Dynamically Tagged Groups of Metal-Poor Stars from the Best and Brightest Survey

We study the dynamics of some 1500 relatively bright very metal-poor stars, specifically those with [Fe/H] < –1.8, derived from medium-resolution (R ~ 2000) spectroscopy of candidate metal-poor stars from the Best and Brightest (BNB) sample. For the stars in this sample for which sufficient kinematic data are available, e.g., GAIA Data Release 2 measures of proper motions, Bayesian StarHorse distances based on Gaia parallaxes, and radial velocities from the BNB spectra, we derive their dynamical parameters with the code Action-based Galaxy Modelling Architecture (AGAMA). The resulting energies and orbital actions (E, J_r, J_φ, J_z) are clustered in dynamical space using the Hierarchical Density-Based Spatial Clustering of Applications with Noise (HDBSCAN) algorithm to identify Dynamically Tagged Groups (DTGs). We consider the dispersions in the abundances of [Fe/H], [C/Fe], and [α/Fe] in order to test the significance of their association with parent dwarf galaxies (and possibly globular clusters) disrupted into the Galactic halo.

This work received partial support from grant PHY 14-30152; Physics Frontier Center/ JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the U.S. National Science Foundation.