Theia 456: Tidally Shredding an Open Cluster

TL;DR

This paper uses dynamical modeling and Gaia data to trace Theia 456 back to a small, coherent cluster 245 million years ago, providing a precise, model-independent age estimate of a dispersed stellar structure.

Contribution

It presents a novel dynamical analysis of Theia 456, deriving its age and properties independently of stellar evolution models, highlighting the importance of tidal disruption in stellar population studies.

Findings

Theia 456 was a small cluster 245 Myr ago.

The structure has a current half-mass radius of 9 pc.

The initial velocity dispersion was about 0.14 km/s.

Abstract

The application of clustering algorithms to the Gaia astrometric catalog has revolutionized our census of stellar populations in the Milky Way, including the discovery of many new, dispersed structures. We focus on one such structure, Theia 456 (COIN-Gaia-13), a loosely bound collection of ~320 stars spanning ~120 pc that has previously been shown to exhibit kinematic, chemical, and gyrochronal coherency, indicating a common origin. We obtain follow-up radial velocities and supplement these with Gaia astrometry to perform an in-depth dynamical analysis of Theia 456. By integrating stellar orbits through a Milky Way potential, we find the currently dispersed structure coalesced into a small cluster in the past. Via Bayesian modeling, we derive a kinematic age of 245 +/- 3 Myr (statistical), a half-mass radius of 9 +/- 2 pc, and an initial one-dimensional velocity dispersion of 0.14 +/- 0.02 km/s. Our results are entirely independent of model isochrones, details of stellar evolution, and internal cluster dynamics, and the statistical precision in our age derivation rivals that of the most precise age-dating techniques known today, though our imperfect knowledge of the Milky Way potential and simple spherical model for Theia 456 at birth add additional uncertainties. Using posterior predictive checking, we confirm these results are robust under reasonable variations to the Milky Way potential. Such low density structures that are disrupted by the Galactic tides before virializing may be ubiquitous, signifying that Theia 456 is a valuable benchmark for studying the dynamical history of stellar populations in the Milky Way.