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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 $\alpha$-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 [$\alpha$/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 [$\alpha$/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., Comment: Accepted version

Context. The local stellar halo of the Milky Way contains the debris from several past accretion events. Aims. Here we study in detail the structure and properties of nearby debris associated with the Helmi streams, which was originally identified as an overdensity in integrals of motion space. Methods. We use 6D phase-space information from Gaia EDR3 combined with spectroscopic surveys, and we analyse the orbits and frequencies of the stars in the streams using various Galactic potentials. We also explore how the Helmi streams constrain the flattening, q, of the Galactic dark matter halo. Results. We find that the streams are split into substructures in integrals of motion space, most notably into two clumps in angular momentum space. The clumps have consistent metallicity distributions and stellar populations, supporting a common progeny. In all the realistic Galactic potentials explored, the Helmi streams’ stars depict a diffuse distribution close to Ωz/ΩR ∼ 0.7. At the same time, the reason for the substructure in angular momentum space appears to be a Ωz : Ωϕ resonance close to 1:1. This resonance is exactly 1:1 in the case where the (density) flattening of the dark halo is q = 1.2. For this halo shape, the substructure in angular momenta is also long-lasting. Conclusions. Our findings suggest that the structure of the Galactic potential leaves a clear imprint on the properties of phase-mixed debris streams.

The debris from past merger events is expected and, to some extent, known to populate the stellar halo near the Sun. We aim to identify and characterise such merger debris using Gaia DR3 data supplemented by metallicity and chemical abundance information from LAMOST LRS and APOGEE for halo stars within 2.5 kpc from the Sun. We utilise a single linkage-based clustering algorithm to identify over-densities in Integrals of Motion space that could be due to merger debris. Combined with metallicity information and chemical abundances, we characterise these statistically significant over-densities. We find that the local stellar halo contains 7 main dynamical groups, some of in-situ and some of accreted origin, most of which are already known. We report the discovery of a new substructure, which we name ED-1. In addition, we find evidence for 11 independent smaller clumps, 5 of which are new: ED-2, 3, 4, 5 and 6 are typically rather tight dynamically, depict a small range of metallicities, and their abundances when available, as well as their location in Integrals of Motion space, suggest an accreted origin. The local halo contains an important amount of substructure, of both in-situ and accreted origin., Comment: 7 pages, 5 figures, submitted to A&A, our catalogue and substructures will be made available online upon acceptance or before upon reasonable request