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

Nidever, D. 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, C.; 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.
Bibliographical reference

The Astrophysical Journal, Volume 796, Issue 1, article id. 38, 16 pp. (2014).

Advertised on:
11
2014
Number of authors
43
IAC number of authors
3
Citations
200
Refereed citations
181
Description
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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