Measuring Radial Orbit Migration in the Milky Way Disk

TL;DR

This paper presents a model to quantify the efficiency of radial orbit migration in the Milky Way disk, using observed age-metallicity data to estimate how far stars migrate from their birthplaces over time.

Contribution

The study introduces a simple, observationally constrained model to measure the strength of radial orbit migration in the Milky Way, providing the first direct estimate of its global efficiency.

Findings

Radial orbit migration efficiency is approximately 3.6 kpc.

Stars migrate about half the disk's radius over their lifetime.

The Sun likely originated around 5.2 kpc from the Galactic center.

Abstract

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.