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