Tracing shock type with chemical diagnostics: an application to L1157

TL;DR

This study investigates molecular signatures of J-type and C-type shocks in protostellar outflows, applying chemical diagnostics to L1157 to identify shock-specific tracers and assess shock types in the B2 clump.

Contribution

It introduces a chemical diagnostic approach to distinguish shock types in protostellar outflows, applying models to L1157 to analyze shock-specific molecular behaviors.

Findings

H2O and HCN show shock-type specific behavior at low velocities and densities.

CH3OH is a reliable pre-shock density probe but does not differentiate shock types.

L1157-B2's molecular abundances are consistent with both C-type and J-type shocks.

Abstract

Aims: The physical structure of a shock wave may take a form unique to its shock type, implying that the chemistry of each shock type is unique as well. We aim to investigate the different chemistries of J-type and C-type shocks in order to identify unique molecular tracers of both shock types. We apply these diagnostics to the protostellar outflow L1157 to establish whether the B2 clump could host shocks exhibiting type-specific behaviour. Of particular interest is the L1157-B2 clump, which has been shown to exhibit bright emission in S-bearing species and HNCO. Methods: We simulate, using a parameterised approach, a planar, steady-state J-type shock wave using UCLCHEM. We compute a grid of models using both C-type and J-type shock models to determine the chemical abundance of shock-tracing species as a function of distance through the shock and apply it to the L1157 outflow. We focus on known shock-tracing molecules such as H2O, HCN, and CH3 OH. Results: We find that a range of molecules including H2O and HCN have unique behaviour specific to a J-type shock, but that such differences in behaviour are only evident at low $v_{s}$ and low $n_{H}$. We find that CH3OH is enhanced by shocks and is a reliable probe of the pre-shock gas density. However, we find no difference between its gas-phase abundance in C-type and J-type shocks. Finally, from our application to L1157, we find that the fractional abundances within the B2 region are consistent with both C-type and J-type shock emission.