High-precision chemical abundances of Galactic building blocks

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.