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Precise abundance ratios are determined for 94 dwarf stars with 5200 < Teff <6300 K, -1.6 < [Fe/H] < -0.4, and distances D < 335 pc. Most of them have halokinematics, but 16 thick-disk stars are included. Equivalent widths of atomiclines are measured from VLT/UVES and NOT/FIES spectra with resolutions R =55000 and R = 40000, respectively. An LTE abundance analysis based on MARCSmodels is applied to derive precise differential abundance ratios of Na, Mg,Si, Ca, Ti, Cr, and Ni with respect to Fe.  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. Differencesin [Na/Fe] and [Ni/Fe] are also present with a remarkably clear correlationbetween these two abundance ratios. The `high-alpha' stars may be ancient diskor bulge stars `heated' to halo kinematics by merging satellite galaxies orthey could have formed as the first stars during the collapse of aproto-Galactic gas cloud. The kinematics of the `low-alpha' stars suggest thatthey have been accreted from dwarf galaxies, and that some of them mayoriginate from the omega Cen progenitor galaxy.

Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics