Presentation #201.08 in the session Star Clusters and Associations — iPoster Session.
A stellar structure named Theia 598 was found in the Gaia astrometric data by Kounkel & Covey (2019) using an unsupervised machine learning algorithm. This collection of 639 stars, at a distance of approximately 210 pc, consists of a central core and two almost symmetric tail-like features spanning 45 degrees in right ascension and 30 degrees in declination across the southern sky. The stars in the central region of Theia 598 were first identified in 2003 as the open cluster Alessi 9. The unique structure of Theia 598 raises several questions. Are the tails really part of a coeval cluster or are they contaminants that the clustering algorithm misidentified as members? If the tails are real, do they represent the dissolution of the Alessi 9 cluster? We produced photometric light curves from TESS Full-Frame Images, then measured rotation periods for 120 candidate members spread across the entire structure. Our preliminary analysis shows that the resulting periods are largely consistent with them belonging to a single-age population; using PARSEC isochrones, we estimate this age to be approximately 350 Myr. We present analyses of Theia 598’s membership using Gaia EDR3 astrometry, literature radial velocities, and rotation periods we measured from TESS light curves. We also outline our plans for using spectra from Gaia’s impending third data release, and describe our follow-up campaign with the high-resolution MIKE spectrograph on the Magellan telescope at Las Campanas Observatory. The expanded sample of radial velocities from these new data sets, together with age-sensitive chromospheric emission and lithium abundances from MIKE, will enable us to determine the true extent of Alessi 9 and establish it as a new benchmark for stellar astrophysics.