In this paper we study a gaseous Bose-Einstein condensate and show the following: (i) A minimum value of the interaction is needed for the existence of stable persistent currents. (ii) Vorticity is not a fundamental invariant of the system, as there exists a conservative mechanism which can destroy a vortex and change its sign. (iii) This mechanism is suppressed by strong interactions.
Garcia-Ripoll, J., Molina, G., Perez-Garcia, V., Torner, L. Structural instability of vortices in Bose-Einstein-Condensates. "Physical review letters", Setembre 2001, vol. 87, núm. 14, p. 40403-40406.