Context : Current models of galaxy formation predict that the Galactic halo was assembled hierarchically .
By measuring abundance ratios in stars it may be possible to identify substructures in the halo resulting from this process .

Aims : A previous study of 94 dwarf stars with -1.6 < [ Fe/H ] < -0.4 in the solar neighborhood 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 .

Methods : Equivalent widths of atomic lines are measured from high resolution VLT/UVES and NOT/FIES spectra and used to derive abundance ratios from an LTE analysis based on MARCS model atmospheres .
The analysis is made relative to two thick-disk stars , HD 22879 and HD 76932 , such that very precise differential values are obtained .

Results : 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 ] .

Conclusions : 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 .