The vertical motions of mono-abundance sub-populations in the Milky Way
  disk
  We present the vertical kinematics of stars in the Milky Way's stellar disk
inferred from SDSS/SEGUE G-dwarf data, deriving the vertical velocity
dispersion, \sigma_z, as a function of vertical height |z| and Galactocentric
radius R for a set of 'mono-abundance' sub-populations of stars with very
similar elemental abundances [\alpha/Fe] and [Fe/H]. We find that all
components exhibit nearly isothermal kinematics in |z|, and a slow outward
decrease of the vertical velocity dispersion: \sigma_z (z,R|[\alpha/Fe],[Fe/H])
~ \sigma_z ([\alpha/Fe],[Fe/H]) x \exp (-(R-R_0)/7 kpc}). The characteristic
velocity dispersions of these components vary from ~ 15 km/s for chemically
young, metal-rich stars, to >~ 50 km/s for metal poor stars. The mean \sigma_z
gradient away from the mid plane is only 0.3 +/- 0.2 km/s/kpc. We find a
continuum of vertical kinetic temperatures (~\sigma^2_z) as function of
([\alpha/Fe],[Fe/H]), which contribute to the stellar surface mass density as
\Sigma_{R_0}(\sigma^2_z) ~ \exp(-\sigma^2_z). The existence of isothermal
mono-abundance populations with intermediate dispersions reject the notion of a
thin-thick disk dichotomy. This continuum of disks argues against models where
the thicker disk portions arise from massive satellite infall or heating;
scenarios where either the oldest disk portion was born hot, or where internal
evolution plays a major role, seem the most viable. The wide range of \sigma_z
([\alpha/Fe],[Fe/H]) combined with a constant \sigma_z(z) for each abundance
bin provides an independent check on the precision of the SEGUE abundances:
\delta_[\alpha/Fe] ~ 0.07 dex and \delta_[Fe/H] ~ 0.15 dex. The radial decline
of the vertical dispersion presumably reflects the decrease in disk
surface-mass density. This measurement constitutes a first step toward a purely
dynamical estimate of the mass profile the disk in our Galaxy. [abridged]
