Tracing Chemical Evolution Over The Extent Of The Milky Way's Disk With APOGEE Red Clump Stars




Nidever, David L.
Bovy, Jo
Bird, Jonathan C.
Andrews, Brett H.
Hayden, Michael
Holtzman, Jon
Majewski, Steven R.
Smith, Verne
Robin, Annie C.
Perez, Ana E. Garcia

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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 approximate to 10,000 kinematically unbiased red-clump stars with similar to 5% distance accuracy as tracers, the [alpha/Fe] versus [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] similar to +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 (similar to 10%). Using simple galactic chemical evolution models, we derive an average star-formation efficiency (SFE) in the high-alpha sequence of similar to 4.5x10(-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 similar to 2Gyr. Finally, while the two alpha-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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Nidever, David L., Jo Bovy, Jonathan C. Bird, Brett H. Andrews, Michael Hayden, Jon Holtzman, Steven R. Majewski et al. "Tracing chemical evolution over the extent of the Milky Way's Disk with APOGEE Red Clump Stars." The Astrophysical Journal, Vol. 796, No. 1 (Nov., 2014): 38.