The B1 shock in the L1157 outflow as seen at high spatial resolution

TL;DR

This study uses high-resolution maps to analyze the B1 shock in the L1157 outflow, revealing the morphology, velocity structure, and physical conditions of the shock and bullets, and suggesting possible origins for the bullets.

Contribution

It provides detailed high-resolution observations and modeling of the B1 shock, offering new insights into the morphology, velocity distribution, and physical conditions of the outflow region.

Findings

High velocity gas is confined in compact bullets.

Lower velocity gas traces the cavity wall.

Models suggest a C-type shock with temperatures above 4000 K.

Abstract

We present high spatial resolution (750 AU at 250 pc) maps of the B1 shock in the blue lobe of the L1157 outflow in four lines: CS (3-2), CH3OH (3_K-2_K), HC3N (16-15) and p-H2CO (2_02-3_01). The combined analysis of the morphology and spectral profiles has shown that the highest velocity gas is confined in a few compact (~ 5 arcsec) bullets while the lowest velocity gas traces the wall of the gas cavity excavated by the shock expansion. A large velocity gradient model applied to the CS (3-2) and (2-1) lines provides an upper limit of 10^6 cm^-3 to the averaged gas density in B1 and a range of 5x10^3< n(H2)< 5x10^5 cm^-3 for the density of the high velocity bullets. The origin of the bullets is still uncertain: they could be the result of local instabilities produced by the interaction of the jet with the ambient medium or could be clump already present in the ambient medium that are excited and accelerated by the expanding outflow. The column densities of the observed species can be reproduced qualitatively by the presence in B1 of a C-type shock and only models where the gas reaches temperatures of at least 4000 K can reproduce the observed HC3N column density.