Measuring Radial Orbit Migration in the Milky Way Disk
  We develop and apply a model to quantify the global efficiency of radial
orbit migration among stars in the Milky Way disk. This model parameterizes the
possible star formation and enrichment histories, radial birth profiles, and
combines them with a migration model that relates present-day orbital radii to
birth radii through a Gaussian probability, broadening with age $\tau$ as
$\sigma_\mathrm{RM8}~\sqrt{\tau/8{~\mathrm{Gyr}}}$. Guided by observations, we
assume that stars are born with an initially tight age--metallicity relation at
given radius, which becomes subsequently scrambled by radial orbit migration,
thereby providing a direct observational constraint on radial orbit migration
strength $\sigma_\mathrm{RM8}$. We fit this model with MCMC to the observed
age--metallicity distribution of low-$\alpha$ red clump stars with
Galactocentric radii between 5 and 14 kpc from APOGEE DR12, sidestepping the
complex spatial selection function and accounting for the considerable age
uncertainties. This simple model reproduces well the observed data, and we find
a global (in radius and time) radial orbit migration efficiency in the Milky
Way of $\sigma_\mathrm{RM8}=3.6\pm 0.1$ kpc when marginalizing over all other
model aspects. This shows that radial orbit migration in the Milky Way's main
disk is indeed rather strong, in line with theoretical expectations: stars
migrate by about a half-mass radius over the age of the disk. The model finds
the Sun's birth radius at $\sim 5.2$ kpc. If such strong radial orbit migration
is typical, this mechanism plays indeed an important role in setting the
structural regularity of disk galaxies.
