High-precision chemical abundances of Galactic building blocks. The
  distinct chemical abundance sequence of Sequoia
  Context: Sequoia is a retrograde kinematic substructures in the nearby
Galactic halo, whose properties are a matter of debate. For example, previous
studies do not necessarily agree on the chemical abundances of Sequoia stars,
which are important for understanding its nature. Aim: We characterize the
chemical properties of a sample of stars from Sequoia by determining
high-precision abundances. Methods: We measure abundances of Na, Mg, Si, Ti,
Cr, Mn, Ni, Zn, Y, and Ba from a differential abundance analysis on high
signal-to-noise ratio, high-resolution spectra from new observations and from
archival data. We compare precisely-measured chemical abundances of 12 Sequoia
candidates with those of typical halo stars from literature, which include also
stars from Gaia-Enceladus. This allows us to characterize Sequoia and compare
it to another Galactic building block. The comparison is made after putting all
the abundances onto the same scale using standard stars. Results: There are
significant difference in [Na/Fe], [Mg/Fe], [Ca/Fe], [Ti/Fe], [Zn/Fe], and
[Y/Fe] between Sequoia and Gaia-Enceladus stars at $-1.8\lesssim
[\mathrm{Fe/H}]\lesssim -1.4$ in the sense that these abundance ratios are
lower in Sequoia. These differences are similar to those seen between
Gaia-Enceladus and in-situ stars at higher metallicity, suggesting that Sequoia
is affected by type~Ia supernovae at lower metallicity than Gaia-Enceladus. We
also confirm that the low [Mg/Fe] of Sequoia is seen in literature and in
surveys, namely APOGEE DR16 and GALAH DR3, if the stars are kinematically
selected in the same way. Conclusion: Sequoia stars have a distinct chemical
abundance pattern and can be chemically separated from in-situ stars or
Gaia-Enceladus stars if abundances are measured with sufficient precision,
namely $\sigma([\mathrm{X/Fe}])\lesssim 0.07\,\mathrm{dex}$.
