  In hierarchical structure formation, metal-poor stars in and around the Milky
Way (MW) originate primarily from mergers of lower-mass galaxies. A common
expectation is therefore that metal-poor stars should have isotropic,
dispersion-dominated orbits that do not correlate strongly with the MW disk.
However, recent observations of stars in the MW show that metal-poor ([Fe/H] <
-2) stars are preferentially on prograde orbits with respect to the disk. Using
the FIRE-2 suite of cosmological zoom-in simulations of MW/M31-mass galaxies,
we investigate the prevalence and origin of prograde metal-poor stars. Almost
all (11 of 12) of our simulations have metal-poor stars preferentially on
prograde orbits today and throughout most of their history: we thus predict
that this is a generic feature of MW/M31-mass galaxies. The typical
prograde-to-retrograde ratio is ~ 2:1, which depends weakly on stellar
metallicity at [Fe/H] < -1. These trends predicted by our simulations agree
well with MW observations. Prograde metal-poor stars originate largely from a
single LMC/SMC-mass gas-rich galaxy merger, typically 7-12.5 Gyr ago, which
deposited both existing metal-poor stars and significant gas on an orbital
vector that sparked the formation of and/or shaped the orientation of a
long-lived stellar disk, giving rise to a prograde bias for all low-metallicity
stars. We also find sub-dominant contributions from in-situ stars formed in the
host galaxy before this merger, and in some cases, additional massive mergers.
We find few clear correlations between any properties of our MW/M31-mass
galaxies at z=0 and the degree of this prograde bias as a result of diverse
merger scenarios.
