SiO Emission in a Jetlike Molecular Outflow toward L1157

Abstract

We report interferometric observations of the SiO (2-1) and CO (1-0) lines toward the blueshifted lobe of the L1157 bipolar outflow. These observations reveal clumpy SiO emission aligned with highly collimated CO emission. The SiO emission is enhanced in two prominent condensations, similar to those seen in VLA images of the NH3(3, 3) line. The SiO condensation closer to the outflow source peaks within 10'' of a bright knot of the 2.1 μm H2 line emission and shows blueshifted emission extending more than 17 km s-1 from the cloud velocity. The close association between the SiO and H2 emission suggests that the SiO emission is excited in the accelerated postshock regions near the head of an underlying jet. The SiO condensation farther downstream shows a smaller velocity extent and no apparent associated H2 emission. We suggest that the two condensations result from separate interactions of an episodic jet with the surrounding cloud core. In this scenario, the emission region farther downstream results from an older event. Both SiO condensations appear to lead the high-velocity CO emission, consistent with the idea that SiO emission is excited close to the head of the jet and the CO accelerated in the oblique outer wings of the bow shock.