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 } , \langle V _ { \Phi } \rangle, \langle e \rangle ) _ { \text { thin } } = ( 18 \pm 0.3 km s ^ { -1 } , 223 \pm 0.4 km s ^ { -1 } , 0.07 \pm 0.07 ) and ( \sigma _ { W } , \langle V _ { \Phi } \rangle, \langle e \rangle ) _ { \text { thick } } = ( 35 \pm 2 km s ^ { -1 } , 163 \pm 2 km s ^ { -1 } , 0.31 \pm 0.16 ) , 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 \langle [ M / H ] \rangle _ { \text { thin } } = +0.03 \pm 0.17 , and \langle [ M / H ] \rangle _ { \text { thick } } = -0.51 \pm 0.23 , in good agreement with previous estimates .
We estimate a radial metallicity gradient in the thin disk of -0.07 dex kpc ^ { -1 } , which is larger than predicted by chemical evolution models where the disk grows inside-out 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 .