  W49B is a mixed-morphology supernova remnant with thermal X-ray emission
dominated by the ejecta. In this remnant, the presence of overionized plasma
has been directly established, with information about its spatial structure.
However, the physical origin of the overionized plasma in W49B has not yet been
understood. We investigate this intriguing issue through a 2D hydrodynamic
model that takes into account, for the first time, the mixing of ejecta with
the inhomogeneous circumstellar and interstellar medium, the thermal
conduction, the radiative losses from optically thin plasma, and the deviations
from equilibrium of ionization induced by plasma dynamics. The model was set up
on the basis of the observational results. We found that the thermal conduction
plays an important role in the evolution of W49B, in- ducing the evaporation of
the circumstellar ring-like cloud (whose presence has been deduced from
previous observations) that mingles with the surrounding hot medium, cooling
down the shocked plasma, and pushes the ejecta backwards to the center of the
remnant, forming there a jet-like structure. During the evolution, a large
region of overionized plasma forms within the remnant. The overionized plasma
originates from the rapid cooling of the hot plasma originally heated by the
shock reflected from the dense ring-like cloud. In particular, we found two
different ways for the rapid cooling of plasma to appear: i) the mixing of
relatively cold and dense material evaporated from the ring with the hot
shocked plasma and ii) the rapid adiabatic expansion of the ejecta. The spatial
distribution of the radiative recombination continuum predicted by the
numerical model is in good agreement with that observed.
