Exploring the impact of a rapidly decelerating bar on transforming bulge orbits into disc-like orbits

TL;DR

This study investigates how a rapidly decelerating galactic bar could influence the orbits of metal-poor stars, suggesting it may partially explain their prograde planar orbits in the Solar neighborhood.

Contribution

It introduces test-particle simulations with a decelerating bar model to explore its effect on bulge star orbits, a novel approach in understanding stellar dynamics.

Findings

Decelerating bar can move some bulge stars outward with prograde orbits.

Most shepherded stars are located in the Galaxy's fourth quadrant.

Decelerating bar explains part of the metal-poor stars' orbits but is not the main cause.

Abstract

The most metal-poor tail of the Milky Way ([Fe/H] $\leq$ $-$2.5) contains a population of stars with very prograde planar orbits, which is puzzling in both their origin and evolution. A possible scenario is that they are shepherded by the bar from the inner Galaxy, where many of the old and low-metallicity stars in the Galaxy are located. To investigate this scenario, we use test-particle simulations with an axisymmetric background potential plus a central bar model. The test particles are generated by an extended distribution function (EDF) model based on the observational constraints of bulge stars. According to the simulation results, a bar with constant pattern speed cannot help bring stars from the bulge to the solar vicinity. In contrast, when the model includes a rapidly decelerating bar, some bulge stars can gain rotation and move outwards as they are trapped in the co-rotation regions of the bar. The resulting distribution of shepherded stars heavily depends on the present-day azimuthal angle between the bar and the Sun. The majority of the low-metallicity bulge stars driven outwards are distributed in the fourth quadrant of the Galaxy with respect to the Sun, and about 10$\%$ of them are within 6 kpc from us. Our experiments indicate that the decelerating bar perturbation can be a contributing process to explain part of the most metal-poor stars with prograde planar orbits seen in the Solar neighborhood but is unlikely to be the dominant one.