The L1157 protostellar outflow imaged with the SMA
  We use the Submillimeter Array to observe, at 1.4 mm, the blue-lobe of the
L1157 outflow at high spatial resolution (~ 3"). We detected SiO, H_2CO, and
CH_3OH lines from several molecular clumps that constitute the outflow. All
three molecules were detected along the wall of the inner cavity that is
supposedly related with the later ejection event. On the other hand, no
emission was detected towards positions related to an old ejection episode,
likely due to space filtering from the interferometer. The H_2CO and CH_3OH
emission is detected only at velocities close to the systemic velocity. The
spatial distributions of the H_2CO and CH_3OH are similar. These emission lines
trace the U-shaped structure seen in the mid-infrared image. In contrast, the
SiO emission is detected in wider velocity range with a peak at ~14 km s/s
blue-shifted 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 ~1500 AU seen in the high-velocity and 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. It is
likely that the high-velocity SiO clump at the B1 position is kinetically
independent from 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 comparable to the density of the bullets
in the extremely high velocity (EHV) jets. It is likely that the high-velocity
SiO clumps in L1157 have the same origin as the EHV bullets.
