  The first stars fundamentally transformed the early Universe through their
production of energetic radiation and the first heavy chemical elements. The
impact on cosmic evolution sensitively depends on their initial mass function
(IMF), which can be empirically constrained through detailed studies of
ancient, metal-poor halo stars in our Galaxy. We compare the lowest magnesium
and iron abundances measured in Galactic halo stars with theoretical
predictions for the minimum stellar enrichment provided by Population III stars
under the assumption of a top-heavy IMF. To demonstrate that abundances
measured in metal-poor stars reflect the chemical conditions at their
formation, and that they can thus be used to derive constraints on the
primordial IMF, we carry out a detailed kinematic analysis of a large sample of
metal-poor stars drawn from the SDSS survey. We assess whether interstellar
accretion has altered their surface abundances. We find that accretion is
generally negligible, even at the extremely low levels where the primordial IMF
can be tested. We conclude that the majority of the first stars were very
massive, but had likely masses below ~140 M.
