Formation of N-rich field stars in the high-density building blocks of
  the Galactic bulge
  Recent observational studies of the Galactic bulge by APOGEE have revealed
that about 1% of the bulge stars have rather high nitrogen abundances
([N/Fe]>0.5). We here numerically investigate in what physical conditions these
N-rich stars (NRS) can be formed in spherical and disky stellar systems with
stellar masses of 10^7-10^9 M_sun that are the bulge's building blocks. The
principal results are as follows. A large fraction (>0.5) of new stars formed
from interstellar medium polluted (ISM) by ejecta of asymptotic giant branch
stars can have [N/Fe]>0.5 within stellar systems, if the gas mass fraction of
ISM (f_g) is low (< 0.03). The mass fraction of NRS among all stars (f_nrs) can
be higher than 1% within 0.5 Gyr timescale of star formation, if the mean
stellar densities (rho_s) of the systems are higher than 0.1 M_sun/ pc^3. The
[N/Fe] distributions depend on rho_s, f_g, and age distributions of their host
stellar systems. NRS have compact and disky spatial distributions within their
host systems and have rotational kinematics. Based on these results, we propose
that the vast majority of the bulge's NRS originate not from globular clusters
(GCs) but from its high-density building blocks. We suggest that NRS in the
Galactic stellar halo have the same origin as those in the bulge. We also
suggest that low-density dwarf spheroidal and gas-rich dwarfs are unlikely to
form NRS. GCs are not only the formation sites of NRS.
