The L 1157 protostellar outflow imaged with the Submillimeter Array

Context. The outflow driven by the class 0 low-mass protostar L1157-mm stands out because of its peculiar chemical richness. However, its complex spatial/velocity structure makes it difficult to interpret observations of different molecular tracers. Aims. We aim to map at high spatial resolution different molecular tracers that are important tools for studying shocks and/or thermal-density structures in outflows. Methods. We used the Submillimeter Array at 1.4 mm to observe the blue lobe of the L1157 outflow at high spatial resolution (similar to 3 ''). Results. We detected SiO, H2CO, and CH3OH lines from several molecular clumps that constitute the outflow. All three molecules were detected along the wall of the inner cavity that is thought to be related to the later ejection event. On the other hand, no emission was detected toward positions related to an old ejection episode, which is very likely due to space filtering from the interferometer. The H2CO and CH3OH emission is detected only at velocities close to the systemic velocity. The spatial distributions of the H2CO and CH3OH are similar. These emission lines trace the U-shaped structure seen in the mid-infrared image. In contrast, the SiO emission is detected in a wider velocity range with a peak at similar to 14 km s(-1) blueshifted from the systemic velocity. The SiO emission is brightest at the B1 position, which corresponds to the apex of the U-shaped structure. There are two compact SiO clumps along the faint arc-like feature to the east of the U-shaped structure. At the B1 position, there are two velocity components; one is a compact clump with a size of similar to 1500 AU seen at high velocity, the other is an extended component with lower velocities. The kinematic structure at the B1 position is different from that expected in a single bow shock. Most likely the high-velocity SiO clump at the B1 position is kinetically independent of the low-velocity gas. The line ratio between SiO (5-4) and SiO (2-1) suggests that the high-velocity SiO clumps consist of high-density gas of n similar to 10(5)-10(6) cm(-3), which is similar to the density of the bullets in extremely high velocity (EHV) jets. The high-velocity SiO clumps in L1157 probably have the same origin as the EHV bullets.