A First Constraint on the Thick Disk Scale Length: Differential Radial
  Abundances in K Giants at Galactocentric Radii 4, 8, and 12 kpc
  Based on high-resolution spectra obtained with the MIKE spectrograph on the
Magellan telescopes we present detailed elemental abundances for 20 red giant
stars in the outer Galactic disk, located at Galactocentric distances between 9
and 13 kpc. The outer disk sample is complemented with samples of red giants
from the inner Galactic disk and the solar neighbourhood, analysed using
identical methods. For Galactocentric distances beyond 10 kpc, we only find
chemical patterns associated with the local thin disk, even for stars far above
the Galactic plane. Our results show that the relative densities of the thick
and thin disks are dramatically different from the solar neighbourhood, and we
therefore suggest that the radial scale length of the thick disk is much
shorter than that of the thin disk. We make a first estimate of the thick disk
scale-length of L_thick=2.0 kpc, assuming L_thin=3.8 kpc for the thin disk. We
suggest that radial migration may explain the lack of radial age, metallicity,
and abundance gradients in the thick disk, possibly also explaining the link
between the thick disk and the metal-poor bulge.
