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

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.

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Publication Details

Journal
Astronomy and Astrophysics
Published
2024-10-08
DOI
https://doi.org/10.1051/0004-6361/202348593
Citations
9
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
2.98

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article

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

G. Kordopatis, Federico Sestito, Rimpei Chiba, V. Hill et al.
9 citations
Astronomy and Astrophysics
Stellar, planetary, and galactic studies
2.98
article

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

G. Kordopatis, Federico Sestito, Rimpei Chiba, V. Hill, Else Starkenburg, Rodrigo Ibata, G. Monari, Zhen Yuan, Akshara Viswanathan, Benoît Famaey, A. Siebert, Guillaume F. Thomas, Anke Arentsen, Nicolas F. Martin, Chengdong Li
article en
9 citations

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.

Astronomy and AstrophysicsVol. 691
Centre National de la Recherche Scientifique (FR), Universidad de La Laguna (ES), University of Groningen (NL), University of Toronto (CA), Université Côte d'Azur (FR), University of Victoria (CA), University of Cambridge (GB), Observatoire astronomique de Strasbourg (FR), Max Planck Institute for Astronomy (DE), Canadian Institute for Theoretical Astrophysics (CA), Observatoire de la Côte d’Azur (FR), Instituto de Astrofísica de Canarias (ES), Université de Strasbourg (FR)
European Space Agency, Agence Nationale de la Recherche, Chinese Academy of Sciences, Nederlandse Organisatie voor Wetenschappelijk Onderzoek, Isaac Newton Trust, National Development and Reform Commission, Natural Sciences and Engineering Research Council of Canada, Science and Technology Facilities Council, European Regional Development Fund, Agencia Estatal de Investigación, National Astronomical Observatories, Chinese Academy of Sciences, Fitzwilliam College, University of Cambridge
Openalex Percentile: Top 9%
Stellar, planetary, and galactic studies
2.98
DOIOpen Access PDFOpenAlex
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