Context :

Aims : Precise abundance ratios are determined for 94 dwarf stars with 5200 < \mbox { $T _ { eff } $ } < 6300 K , -1.6 < [ Fe/H ] < -0.4 , and distances D \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 335 pc .
Most of them have halo kinematics , but 16 thick-disk stars are included .

Methods : Equivalent widths of atomic lines are measured from VLT/UVES and NOT/FIES spectra with resolutions R \simeq 55 000 and R \simeq 40 000 , respectively .
An LTE abundance analysis based on MARCS models is applied to derive precise differential abundance ratios of Na , Mg , Si , Ca , Ti , Cr , and Ni with respect to Fe .

Results : The halo stars fall into two populations , clearly separated in [ \alpha /Fe ] , where \alpha refers to the average abundance of Mg , Si , Ca , and Ti .
Differences in [ Na/Fe ] and [ Ni/Fe ] are also present with a remarkably clear correlation between these two abundance ratios .

Conclusions : The ‘ high- \alpha ’ stars may be ancient disk or bulge stars ‘ heated ’ to halo kinematics by merging satellite galaxies or they could have formed as the first stars during the collapse of a proto-Galactic gas cloud .
The kinematics of the ‘ low- \alpha ’ stars suggest that they have been accreted from dwarf galaxies , and that some of them may originate from the \omega Cen progenitor galaxy .