Letter to the Editor

Could very low-metallicity stars with rotation-dominated orbits have been driven by the bar?

¹ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France
² Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
³ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St George Street, Toronto, ON, M5S 3H8 Canada
⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA UK
⁵ Dept. of Physics and Astronomy, University of Victoria, P.O. Box 3055, STN CSC Victoria, BC, V8W 3P6 Canada
⁶ Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
⁷ Universidad de La Laguna, Dept. Astrofísica, E-38206 La Laguna, Tenerife, Spain
⁸ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
⁹ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands

^⋆ Corresponding authors; zhen.yuan@astro.unistra.fr, chengdong.li@astro.unistra.fr

Received: 13 November 2023
Accepted: 25 July 2024

Abstract

The most metal-poor stars (e.g., [Fe/H] ≤ –2.5) are the ancient fossils from the early assembly epoch of our Galaxy. They very likely formed before the the thick disk. Recent studies have shown that a non-negligible fraction of them have prograde planar orbits, which means that their origin is a puzzle. It has been suggested that a later-formed rotating bar could have driven these old stars from the inner Galaxy outward and transformed their orbits so that they became more dominated by rotation. However, it is unclear whether this mechanism can explain these stars as observed in the solar neighborhood. We explore whether this scenario is feasible by tracing these stars backward in an axisymmetric Milky Way potential with a bar as perturber. We integrated their orbits backward for 6 Gyr under two bar models: one model with a constant pattern speed, and the other with a decelerating speed. Our experiments show that for the constantly rotating bar model, the stars of interest are little affected by the bar and cannot have been driven from a spheroidal inner Milky Way to their current orbits. In the extreme case of a decelerating bar, some of the very metal-poor stars on planar and prograde orbits can be brought from the inner Milky Way, but ∼90% of them were nevertheless already dominated by rotation (J_ϕ ≥ 1000 km s⁻¹ kpc) 6 Gyr ago. The chance that these stars started with spheroid-like orbits with low rotation (J_ϕ ≲ 600 km s⁻¹ kpc) is very low (< 3%). We therefore conclude that within the solar neighborhood, the bar is unlikely to have shepherded a significant fraction of spheroid stars in the inner Galaxy to produce the overdensity of stars on prograde planar orbits that is observed today.