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