Keeping it Cool: Much Orbit Migration, yet Little Heating, in the
  Galactic Disk
  A star in the Milky Way's disk can now be at a Galactocentric radius quite
distant from its birth radius for two reasons: either its orbit has become
eccentric through radial heating, which increases its radial action $J_R$
(`blurring'); or merely its angular momentum $L_z$ has changed and thereby its
guiding radius (`churning'). We know that radial orbit migration is strong in
the Galactic low-$\alpha$ disk and set out to quantify the relative importance
of these two effects, by devising and applying a parameterized model for the
distribution $p(L_z, J_R, \tau, \mathrm[Fe/H])$ in the stellar disk. This model
describes the orbit evolution for stars of age $\tau$ and metallicity [Fe/H],
presuming coeval stars were initially born on (near-)circular orbits, and with
a unique [Fe/H] at a given birth angular momentum and age. We fit this model to
APOGEE red clump stars, accounting for the complex selection function of the
survey. The best fit model implies changes of angular momentum of
$\sqrt{\langle \Delta L_z \rangle^2} \approx 619\,
\mathrm{kpc~km/s~}(\tau/\mathrm{6~Gyr})^{0.5}$, and changes of radial action as
$\sqrt{\langle \Delta J_R \rangle^2} \approx 63\, \mathrm{kpc~km/s~}
(\tau/\mathrm{6~Gyr})^{0.6}$ at 8 kpc. This suggests that the secular orbit
evolution of the disk is dominated by diffusion in angular momentum, with
radial heating being an order of magnitude lower.
