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 cm–3) gas associated with the large (20''-25'') cavities opened by the protostellar wind. We find an N2H+ column density of a few 1012 cm–2 corresponding to an abundance of (2-8) × 10–9. 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 cm–3, and then further compressed and accelerated by the shock.
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