  Detailed elemental-abundance patterns of giant stars in the Galactic halo
measured by APOGEE-2 have revealed the existence of a unique and significant
stellar sub-population of silicon-enhanced ([Si/Fe]$\gtrsim +0.5$) metal-poor
stars, spanning a wide range of metallicities
($-1.5\lesssim$[Fe/H]$\lesssim-0.8$). Stars with over-abundances in [Si/Fe] are
of great interest because these have very strong silicon ($^{28}$Si) spectral
features for stars of their metallicity and evolutionary stage, offering clues
about rare nucleosynthetic pathways in globular clusters (GCs). Si-rich field
stars have been conjectured to have been evaporated from GCs, however, the
origin of their abundances remains unclear, and several scenarios have been
offered to explain the anomalous abundance ratios. These include the hypothesis
that some of them were born from a cloud of gas previously polluted by a
progenitor that underwent a specific and peculiar nucleosynthesis event, or due
to mass transfer from a previous evolved companion. However, those scenarios do
not simultaneously explain the wide gamut of chemical species that are found in
Si-rich stars. Instead, we show that the present inventory of such unusual
stars, as well as their relation to known halo substructures (including the
in-situ halo, \textit{Gaia}-Enceladus, the Helmi Stream(s), and Sequoia, among
others), is still incomplete. We report the chemical abundances of the
iron-peak (Fe), the light- (C and N), the $\alpha-$ (O and Mg), the odd-Z (Na
and Al), and the \textit{s}-process (Ce and Nd) elements of 55 newly identified
Si-rich field stars (among more than $\sim$600,000 APOGEE-2 targets), that
exhibit over-abundance of [Si/Fe] as extreme as those observed in some Galactic
GCs, and are relatively cleanly from other stars in the [Si/Fe]-[Fe/H] plane.
This new census confirms the presence of a statistically significant ...
