High-velocity SiO emission in the L1448 outflow. Evidence for dense shocked gas in the molecular bullets.

The authors present strip maps in the J = 2→1, J = 3→2, and J = 5→4 lines of v = 0 SiO along the main axis of the extremely high-velocity molecular outflow L1448. The flow exhibits strong SiO emission, while the ambient cloud does not emit in the SiO lines. Every high-velocity molecular bullet previously detected in CO by Bachiller et al. (1990) is well observed in SiO J = 2→1. In addition, toward the source driving the outflow the J = 3→2 and J = 5→4 lines are particularly intense, indicating extreme excitation conditions. The unusually high abundance of SiO in the L1448 outflow is very likely generated by shocks that are removing silicon from dust grains. The SiO abundance in the bullets appears to be enhanced by a factor ≥10⁴ with respect to the ambient cloud. The observations of the young L1448 outflow, when contrasted with observations of SiO in other more evolved flows, indicate that considerable amounts of SiO can be formed in the first evolutionary stages of the bipolar outflows and that, afterwards, SiO is quickly depleted onto the dust grains. The high abundance of SiO in the L1448 high-velocity molecular clumps indicate that these are shocked bullets. It is proposed that this kind of dense shocked bullets are the precursors of the Herbig-Haro objects, which are often observed at the end of the main axes of the bipolar outflows.