2013ApJ...776...52C

We present the first detection of N₂H⁺ toward a low-mass protostellar outflow, namely, the L1157-B1 shock, at ∼0.1 pc from the protostellar cocoon. The detection was obtained with the IRAM 30 m antenna. We observed emission at 93 GHz due to the J = 1-0 hyperfine lines. Analysis of this emission coupled with HIFI CHESS multiline CO observations leads to the conclusion that the observed N₂H⁺(1-0) line originated from the dense ( ≥ 10⁵/cm3) gas associated with the large (20''-25'') cavities opened by the protostellar wind. We find an N₂H⁺ column density of a few 10¹²/cm2 corresponding to an abundance of (2-8)x10^–9. The N₂H⁺ abundance can be matched by a model of quiescent gas evolved for more than 10⁴ yr, i.e., for more than the shock kinematical age (≃2000 yr). Modeling of C-shocks confirms that the abundance of N₂H⁺ is not increased by the passage of the shock. In summary, N₂H⁺ is a fossil record of the pre-shock gas, formed when the density of the gas was around 10⁴/cm3, and then further compressed and accelerated by the shock.