Chemo-dynamics and asteroseismic ages of seven metal-poor red giants from the Kepler field

We use accurate estimates of aluminium abundance from the APOGEE Data Release 17 and Gaia Early Data Release 3 astrometry to select a highly pure sample of stars with metallicity −1.5 ≲ [Fe/H] ≲ 0.5 born in-situ in the Milky Way proper. The low-metallicity ([Fe/H]≲ −1.3) in-situ component we dub Aurora is kinematically hot with an approximately isotropic velocity ellipsoid and a modest net rotation. Aurora stars exhibit large scatter in metallicity and in many element abundance ratios. The median tangential velocity of the in-situ stars increases sharply with metallicity between [Fe/H]=−1.3 and −0.9, the transition that we call the spin-up. The observed and theoretically expected age-metallicity correlations imply that this increase reflects a rapid formation of the MW disk over ≈1 − 2 Gyrs. The transformation of the stellar kinematics as a function of [Fe/H] is accompanied by a qualitative change in chemical abundances: the scatter drops sharply once the Galaxy builds up a disk during later epochs corresponding to [Fe/H]>−0.9. Results of galaxy formation models presented in this and other recent studies strongly indicate that the trends observed in the MW reflect generic processes during the early evolution of progenitors of MW-sized galaxies: a period of chaotic pre-disk evolution, when gas is accreted along cold narrow filaments and when stars are born in irregular configurations, and subsequent rapid disk formation. The latter signals formation of a stable hot gaseous halo around the MW progenitor, which changes the mode of gas accretion and allows development of coherently rotating disk.

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From dawn till disc: Milky Way’s turbulent youth revealed by the APOGEE+Gaia data

Belokurov

Kravtsov

2022

Monthly Notices of the Royal Astronomical Society

102

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The odd bunch: chrono-chemo-dynamics of sixteen unusual stars from Kepler

Puls

Casagrande

Monty

et al. 2023

Monthly Notices of the Royal Astronomical Society

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In this study we combine asteroseismic, spectroscopic and kinematic information to perform a detailed analysis of a sample of 16 stars from the Kepler field. Our selection focuses on stars that appear to contradict Galactic chemical evolution models: young and α-rich, old and metal-rich, as well as other targets with unclear classification in past surveys. Kinematics are derived from Gaia DR3 parallaxes and proper motions, and high-resolution spectra from HIRES/Keck are used to calculate chemical abundances for over 20 elements. This information is used to perform careful checks on asteroseismic masses and ages derived via grid-based modelling. Among the seven stars previously classified as young and α-rich, only one seems to be an unambiguously older object masking its true age. We confirm the existence of two very old (≥11 Gyr), super metal rich (≥0.1 dex) giants. These two stars have regular thin disc chemistry and in-plane solar circle orbits which fit well in the picture of radial migration via the churning mechanism. The alternative explanation that these stars have younger ages would require mass-loss rates which strongly increases with increasing metallicity. Finally, we suggest further investigations to explore the suitability of Zn as a chemical clock in red giants.

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The ratio of [Eu/α] differentiates accreted/in situ Milky Way stars across metallicities, as indicated by both field stars and globular clusters

Monty,

Belokurov,

Sanders

et al. 2024

Monthly Notices of the Royal Astronomical Society

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We combine stellar orbits with the abundances of the heavy, r-process element europium and the light, α-element, silicon to separate in-situ and accreted populations in the Milky Way across all metallicities. At high orbital energy, the accretion-dominated halo shows elevated values of [Eu/Si], while at lower energies, where many of the stars were born in-situ, the levels of [Eu/Si] are lower. These systematically different levels of [Eu/Si] in the MW and the accreted halo imply that the scatter in [Eu/α] within a single galaxy is smaller than previously thought. At the lowest metallicities, we find that both accreted and in-situ populations trend down in [Eu/Si], consistent with enrichment via neutron star mergers. Through compiling a large dataset of abundances for 54 globular clusters (GCs), we show that differences in [Eu/Si] extend to populations of in-situ/accreted GCs. We interpret this consistency as evidence that in r-process elements GCs trace the star formation history of their hosts, motivating their use as sub-Gyr timers of galactic evolution. Furthermore, fitting the trends in [Eu/Si] using a simple galactic chemical evolution model, we find that differences in [Eu/Si] between accreted and in-situ MW field stars cannot be explained through star formation efficiency alone. Finally, we show that the use of [Eu/Si] as a chemical tag between GCs and their host galaxies extends beyond the Local Group, to the halo of M31 - potentially offering the opportunity to do Galactic Archaeology in an external galaxy.

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Chemo-dynamics and asteroseismic ages of seven metal-poor red giants from the Kepler field

Cited by 5 publications

References 115 publications

From dawn till disc: Milky Way’s turbulent youth revealed by the APOGEE+Gaia data

From dawn till disc: Milky Way’s turbulent youth revealed by the APOGEE+Gaia data

The odd bunch: chrono-chemo-dynamics of sixteen unusual stars from Kepler

The ratio of [Eu/α] differentiates accreted/in situ Milky Way stars across metallicities, as indicated by both field stars and globular clusters

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