We present the first detection of N _ { 2 } H ^ { + } towards a low-mass protostellar outflow , namely the L1157-B1 shock , at \sim 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 .
The analysis of the emission coupled with the HIFI CHESS multiline CO observations leads to the conclusion that the observed N _ { 2 } H ^ { + } ( 1–0 ) line originates from the dense ( \geq 10 ^ { 5 } cm ^ { -3 } ) gas associated with the large ( 20 \arcsec –25 \arcsec ) cavities opened by the protostellar wind .
We find a N _ { 2 } H ^ { + } column density of few 10 ^ { 12 } cm ^ { -2 } corresponding to an abundance of ( 2–8 ) \times 10 ^ { -9 } .
The N _ { 2 } H ^ { + } abundance can be matched by a model of quiescent gas evolved for more than 10 ^ { 4 } yr , i.e .
for more than the shock kinematical age ( \simeq 2000 yr ) .
Modelling of C-shocks confirms that the abundance of N _ { 2 } H ^ { + } is not increased by the passage of the shock .
In summary , N _ { 2 } H ^ { + } is a fossil record of the pre-shock gas , formed when the density of the gas was around 10 ^ { 4 } cm ^ { -3 } , and then further compressed and accelerated by the shock .