EVIDENCE FOR PROTOSTELLAR INFALL IN NGC 1333-IRAS2

Observations are presented of HCO+ (J=4-->3) emission towards the Clas s 0 protostar NGC 1333-IRAS2, showing a self-absorbed, asymmetric, dou ble-peaked spectral line profile, with the blue peak more intense than the red. Such a profile is expected in the infalling envelopes of pro tostars. A non-local, spherically symmetric, exact Lambda-iteration ra diative transfer code is used to generate a model HCO+ spectrum for an infalling envelope, which is found to be in good agreement with the o bservations, supporting the protostellar infall interpretation. The to tal mass and luminosity of the protostar required by the model fit to the spectral line data are consistent with those previously derived, a lthough the exact radial dependence of parameters such as density and velocity is not well constrained. This source therefore becomes one of only a small number of protostars in which infall has been directly o bserved.