We present the detection of the SiO \nu =0 J =2–1 and J =3–2 lines , and of the HCO 1 _ { 01 } –0 _ { 00 } J =3/2–1/2 F =2–1 line at ambient velocities towards the molecular outflows in L1448–mm and L1448–IRS3 .
This is the first detection of HCO in a dark cloud .
We have also measured lines of H ^ { 13 } CO ^ { + } , H ^ { 13 } CN , HN ^ { 13 } C , CH _ { 3 } OH , and N _ { 2 } H ^ { + } .
While the HCO and the SiO lines have the narrowest profiles with linewidths of \sim 0.5 km s ^ { -1 } , the other lines have widths of \sim 1 km s ^ { -1 } .
Towards L1448–mm , all lines except those of SiO and HCO , show two distinct velocity components centered at 4.7 and 5.2 km s ^ { -1 } .
HCO is only observed in the 4.7 km s ^ { -1 } cloud , and SiO in the 5.2 km s ^ { -1 } component .
The SiO abundance is \sim 10 ^ { -11 } in the 5.2 km s ^ { -1 } clouds , one order of magnitude larger than in the 4.7 km s ^ { -1 } component and in other dark clouds .
The HCO abundance is \sim 10 ^ { -11 } , similar to that predicted by the ion–molecule reactions models for the quiescent gas in dark clouds .
The large change in the SiO/HCO abundance ratio ( > 150 ) from the 4.7 to the 5.2 km s ^ { -1 } component , and the distribution and kinematics of the SiO emission towards L1448–mm suggest that the ambient SiO is associated with the molecular outflows .
We propose that the narrow linewidths and the abundances of SiO in the ambient gas are produced by the interaction of the magnetic and/or radiative precursors of the shocks with the clumpy pre–shocked ambient gas .