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High-precision chemical abundances of Galactic building blocks. II. Revisiting the chemical distinctness of the Helmi streams

Abstract : Context. The Helmi streams are a kinematic substructure whose progenitor is likely a dwarf galaxy. Although 20 years have passed since their discovery, it is still unclear whether their members are chemically distinguishable from other halo stars in the Milky Way.
Aims: We aim to precisely characterize the chemical properties of the Helmi streams.
Methods: We analyzed high-resolution, high signal-to-noise ratio spectra for 11 Helmi stream stars through a line-by-line abundance analysis. We compared the derived abundances to homogenized literature abundances of the other halo stars, including those belonging to other kinematic substructures, such as Gaia-Enceladus and Sequoia.
Results: Compared to typical halo stars, the Helmi stream members clearly show low values of [X/Fe] in elements produced by massive stars, such as Na and α-elements. This tendency is seen down to metallicities of at least [Fe/H] ∼ − 2.2, suggesting type Ia supernovae already started to contribute to the chemical evolution at this metallicity. We find that the [α/Fe] ratio does not evolve significantly with metallicity, making the Helmi stream stars less distinguishable from Gaia-Enceladus stars at [Fe/H] ≳ − 1.5. The almost constant but low value of [α/Fe] might be indicative of quiescent star formation with low efficiency at the beginning and bursty star formation at later times. We also find extremely low values of [Y/Fe] at low metallicity, providing further support for the claim that light neutron-capture elements are deficient in Helmi streams. While Zn is deficient at low metallicity, it shows a large spread at high metallicity. The origin of the extremely low Y abundances and Zn variations remains unclear.
Conclusions: The Helmi stream stars are distinguishable from the majority of the halo stars if homogeneously derived abundances are compared.

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Contributor : Olivier Hauss Connect in order to contact the contributor
Submitted on : Tuesday, September 20, 2022 - 3:44:38 PM
Last modification on : Wednesday, September 21, 2022 - 3:38:30 AM


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Tadafumi Matsuno, Emma Dodd, Helmer H. Koppelman, Amina Helmi, Miho N. Ishigaki, et al.. High-precision chemical abundances of Galactic building blocks. II. Revisiting the chemical distinctness of the Helmi streams. Astronomy & Astrophysics, 2022, 665, ⟨10.1051/0004-6361/202243609⟩. ⟨insu-03781617⟩



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