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-α disk and set out to quantify the relative importance of these two effects, by devising and applying a parameterized model ( ${{\boldsymbol{p}}}_{{\boldsymbol{m}}}$ ) for the distribution $p({L}_{z},{J}_{R},\tau ,\left[\mathrm{Fe}/{\rm{H}}\right]| {{\boldsymbol{p}}}_{{\boldsymbol{m}}})$ in the stellar disk. This model describes the orbit evolution for stars of age τ and metallicity $\left[\mathrm{Fe}/{\rm{H}}\right]$ , presuming that coeval stars were initially born on (near-)circular orbits, and with a unique $\left[\mathrm{Fe}/{\rm{H}}\right]$ 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 $\sqrt{\langle {\rm{\Delta }}{L}_{z}{\rangle }^{2}}\approx 619\,\mathrm{kpc}\,\mathrm{km}\,{{\rm{s}}}^{-1}\ {(\tau /6\mathrm{Gyr})}^{0.5}$ and changes of radial action as $\sqrt{\langle {\rm{\Delta }}{J}_{R}{\rangle }^{2}}\approx 63\,\mathrm{kpc}\,\mathrm{km}\,{{\rm{s}}}^{-1}{(\tau /6\mathrm{Gyr})}^{0.6}$ 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.