  Low-mass planets migrate in the type-I regime. In the inviscid limit, the
contrast between the vortensity trapped inside the planet's corotating region
and the background disk vortensity leads to a dynamical corotation torque,
which is thought to slow down inward migration. We investigate the effect of
radiative cooling on low-mass planet migration using inviscid 2D hydrodynamical
simulations. We find that cooling induces a baroclinic forcing on material
U-turning near the planet, resulting in vortensity growth in the corotating
region, which in turn weakens the dynamical corotation torque and leads to 2-3x
faster inward migration. This mechanism is most efficient when cooling acts on
a timescale similar to the U-turn time of material inside the corotating
region, but is nonetheless relevant for a substantial radial range in a typical
disk (5-50 au). As the planet migrates inwards, the contrast between the
vortensity inside and outside the corotating region increases and partially
regulates the effect of baroclinic forcing. As a secondary effect, we show that
radiative damping can further weaken the vortensity barrier created by the
planet's spiral shocks, supporting inward migration. Finally, we highlight that
a self-consistent treatment of radiative diffusion as opposed to local cooling
is critical in order to avoid overestimating the vortensity growth and the
resulting migration rate.
