  We consider the effects of finite temperature on the scattering of impurity
atoms in a BoseEinstein condensate, showing that the scattering rate is
enhanced by the thermal atoms. Collisions can increase or decrease the impurity
energy. Below the Landau velocity only the first process occurs, i.e., the
collisions cool the condensate. Above the critical velocity the dissipative
collisions prevail over the cooling ones for sufficiently low temperatures.
These considerations are applied to a recent experiment.
