2004ApJ...603L..49J

We present the detection of the SiO ν=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 toward the molecular outflows in L1448-mm and L1448 IRS 3. 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 line widths of ∼0.5 km/s, the other lines have widths of ∼1 km/s. Toward L1448-mm, all lines except those of SiO and HCO show two distinct velocity components centered at 4.7 and 5.2 km/s. HCO is only observed in the 4.7 km.s^–1 cloud and SiO in the 5.2 km/s component. The SiO abundance is ∼10^–11 in the 5.2 km/s clouds, 1 order of magnitude larger than in the 4.7 km/s component and in other dark clouds. The HCO abundance is ∼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 component, and the distribution and kinematics of the SiO emission toward L1448-mm suggest that the ambient SiO is associated with the molecular outflows. We propose that the narrow line widths 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 preshocked ambient gas.