2004A&A...419L..35B

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<1600AU, n_H2>5x10⁵cm^-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⁺.