Star clusters disperse over time, with their gas ejected by jets, winds, radiation and supernovae and their stellar members blending into their host galaxy over tens of millions of years as they become gravitationally unbound. However, the stars can remain spatially and kinematically coherent for much longer than this, which means that, in theory, one can wind back the clock to understand their birth location. Of course, the further back in history the cluster was born, the more challenging it is to reconstruct using purely spatial or kinematic clustering, and the Pleiades stars are ~125 Myr old. One alternative mechanism to find stars with similar ages is to select them based on rotation rates, since for cool stars rotation periods increase with age because of a process called magnetic braking. In an earlier paper, Andrew Boyle and colleagues investigated whether, and how reliably, stellar rotation periods could be calculated from TESS photometry. Building on that work by adding Gaia kinematics, they have now developed a rotation-based Bayesian membership framework for piecing together diffused and dissolved cluster systems.
Potential members of the larger Pleiades family — called the ‘Greater Pleiades Complex’ by the authors — were selected on the basis of their velocity offset from the core of the Pleiades, a rotational period less than 12 days, available Gaia photometry and an effective temperature 3,000 K < Teff < 6,500 K (because hotter stars are not rotationally braked in the same way as cooler stars). These criteria reduced nearly 7 million stars with TESS and Gaia data to just several thousand candidate Pleiades members. The Bayesian membership framework then assigned membership probability based on kinematics and age and, with a final selection threshold of 50% probability, yielded 3,091 stars (the positions of more than half of which are shown in the image).
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