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

Aims. Precise abundance ratios are determined for 94 dwarf stars with 5200 < T(eff) < 6300K, -1.6 < [Fe/H] < -0.4, and distances D less than or similar to 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 similar or equal to 55 000 and R similar or equal to 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 a 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.