The occurrence of nitrogen-enhanced metal-poor stars: implications for
  the initial mass function in the early Galactic halo
  Most carbon-enhanced metal-poor (CEMP) stars are thought to result from past
mass transfer of He-burning material from an asymptotic giant branch (AGB) star
to a low-mass companion star, which we now observe as a CEMP star. Because AGB
stars of intermediate mass efficiently cycle carbon into nitrogen in their
envelopes, the same evolution scenario predicts the existence of a population
of nitrogen-enhanced metal-poor (NEMP) stars, with [N/Fe] > 1 and [N/C] > 0.5.
Such NEMP stars are rare, although their occurrence depends on metallicity:
they appear to be more common at [Fe/H] < -2.8 by about a factor of 10 compared
to less metal-poor stars. We analyse the observed sample of metal-poor stars
with measurements of both carbon and nitrogen to derive firm constraints on the
occurrence of NEMP stars as a function of metallicity. We compare these
constraints to binary population synthesis calculations in which we vary the
initial distributions of mass, mass ratio and binary orbital periods. We show
that the observed paucity of NEMP stars at [Fe/H] > -2.8 does not allow for
large modifications in the initial mass function, as have been suggested in the
literature to account for the high frequency of CEMP stars. The situation at
lower metallicity is less clear, and we do not currently have stellar models to
perform this comparison for [Fe/H] < -2.8. However, unless intermediate-mass
AGB stars behave very differently at such low metallicity, the observed NEMP
frequency at [Fe/H] < -2.8 appears incompatible with the top-heavy forms of the
initial mass function suggested in the literature.
