Observational Constraints from the Chemically Divided Galactic Disks

Details Observational Constraints from the Chemically Divided Galactic Disks Observational Constraints from the Chemically Divided Galactic Disks

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21920505l&link_type=abstract

American Astronomical Society, AAS Meeting #219, #205.05

Astronomy and Astrophysics

Astrophysics

Scientific paper

We present measurements of the [α/Fe] ratio, obtained from SDSS/SEGUE medium-resolution spectra of about 17,000 G-type dwarfs, and employ this ratio to separate these stars into likely thin- and thick-disk subsamples. We examine these subsamples for trends of rotational velocity and eccentricity with metallicity, [Fe/H], distance from the Galactic plane, |Z|, and Galactocentric distance, R. Some of the notable findings are that there is a rather strong rotational velocity gradient of -20 to -30 km/s/dex with [Fe/H] for the thin-disk population, and +40 to +50 km/s/dex for the thick-disk population. The rotational velocity decreases with |Z| for both disk components, with similar slopes (-9.0 +\- 1.0 km/s/kpc). A relatively strong gradient of orbital eccentricity with [Fe/H] (about -0.2/dex) is observed for the thick-disk stars, whereas the eccentricity is independent of [Fe/H] for the thin-disk subsample. The shapes of the eccentricity distributions for the thin- and thick-disk populations remain roughly unchanged with |Z|. Comparison with several contemporary models of disk evolution indicates that radial migration seems to have played a vital role in the evolution of the thin-disk population, but probably less so for the thick disk, relative to the gas-rich merger or disk heating scenarios. We emphasize that more physically realistic models and simulations are required to perform detailed quantitative comparisons. This work was supported in part by grants PHY 02-16783 and PHY 08-22648: Physics Frontiers Center/Joint Institute for Nuclear Astrophysics (JINA), awarded by the U.S. National Science Foundation.

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