The tale of the tail -- disentangling the high transverse velocity stars in Gaia DR2

TL;DR

This study analyzes high transverse velocity stars in Gaia DR2, revealing a bifurcation in the local stellar halo, and refines Galactic models to better understand the origins and composition of these stars, including thick-disc and accreted populations.

Contribution

The paper updates the Besançon/Galaxia model with new observational data and kinematic descriptions, providing improved constraints on the Milky Way's halo and disc properties, and clarifies the nature of high velocity stars.

Findings

Thick disc contributes ~13% to high velocity tail.

Identification of retrograde stars with thick-disc chemistry.

Resonant orbits likely explain dynamical substructures.

Abstract

Although the stellar halo accounts for just ~1% of the total stellar mass of the Milky Way, the kinematics of halo stars encode valuable information about the origins and evolution of our Galaxy. It has been shown that the high transverse velocity stars in Gaia DR2 reveal a double sequence in the Hertzsprung-Russell (HR) diagram, indicating a bifurcation in the local stellar halo within 1 kpc. We fit these stars by updating the popular Besan\c{c}on/Galaxia model, incorporating the latest observational results for the stellar halo and an improved kinematic description for the thick-disk from Sch\"onrich & Binney (2012). We are able to obtain a good match to the Gaia data and provide new constraints on the properties of the Galactic disc and stellar halo. In particular, we show that the kinematically defined thick disc contribution to this high velocity tail is ~13%. We look in greater detail using chemistry from LAMOST DR5, identifying a population of retrograde stars with thick-disc chemistry. Our thick disc kinematic model cannot account for this population and so we conclude there is likely to be a contribution from heated or accreted stars in the Solar Neighbourhood. We also investigate proposed dynamical substructures in this sample, concluding that they are probably due to resonant orbits rather than accreted populations. Finally we provide new insights on the nature of the two sequences and their relation with past accretion events and the primordial Galactic disc.