Disappearance of N2H+ from the Gas Phase in the Class 0 Protostar IRAM 04191

We present a high-resolution spectroscopic study of the envelope of the young Class 0 protostar IRAM 04191+1522 in Taurus. N2H+(1-0) observations with the Plateau de Bure Interferometer and the IRAM 30m telescope demonstrate that the molecular ion N2H+ disappears from the gas phase in the inner envelope (r < 1600 AU, n(H2) > 4-7 * 1e5 cm-3). This may result from N2 depletion on polar ice mantles and enhanced grain chemistry.