A First Constraint on the Thick Disk Scale Length: Differential Radial Abundances in K Giants at Galactocentric Radii 4, 8, and 12 kpc

TL;DR

This study uses high-resolution spectra of red giant stars at various galactocentric radii to constrain the scale length of the Galactic thick disk, suggesting it is significantly shorter than the thin disk, with implications for Galactic formation models.

Contribution

First estimate of the thick disk scale length based on differential chemical abundances, indicating a much shorter scale length than the thin disk, and proposing radial migration as an explanatory mechanism.

Findings

Thick disk scale length estimated at 2.0 kpc.

Outer disk stars show thin disk chemical patterns.

Radial migration may explain the lack of gradients.

Abstract

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.