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The detection of narrow SiO thermal emission toward young outflows has beenproposed to be a signature of the magnetic precursor of C-shocks. Recentmodeling of the SiO emission across C-shocks predicts variations in the SiOline intensity and line shape at the precursor and intermediate-velocityregimes in only few years. We present high-angular resolution (3.8"x3.3")images of the thermal SiO J=2-1 emission toward the L1448-mm outflow in twoepochs (November 2004-February 2005, March-April 2009). Several SiOcondensations have appeared at intermediate velocities (20-50 km/s) toward thered-shifted lobe of the outflow since 2005. Toward one of the condensations(clump D), systematic differences of the dirty beams between 2005 and 2009could be responsible for the SiO variability. At higher velocities (50-80km/s), SiO could also have experienced changes in its intensity. We proposethat the SiO variability toward L1448-mm is due to a real SiO enhancement byyoung C-shocks at the internal working surface between the jet and the ambientgas. For the precursor regime (5.2-9.2 km/s), several narrow and faint SiOcomponents are detected. Narrow SiO tends to be compact, transient and showselongated (bow-shock) morphologies perpendicular to the jet. We speculate thatthese features are associated with the precursor of C-shocks appearing at theinterface of the new SiO components seen at intermediate velocities.

Variability of the SiO thermal line emission toward the young L1448-mm outflow