Surface modes and critical velocity in trapped Bose-condensates

We propose the novel mechanism leading to superfluidity breakdown in dilute Bose-condensed gases. We discuss the properties and highlight the role played by surface excitations in trapped condensates. We show that the critical velocity measured in recent experiments is nothing else but the critical velocity associated with the spontaneous creation of surface modes. The latter turns out to be larger than that found by the analysis of a superflow stability with respect to vortex nucleation.