A distinct halo population revealed from 3D non-LTE magnesium abundances
arxiv(2024)
摘要
Magnesium is one of the important elements in stellar physics as an electron
donor and in Galactic Archaeology as a discriminator of different stellar
populations. However, previous studies of Mg I and Mg II lines in metal-poor
benchmark stars have flagged problems with magnesium abundances inferred from
one-dimensional (1D), hydrostatic models of stellar atmospheres, both with or
without the local thermodynamic equilibrium (LTE) approximation. We here
present 3D non-LTE calculations for magnesium in FG-type dwarfs, and provide
corrections for 1D LTE abundances. The 3D non-LTE corrections reduce the
ionisation imbalances in the benchmark metal-poor stars HD84937 and HD140283
from -0.16 dex and -0.27 dex in 1D LTE, to just -0.02 dex and -0.09 dex
respectively. We then applied our abundance corrections to 1D LTE literature
results for stars in the thin disc, thick disc, α-rich halo, and
α-poor halo. We find that the 3D non-LTE results show a richer
substructure in [Mg/Fe]-[Fe/H] in the α-poor halo, revealing two
subpopulations at the metal-rich end. These two subpopulations are also
separated in kinematics, supporting the astrophysical origin of the separation.
While the more magnesium-poor subpopulation is likely to be debris from a
massive accreted galaxy, Gaia-Enceladus, the other subpopulation may be related
to a previous identified group of stars, called Eos. The presence of additional
separation in [Mg/Fe] suggests that previous Mg abundance measurements may have
been limited in the precision by the 1D and LTE approximations, highlighting
the importance of 3D non-LTE modelling.
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