Tracing the shock precursors in the L1448-mm/IRS3 outflows
  We present the detection of the SiO v=0 J=2-1 and J=3-2 lines, and of the HCO 101-000 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 H13CO+, H13CN, HN13C, CH3OH, and N2H+. While the HCO and the SiO lines have the narrowest profiles with linewidths of 0.5 km s-1, the other lines have widths of 1km 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 10E-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 10E-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. 
I. Jimenez-Serra, J. Martin-Pintado, A. Rodriguez-Franco, N. Marcelino