The GALAH survey: Tracing the Milky Way's formation and evolution through RR Lyrae stars
arxiv(2024)
摘要
Stellar mergers and accretion events have been crucial in shaping the
evolution of the Milky Way (MW). These events have been dynamically identified
and chemically characterised using red giants and main-sequence stars. RR Lyrae
(RRL) variables can play a crucial role in tracing the early formation of the
MW since they are ubiquitous, old (t≥10 Gyr) low-mass stars and accurate
distance indicators. We exploited Data Release 3 of the GALAH survey to
identify 78 field RRLs suitable for chemical analysis. Using synthetic spectra
calculations, we determined atmospheric parameters and abundances of Fe, Mg,
Ca, Y, and Ba. Most of our stars exhibit halo-like chemical compositions, with
an iron peak around [Fe/H]≈ -1.40, and enhanced Ca and Mg content.
Notably, we discovered a metal-rich tail, with [Fe/H] values ranging from -1
to approximately solar metallicity. This sub-group includes almost 1/4 of the
sample, it is characterised by thin disc kinematics and displays sub-solar
α-element abundances, marginally consistent with the majority of the MW
stars. Surprisingly, they differ distinctly from typical MW disc stars in terms
of the s-process elements Y and Ba. We took advantage of similar data available
in the literature and built a total sample of 535 field RRLs for which we
estimated kinematical and dynamical properties. We found that metal-rich RRLs
(1/3 of the sample) likely represent an old component of the MW thin disc. We
also detected RRLs with retrograde orbits and provided preliminary associations
with the Gaia-Sausage-Enceladus, Helmi, Sequoia, Sagittarius, and Thamnos
stellar streams.
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