Chemical trends in the Galactic halo from APOGEE data
The galaxy formation process in the \Lambda$ Cold Dark Matter scenario can be constrained from the analysis of stars in the Milky Way's halo system. We examine the variation of chemical abundances in distant halo stars observed by the Apache Point Galactic Evolution Experiment (APOGEE) as a function of distance from the Galactic center $r$ and iron abundance [M/H] in the range 5 \lesssim r \lesssim$ 30 kpc and 2.5 15$ kpc and [M/H] 1.1$ (larger in the case of O, Mg and S) with respect to the nearest halo stars. This result confirms previous claims for low \alpha$ stars found at larger distances. Chemical differences in elements with other nucleosynthetic origins (Ni, K, Na, and Al) are also detected. C and N do not provide reliable information about the interstellar medium from which stars formed because our sample comprises RGB and AGB stars and can experience mixing of material to their surfaces.