiguous due to data limitations. The study demonstrates a scalable method for tracing the genealogies of nearby clusters, enabling a more complete view of local star formation history and setting the stage for applying this approach to additional associations with upcoming data releases. It also highlights the role of rotational ages as a powerful prior to recover diffuse structures that traditional spatial-kinematic clustering may miss, particularly for ages in the 80–200 Myr range.

Abstract

Most star clusters dissolve into the Galaxy over tens to hundreds of millions of years after they form. While recent Gaia studies have honed our view of cluster dispersal, the exact chronology of which star formation events begat which star cluster remnants remains unclear. This problem is acute after 100 Myr, when cluster remnants have spread over hundreds of parsecs and most age estimates for main sequence stars are too imprecise to link the stars to their birth events. Here we develop a Bayesian framework that combines TESS stellar rotation rates with Gaia kinematics to identify diffuse remnants of open clusters. We apply our method to the Pleiades, which previous studies have noted shows kinematic similarities to other nearby young stellar groups. We find that the Pleiades constitutes the bound core of a much larger, coeval structure that contains multiple known clusters distributed over 600 pc. We refer to this structure as the Greater Pleiades Complex. On the basis of uniform ages, coherent space velocities, detailed elemental abundances, and traceback histories, we conclude that most stars in this complex originated from the same giant molecular cloud. This work establishes a scalable approach for tracing the genealogies of nearby clusters and further cements the Pleiades as a cornerstone of stellar astrophysics. We aim to apply this methodology to other associations as part of the upcoming TESS All-Sky Rotation Survey.

Figures (15)

• Figure 1: TESS's near all-sky coverage enables studies of large stellar complexes.Left:TESS coverage from 2018 July to 2026 August (Sectors 1-107), color coded by the number of months over which each star has been observed by TESS. Right: Galactocentric X and Y positions of Orion ($t<12$ Myr), Sco-Cen ($t\sim17$ Myr), and Vela ($t\sim19$ Myr), with the Sun at the origin. Even at young ages, these hierarchically-structured groups span hundreds of parsecs in X and Y. Gray points are all stars in our $T < 16$, $d < 500$ pc target list. The membership list for Orion is taken from 2018AJ....156...84K (excluding stars in that work marked as field stars or not assigned to an Orion sub-group), Sco-Cen from 2023ApJ...954..134K, and Vela from 2019AA...626A..17C (including only stars with a $>99$% probability from that work of being part of a Vela sub-group). Positive Y is in the direction of Galactic rotation and positive X is in the direction of the Galactic center. This will be the orientation of our plots for the rest of this work.
• Figure 2: Stellar kinematics and rotation isolate the Greater Pleiades Complex. Top Left: Stars with $T < 16$ and $d < 500$ pc (gray), highlighting those within $5~\mathrm{km~s^{-1}}$ of the Pleiades' median 3D velocity (black). The Sun is marked by a yellow point at the origin. Top right: Subset of stars consistent with the Pleiades in velocity, with reliable TESS rotation periods ($P_{\rm rot}$$<12$ days and reliability $>0.7$). Bottom left: Six-dimensional clustering results from HDBSCAN applied to the velocity and rotation-selected sample. Bottom right: High-confidence members ($N$=1,075) with 3D velocities, selected for spatiokinematic coherence, rotation consistency, and placement on the cluster CMD. These stars define a co-moving and coeval population that we will argue comprises the Greater Pleiades Complex.
• Figure 3: The Greater Pleiades Complex connects multiple known associations. Left: An XY projection of the stars in our final membership list with a probability $P\bigl(M \mid P_{\rm rot}\bigr) > 0.5$. This includes stars without radial velocity measurements that were not present in Figure \ref{['fig:quality_cuts']}. Right: The results of cross-matching our membership list with the groups from kounkelUntanglingGalaxyLocal2019 and Hunt2023, keeping only matches with more than 50 stars. Stars from AB Dor are taken from the Montreal Open Clusters and Associations (MOCA) database (J. Gagné et al., in preparation; 2024PASP..136f3001G). Multiple known groups in the region of the Pleiades appear to be connected by a bridge of coeval, comoving stars.
• Figure 4: UPK 303 and surrounding stars compared with the Pleiades. Galactocentric positions (XY, XZ, YZ; top row) show that UPK 303 lies along a continuous spatial sequence extending from the Pleiades core. The second row shows Galactocentric velocities (UVW). The color–magnitude diagram (bottom left) and the rotation period–$T_{\mathrm{eff}}$ plane (bottom right) demonstrate that both structures lie along the same stellar sequence, consistent with a common age. Metallicity spread (bottom middle) show that UPK 303 members differ from a random sample of nearby field stars and are comparable to stars in the Pleiades core. The bottom-right panel shows the past separation between UPK 303 and the Pleiades, computed via 5000 Monte Carlo orbit integrations. Together, the spatial, kinematic, photometric, rotational, chemical, and dynamical similarities support a physical association between UPK 303 and the Pleiades.
• Figure 5: HSC 1964 and surrounding stars compared with the Pleiades. The structure of the plot is the same as in Figure \ref{['fig:upk_303']}. Like UPK 303, kinematics, age diagnostics, metallicity, and back-integrations all provide independent lines of evidence that HSC 1964 and the core of the Pleiades share a common origin.