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

Abstract

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, nH2 > 5 × 105 cm-3). This result departs from the predictions of current chemical models. It suggests either that N2 is more depleted than the models predict, owing to a higher binding energy on polar ice or an enhanced grain chemistry transforming N2 to less volatile species, or that strong deuterium fractionation enhances N2D+ to the detriment of N2H +.