Two distinct halo populations in the solar neighborhood. II. Evidence   from stellar abundances of Mn, Cu, Zn, Y, and Ba
  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. 
Poul E. Nissen and William J. Schuster
astro-ph.GA