Kinematics of Stellar Populations with RAVE Data

TL;DR

This study analyzes the kinematics and metallicity of thin and thick disk stars in the Milky Way using RAVE survey data, revealing their velocity dispersions, orbital properties, and implications for disk formation and evolution.

Contribution

It provides new measurements of kinematic and chemical properties of disk stars, and discusses their implications for the formation mechanisms of the Galactic thick disk.

Findings

Thick disk stars have higher velocity dispersion and orbital eccentricity than thin disk stars.

The thick disk scale length is estimated to be 1.5-2.2 kpc.

Radial metallicity gradient in the thin disk is -0.07 dex/kpc.

Abstract

We study the kinematics of the Galactic thin and thick disk populations using stars from the RAVE survey's second data release together with distance estimates from Breddels et al. (2009). The velocity distribution exhibits the expected moving groups present in the solar neighborhood. We separate thick and thin disk stars by applying the X (stellar-population) criterion of Schuster et al. (1993), which takes into account both kinematic and metallicity information. For 1906 thin disk and 110 thick disk stars classified in this way, we find a vertical velocity dispersion, mean rotational velocity and mean orbital eccentricity of (sigma_W, Vphi, e)_thin = (18\pm0.3 km/s, 223\pm0.4 km/s, 0.07\pm0.07) and (sigma_W, Vphi, e)_thick = (35\pm2 km/s, 163\pm2 km/s, 0.31\pm0.16), respectively. From the radial Jeans equation, we derive a thick disk scale length in the range 1.5-2.2 kpc, whose greatest uncertainty lies in the adopted form of the underlying potential. The shape of the orbital eccentricity distribution indicates that the thick disk stars in our sample most likely formed in situ with minor gas-rich mergers and/or radial migration being the most likely cause for their orbits. We further obtain mean metal abundances of <[M/H]>_thin = +0.03 \pm 0.17, and <[M/H]>_thick = -0.51\pm0.23, in good agreement with previous estimates. We estimate a radial metallicity gradient in the thin disk of -0.07 dex/kpc, which is larger than predicted by chemical evolution models where the disk grows insideout from infalling gas. It is, however, consistent with models where significant migration of stars shapes the chemical signature of the disk, implying that radial migration might play at least part of a role in the thick disk's formation.