http://arxiv.org/abs/2203.11808
Context: The Helmi streams are kinematic substructures whose progenitor is likely a dwarf galaxy. Although 20 years have past 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 analysed high-resolution, high-signal-to-noise ratio spectra for eleven Helmi streams’ 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 streams’ members clearly show low values of [X/Fe] in elements produced by massive stars, such as Na and $\alpha$-elements. This tendency is seen down to metallicities of at least $[\mathrm{Fe/H}]\sim -2.2$, suggesting type~Ia supernovae already started to contribute to the chemical evolution at this metallicity. We found that the [$\alpha$/Fe] ratio does not evolve much with metallicity, making the Helmi streams’ stars less distinguishable from Gaia-Enceladus stars at $[\mathrm{Fe/H}]\gtrsim -1.5$. The almost constant but low value of [$\alpha$/Fe] might be indicative of quiescent star formation with low efficiency at the beginning and bursty star formation at later times. We also found extremely low values of [Y/Fe] at low metallicity, providing further support 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 remain unclear. Conclusion: The Helmi streams’ stars are distinguishable from the majority of the halo stars if homogeneously derived abundances are compared.
T. Matsuno, E. Dodd, H. Koppelman, et. al.
Wed, 23 Mar 22
58/76
Comments: 15 pages. Submitted to A&A. The entity of Table 5 is included in the source. Comments welcome!
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