The L1157 protostellar outflow imaged with the SMA

TL;DR

This study uses high-resolution SMA observations to analyze the molecular composition and kinematic structure of the L1157 protostellar outflow, revealing distinct high-velocity SiO clumps likely originating from EHV bullets.

Contribution

First high-resolution imaging of the L1157 outflow's molecular and kinematic structure, identifying independent high-velocity SiO clumps with densities similar to EHV bullets.

Findings

SiO, H2CO, and CH3OH detected along the inner cavity wall.

High-velocity SiO clumps are spatially and kinematically distinct from low-velocity gas.

High-velocity SiO clumps have densities of 10^5 - 10^6 cm^-3, similar to EHV bullets.

Abstract

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.