The metal-rich halo component extended in z: a characterization with Gaia DR2 and APOGEE

TL;DR

This study characterizes the metal-rich halo stars extending up to 10 kpc from the Galactic plane using Gaia DR2 and APOGEE data, revealing three chemically and kinematically distinct populations with different orbital properties.

Contribution

It identifies and characterizes a third intermediate-[Mg/Fe] stellar population in the metal-rich halo, linking it to known stellar overdensities and suggesting a complex halo composition.

Findings

Three distinct stellar populations with different [Mg/Fe] ratios.

Different kinematic and orbital properties among the populations.

The intermediate population shows prograde rotation and less-bound orbits.

Abstract

We report an analysis of the metal-rich tail ([Fe/H] $> -0.75$) of halo stars located at distances from the Galactic plane $z$ up to $|z| \sim 10$ kpc, observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). We examine the chemistry, kinematics, and dynamics of this metal-rich halo sample using chemical abundances and radial velocities provided by the fourteenth APOGEE data release (DR14) and proper motions from the second Gaia data release (DR2). The analysis reveals three chemically different stellar populations in the [Mg/Fe] vs. [Fe/H] space -- the two distinct halo populations already reported in the literature, and a third group with intermediate [Mg/Fe] $\sim$+0.1. We derive the $U$, $V$ and $W$ velocity components with respect to the Local Standard of Rest, as well as orbits for the three stellar groups, and find that they differ also in their kinematical and dynamical properties. The high-[Mg/Fe] population exhibits a mean prograde rotation, as well as orbits that are more bound and closer to the plane, whereas the low-[Mg/Fe] population has $<V>$ closer to 0, and stars that move in less-bound orbits reaching larger distances from the centre and the Galactic plane. The intermediate-Mg stars exhibit different orbital characteristics, moving with a strong prograde rotation and low excentricity, but in less-bound orbits. This stellar population resembles the two stellar overdensities lying about $|z| \sim 5$ kpc recently reported in the literature, for which a disc origin has been claimed.