We employ the first two years of data from the near-infrared , high-resolution SDSS-III/APOGEE spectroscopic survey to investigate the distribution of metallicity and \alpha -element abundances of stars over a large part of the Milky Way disk .
Using a sample of \approx 10 , 000 kinematically-unbiased red-clump stars with \sim 5 % distance accuracy as tracers , the [ \alpha /Fe ] vs. [ Fe/H ] distribution of this sample exhibits a bimodality in [ \alpha /Fe ] at intermediate metallicities , -0.9 < [ Fe/H ] < -0.2 , but at higher metallicities ( [ Fe/H ] \sim +0.2 ) the two sequences smoothly merge .
We investigate the effects of the APOGEE selection function and volume filling fraction and find that these have little qualitative impact on the \alpha -element abundance patterns .
The described abundance pattern is found throughout the range 5 < R < 11 kpc and 0 < |Z| < 2 kpc across the Galaxy .
The [ \alpha /Fe ] trend of the high- \alpha sequence is surprisingly constant throughout the Galaxy , with little variation from region to region ( \sim 10 % ) .
Using simple galactic chemical evolution models we derive an average star formation efficiency ( SFE ) in the high- \alpha sequence of \sim 4.5 \times 10 ^ { -10 } yr ^ { -1 } , which is quite close to the nearly-constant value found in molecular-gas-dominated regions of nearby spirals .
This result suggests that the early evolution of the Milky Way disk was characterized by stars that shared a similar star formation history and were formed in a well-mixed , turbulent , and molecular-dominated ISM with a gas consumption timescale ( SFE ^ { -1 } ) of \sim 2 Gyr .
Finally , while the two \alpha -element sequences in the inner Galaxy can be explained by a single chemical evolutionary track this can not hold in the outer Galaxy , requiring instead a mix of two or more populations with distinct enrichment histories .