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A star in the Milky Way's disk can now be at a Galactocentric radius quitedistant from its birth radius for two reasons: either its orbit has becomeeccentric through radial heating, which increases its radial action $J_R$(`blurring'); or merely its angular momentum $L_z$ has changed and thereby itsguiding radius (`churning'). We know that radial orbit migration is strong inthe Galactic low-$\alpha$ disk and set out to quantify the relative importanceof these two effects, by devising and applying a parameterized model for thedistribution $p(L_z, J_R, \tau, \mathrm[Fe/H])$ in the stellar disk. This modeldescribes the orbit evolution for stars of age $\tau$ and metallicity [Fe/H],presuming coeval stars were initially born on (near-)circular orbits, and witha unique [Fe/H] at a given birth angular momentum and age. We fit this model toAPOGEE red clump stars, accounting for the complex selection function of thesurvey. 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 orbitevolution of the disk is dominated by diffusion in angular momentum, withradial heating being an order of magnitude lower.

Keeping it Cool: Much Orbit Migration, yet Little Heating, in the Galactic Disk