Disappearance of N2H+ from the gas phase in the class 0 protostar IRAM 04191

We present a high-resolution millimeter study of the very young class 0 protostar IRAM 04191+1522 in the Taurus molecular cloud. N2H+(1-0) observations with the IRAM Plateau de Bure Interferometer and 30 m telescope demonstrate that the molecular ion N2H+ disappears from the gas phase in the inner part of the protostellar envelope (r < 1600 AU, n(H2) > 5 x 10(5) cm(-3)). This result departs from the predictions of current chemical models. It suggests either that N-2 is more depleted than the models predict, owing to a higher binding energy on polar ice or an enhanced grain chemistry transforming N-2 to less volatile species, or that strong deuterium fractionation enhances N2D+ to the detriment of N2H+.