Second sound in a laser cooled gas

We show that the second sound can exist in the ultra-cold gas of atoms confined and cooled in a magneto-optical trap. Our approach is based on a two-fluids model, similar to that used for superfluidity. Each of the fluids (the atomic gas and the phonon gas) is described by similar kinetic and fluid equation, which are coupled. The second sound modes are elementary excitations of these two coupled fluids. The ultra-cold gas could be used as an experimental test bed for detailed studies of the dispersion properties of the first and second sound.