Tracing Chemical Evolution over the Extent of the Milky Way's Disk with APOGEE Red Clump Stars

Nidever, David L. and Bovy, Jo and Bird, Jonathan C. and Andrews, Brett H. and Hayden, Michael and Holtzman, Jon and Majewski, Steven R. and Smith, Verne and Robin, Annie C. and Mészáros, Szabolcs (2014) Tracing Chemical Evolution over the Extent of the Milky Way's Disk with APOGEE Red Clump Stars. ASTROPHYSICAL JOURNAL, 796 (1). ISSN 1538-4357

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Abstract

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 α-element abundances of stars over a large part of the Milky Way disk. Using a sample of ≈ 10, 000 kinematically-unbiased red- clump stars with ∼5% distance accuracy as tracers, the [α/Fe] vs. [Fe/H] distribution of this sample exhibits a bimodality in [α/Fe] at intermediate metallicities, −0.9 <[Fe/H]< −0.2, but at higher metallicities ([Fe/H]∼+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 α-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 [α/Fe] trend of the high-α sequence is surprisingly constant throughout the Galaxy, with little variation from region to region (∼10%). Using simple galactic chemical evolution models we derive an average star formation efficiency (SFE) in the high-α sequence of ∼4.5×10−10 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 ∼ 2 Gyr. Finally, while the two α-element sequences in the inner Galaxy can be explained by a single chemical evolutionary track this cannot hold in the outer Galaxy, requiring instead a mix of two or more populations with distinct enrichment histories.

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