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

November 2014 • 2014ApJ...796...38N

Authors • Nidever, David L. • Bovy, Jo • Bird, Jonathan C. • Andrews, Brett H. • Hayden, Michael • Holtzman, Jon • Majewski, Steven R. • Smith, Verne • Robin, Annie C. • García Pérez, Ana E. • Cunha, Katia • Allende Prieto, Carlos • Zasowski, Gail • Schiavon, Ricardo P. • Johnson, Jennifer A. • Weinberg, David H. • Feuillet, Diane • Schneider, Donald P. • Shetrone, Matthew • Sobeck, Jennifer • García-Hernández, D. A. • Zamora, O. • Rix, Hans-Walter • Beers, Timothy C. • Wilson, John C. • O'Connell, Robert W. • Minchev, Ivan • Chiappini, Cristina • Anders, Friedrich • Bizyaev, Dmitry • Brewington, Howard • Ebelke, Garrett • Frinchaboy, Peter M. • Ge, Jian • Kinemuchi, Karen • Malanushenko, Elena • Malanushenko, Viktor • Marchante, Moses • Mészáros, Szabolcs • Oravetz, Daniel • Pan, Kaike • Simmons, Audrey • Skrutskie, Michael F.

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] versus [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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Jennifer Sobeck

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