  A system of bosons in a harmonic trap is cooled via their interactions with a
thermal reservoir. We derive the master equation that governs the evolution of
the system and may describe diverse physical situations: laser cooling,
symphatetic cooling, etc. We investigate in detail the dynamics of the gas in
the Lamb-Dicke limit, whereby the size of the trap is small in comparison to
the de Broglie wavelength of the reservoir quanta. In this case, the dynamics
is characterized by two time scales. First, an equilibrium is reached on a fast
time scale within the degenerated subspaces of the system. Then, an equilibrium
between these subspaces is reached on a slow time scale.
