L1157 blue lobe SiO, H2CO, and CH3OH maps

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 ~14km/s 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 ~1500AU 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~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.