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arXiv · 2203.11808

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

Tadafumi Matsuno·Emma Dodd·Helmer H. Koppelman·Amina Helmi·Miho N. Ishigaki·Wako Aoki·Jingkun Zhao·Zhen Yuan·Kohei Hattori
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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. Aim: 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]$\sim -2.2$, 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]$\gtrsim -1.5$. 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. Conclusion: The Helmi stream stars are distinguishable from the majority of the halo stars if homogeneously derived abundances are compared.

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Tadafumi Matsuno, Emma Dodd, Helmer H. Koppelman, Amina Helmi, Miho N. Ishigaki, Wako Aoki, Jingkun Zhao, Zhen Yuan, Kohei Hattori. 2023-07-06. High-precision chemical abundances of Galactic building blocks. II. Revisiting the chemical distinctness of the Helmi streams. https://doi.org/10.1051/0004-6361%2F202243609

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