The oldest low-α thin disc stars
Abstract
The formation pathway of the α-poor Milky Way thin disc remains under debate. The latest observational data indicates the presence of comparable numbers of old stars in both the low- and high-α disc components, suggesting either coeval or overlapping formation epochs. Using GALAH DR4, we provide evidence that the α-poor stars coexist with α-rich stars up to ∼12.5 Gyr. Stellar ages of 0.5 Gyr precision were re-derived for nearly 30 000 subgiant stars using accurate effective temperatures (Teff) from the infrared flux method (IRFM) together with distances based on Gaia parallaxes. About 22\% of the sample constitutes a population peaking at 9.2 Gyr, with evidence for an extended old-age tail reaching 12.5 Gyr. Among stars older than 10 and 11 Gyr, between 20 and 30\% have relative low [α/Fe] and solar-like metallicities (−0.5 ≤ [Fe/H] ≤ +0.5 dex). These stars exhibit kinematics consistent with those of near solar-age α-poor stars in the thin disc. These stars can be evolved Sun-like stars, i.e. old solar analogues, whose Teff and gravity modified with the time. Comparison with a well-tested Galactic disc chemical evolution model supports the presence of a significant fraction of old, α-poor disc stars, confirming the plausibility of our selection criteria. We provide a catalogue with the selected sample to facilitate spectroscopic follow-ups at high resolution.
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BibTeX
@misc{giribaldi2026oldest,
title = {{The oldest low-$α$ thin disc stars}},
author = {Riano E. Giribaldi and Laura Magrini and Marco Palla and Carlos Viscasillas Vázquez},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.25235},
eprint = {2609.25235},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.25235},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The oldest low-α thin disc stars
AU - Riano E. Giribaldi
AU - Laura Magrini
AU - Marco Palla
AU - Carlos Viscasillas Vázquez
PY - 2026
DA - 2026/09/21
PB - arXiv
DO - 10.48550/arxiv.2609.25235
UR - https://arxiv.org/abs/2609.25235
AB - The formation pathway of the α-poor Milky Way thin disc remains under debate. The latest observational data indicates the presence of comparable numbers of old stars in both the low- and high-α disc components, suggesting either coeval or overlapping formation epochs. Using GALAH DR4, we provide evidence that the α-poor stars coexist with α-rich stars up to ∼12.5 Gyr. Stellar ages of 0.5 Gyr precision were re-derived for nearly 30 000 subgiant stars using accurate effective temperatures (T_eff) from the infrared flux method (IRFM) together with distances based on Gaia parallaxes. About 22\% of the sample constitutes a population peaking at 9.2 Gyr, with evidence for an extended old-age tail reaching 12.5 Gyr. Among stars older than 10 and 11 Gyr, between 20 and 30\% have relative low [α/Fe] and solar-like metallicities (−0.5 ≤ [Fe/H] ≤ +0.5 dex). These stars exhibit kinematics consistent with those of near solar-age α-poor stars in the thin disc. These stars can be evolved Sun-like stars, i.e. old solar analogues, whose T_eff and gravity modified with the time. Comparison with a well-tested Galactic disc chemical evolution model supports the presence of a significant fraction of old, α-poor disc stars, confirming the plausibility of our selection criteria. We provide a catalogue with the selected sample to facilitate spectroscopic follow-ups at high resolution.
ER -