  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.
