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 { RM 8 } } ~ { } \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 { RM 8 } } .
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 { RM 8 } } = 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 .