Magnetic precursors of C–shocks accelerate , compress and heat molecular ions , modifying the kinematics and the physical conditions of the ion fluid with respect to the neutral one .
Electron densities are also expected to be significantly enhanced in shock precursors .
In this Letter , we present observations of strongly polar ion and neutral molecules such as SiO , H ^ { 13 } CO ^ { + } , HN ^ { 13 } C and H ^ { 13 } CN , which reveal the electron density enhancements associated with the precursor of the young L1448–mm outflow .
While in the ambient gas the excitation of the ions and neutrals is explained by collisional excitation by H _ { 2 } with a single density of \sim 10 ^ { 5 } cm ^ { -3 } , H ^ { 13 } CO ^ { + } shows an over excitation in the shock precursor component that requires H _ { 2 } densities of a factor of \geq 10 larger than those derived from the neutral species .
This over excitation in H ^ { 13 } CO ^ { + } can be explained if we consider an additional excitation by collisions with electrons and an electron density enhancement in the precursor stage by a factor of \sim 500 , i.e .
a fractional ionization of 5 \times 10 ^ { -5 } .
These results show that multiline observations can be used to study the evolution of the ion and electron fluids at the first stages of the C–shock interaction .