The critical temperature of a trapped, weakly interacting Bose gas

version 2, 20 octobre 2003.International audienceWe report on measurements of the critical temperature of aharmonically trapped, weakly interacting Bose gas as a function ofatom number. Our results exclude ideal-gas behavior by more thantwo standard deviations, and agree quantitatively with mean-fieldtheory. At our level of sensitivity, we find no additional shiftdue to critical fluctuations. In the course of this measurement,the onset of hydrodynamic expansion in the thermal component hasbeen observed. Our thermometry method takes this feature intoaccount