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-a disk and set out to quantify the relative importance of these two effects, by devising and applying a parameterized model (p(m)) for the distribution p(L-z, J(R), tau, [Fe H]vertical bar p(m)) in the stellar disk. This model describes the orbit evolution for stars of age tau and metallicity [Fe H], presuming that 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 root <Delta L-z >(2) approximate to 619 kpc km s(-1) (tau/6 Gyr)(0.5) and changes of radial action as root <Delta L-R >(2) approximate to 63 kpc km s(-1) (tau/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.