Strongly induced collapse in the Class 0 protostar NGC 1333 IRAS 4A

Context. The onset of gravitational collapse in cluster-forming clouds is still poorly known. Aims. Our goal is to use the Class 0 protostar IRAS 4A, which is undergoing collapse in the active molecular cloud NGC 1333, to set constraints on this process. In particular we want to measure the mass infall rate and investigate whether the collapse could have been triggered by a strong external perturbation. Methods. We analyze existing continuum observations to derive the density structure of the envelope, and use our new molecular line observations with the IRAM 30 m telescope to probe its velocity structure. We perform a detailed comparison of this set of data with a numerical model of collapse triggered by a fast external compression. Results. Both the density and velocity structures of the envelope can be well fitted by this model of collapse induced by a fast external compression for a time of 1-2 x 10(4) yr after the formation of the central protostar. We deduce a high mass infall rate of 0.7-2 x 10-4 M-circle dot yr(-1). The momentum required for the perturbation to produce this high mass infall rate is on the same order as the momenta measured for the NGC 1333 numerous outflows. Our analysis also shows that the turbulence is highly non uniform in the envelope, dropping from supersonic to subsonic values toward the center. The inner subsonic turbulence is most likely a relic of the conditions prevailing in the dense core before the onset of collapse. Conclusions. The vigorous collapse undergone by IRAS 4A was triggered by a fast external compression, probably related to the expansion of a nearby cavity, which could have simultaneously triggered the collapse of the nearby Class 0 protostar IRAS 4B. This cavity could have been generated by an outflow but we have not found a good protostellar candidate yet.