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

Abstract

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 ( ) for the distribution in the stellar disk. This model describes the orbit evolution for stars of age τ and metallicity , presuming that coeval stars were initially born on (near-)circular orbits, and with a unique 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 and changes of radial action as 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.