  A previous study (Nissen & Schuster 2010) of 94 dwarf stars with -1.6 <
[Fe/H] < -0.4 has revealed the existence of two distinct halo populations with
a systematic difference in [alpha/Fe] at a given metallicity. In continuation
of that work, abundances of Mn, Cu, Zn, Y, and Ba are determined for the same
sample of stars. Equivalent widths of atomic lines are measured from high
resolution VLT/UVES and NOT/FIES spectra and used to derive precise abundance
ratios from an LTE analysis based on MARCS model atmospheres. Systematic
differences between the `high-alpha' and `low-alpha' halo populations are found
for [Cu/Fe], [Zn/Fe], and [Ba/Y], whereas there is no significant difference in
the case of [Mn/Fe]. At a given metallicity, [Cu/Fe] shows a large scatter that
is closely correlated with a corresponding scatter in [Na/Fe] and [Ni/Fe]. The
metallicity trends of [Cu/Fe], [Zn/Fe], and [Ba/Y] can be explained from
existing nucleosynthesis calculations if the high-alpha stars formed in regions
with such a high star formation rate that only massive stars and Type II
supernovae contributed to the chemical enrichment. The low-alpha stars, on the
other hand, most likely originate from systems with a slower chemical
evolution, characterized by additional enrichment from Type Ia supernovae and
low-mass AGB stars.
