ar X iv : a st ro - p h / 04 02 01 6 v 1 1 Fe b 20 04 Disappearance of N 2 H + 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. N 2 H + (1-0) observations with the Plateau de Bure Interferometer and the IRAM 30m telescope demonstrate that the molecular ion N 2 H + disappears from the gas phase in the inner envelope (r < 1600 AU, n H 2 > 4 − 7 × 10 5 cm −3). This may result from N 2 depletion on polar ice mantles and enhanced grain chemistry. With an age t ∼ 1 − 3 × 10 4 yr measured from the beginning of the accretion phase, IRAM 04191+1522 is the youngest Class 0 protostar known so far in the Taurus molecular cloud (d = 140 pc). It has a prominent envelope (1.5 M ⊙) and an associated powerful bipolar outflow [3]. Belloche et al. [4] showed that the envelope is undergoing both extended infall motions and fast, differential rotation. They proposed that the rapidly rotating inner part of the envelope (r < 3500 AU) corresponds to a magnetically supercritical core decoupling from an environment still supported by magnetic fields and strongly affected by magnetic braking. We recently carried out new observations in the N 2 H + (1-0) line with the Plateau de Bure Interferometer (PdBI) to probe the inner structure of the IRAM 04191 envelope. Here, we present and discuss the results of these high-resolution (∼ 5 ′′) observations of the small-scale distribution of N 2 H + .