Chemical enrichment of stars due to accretion from ISM during the
  Galaxy's assembly
  Using the Eris zoom-in cosmological simulation of assembly of a Milky Way
analog, we study chemical enrichment of stars due to accretion of
metal-enriched gas from the interstellar medium during the Galaxy's
development. We consider metal-poor and old stars in both Galactic halo and
bulge and make use of stellar orbits, gas density and metallicity distributions
in Eris. Assuming spherically symmetric Bondi-Hoyle accretion, we find that
halo and bulge stars accrete metals at the rate of about 10^-24 solar mass per
year and 10^-22 solar mass per year, respectively, at redshifts z < 3, but this
accretion rate increases hundred-fold to about 10^-20 solar mass per year at
higher redshifts due to increased gas density. Bulge and halo stars accrete
similar amounts of metals at high redshifts as kinematically distinct bulge and
halo are not yet developed at these redshifts and both sets of stars encounter
similar metal distribution in the ISM on average. Accretion alone can enrich
main-sequence stars up to [Fe/H] -2 in extreme cases.Median enrichment level
due to accretion in these stars is about [Fe/H]~-6 to -5.Because accretion
mostly takes place at high redshifts, it is alpha-enriched to [alpha/Fe]~0.5.
We find that accretive metal enrichment is significant enough to affect the
predicted metallicity distribution function of halo stars at [Fe/H] < -5.This
suggests that attempts to infer the natal chemical environment of the most
metal-poor stars from their observed enrichment today can be hindered due to
metal accretion. Peculiar enrichment patterns such as those predicted to arise
from pair-instability supernovae could help in disentangling natal and accreted
metal content of stars.
