High-velocity SiO emission in the L 1448 outflow - Evidence for dense shocked gas in the molecular bullets

Astronomy and Astrophysics – Astrophysics

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Emission Spectra, Herbig-Haro Objects, Molecular Flow, Silicon Oxides, Star Formation, Abundance, Evolution (Development), High Speed, Stars: Formation Of, Interstellar Medium: Clouds: L1448, Interstellar Medium: Molecules, Millimeter Lines, Radio Lines: Molecular

Scientific paper

We present strip maps in the J=2→1 , J=3→2 , and J=5→4 lines of υ=0 SiO along the main axis of the extremely high-velocity molecular outflow L 1448. 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 L 1448 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. Our observations of the young L 1448 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 L 1448 high-velocity molecular clumps indicate that these are shocked bullets. We propose 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.

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