# When Cold Radial Migration is Hot: Constraints from Resonant Overlap

## TL;DR

This paper investigates how cold radial migration, traditionally considered kinematically cold, can cause heating during resonant overlap, providing new constraints on the extent and effects of stellar radial migration in galactic disks.

## Contribution

It demonstrates that cold torquing can induce kinematic heating during resonant overlap, challenging previous assumptions of its cold nature and quantifying the maximum radial excursions.

## Key findings

- Resonant overlap causes kinematic heating of cold torqued stars.
- Up to 40% of cold torqued stars in the solar neighborhood experience resonant overlap.
- Maximum radial excursions depend on spiral strength and rotation curve, constrained by ultraharmonic spacing.

## Abstract

It is widely accepted that stars in a spiral disk, like the Milky Way's, can radially migrate on order a scale length over the disk's lifetime. With the exception of cold torquing, also known as "churning," processes that contribute to the radial migration of stars are necessarily associated with kinematic heating. Additionally, it is an open question whether or not an episode of cold torquing is kinemically cold over long radial distances. This study uses a suite of analytically based simulations to investigate the dynamical response when stars are subject to cold torquing and are also resonant with an ultraharmonic. Model results demonstrate that these populations are kinematically heated and have RMS changes in orbital angular momentum around corotation that can exceed those of populations that do not experience resonant overlap. Thus, kinematic heating can occur during episodes of cold torquing. In a case study of a Milky Way-like disk with an exponential surface density profile and flat rotation curve, up to 40% of cold torqued stars in the solar cylinder experience resonant overlap. This fraction increases toward the galactic center. To first approximation, the maximum radial excursions from cold torquing depend only on the strength of the spiral pattern and the underlying rotation curve. This work places an upper limit to these excursions to be the distance between the ultraharmonics, otherwise radial migration near corotation can kinematically heat. The diffusion rate for kinematically cold radial migration is thus constrained by limiting the step size in the random walk approximation.