The formation of C-shocks: structure and signatures

TL;DR

This paper uses numerical simulations to study the evolution and signatures of C-shocks in molecular clouds, revealing how different shock types develop and their observable emission features over time.

Contribution

It extends ZEUS code for simulating C-shock evolution, analyzing the transition from jump shocks to steady C-shock sub-types with detailed observational predictions.

Findings

C-shocks evolve through four distinct stages with unique signatures.

Most shocks transform into C-type structures, except certain slow shocks that remain J-type.

Emission signatures depend on shock speed and ion density, with observable spectral changes over time.

Abstract

Shock waves in molecular clouds should evolve into continuous or C-type structures due to the magnetic field and ion-neutral friction. We here determine whether and how this is achieved through plane-parallel numerical simulations using an extended version of ZEUS. We first describe and test the adapted code against analytical results, laying the necessary foundations for subsequent works on supersonic ambipolar diffusion, including C-type jets and shock instability. The evolution away from jump shocks toward the numerous steady C-shock sub-types is then investigated. The evolution passes through four stages, which possess distinctive observational properties. The time scales and length scales cover broad ranges. Specific results are included for shock types including switch, absorber, neutralised, oblique, transverse and intermediate. Only intermediate Type II shocks and `slow shocks', including switch-off shocks, remain as J-type under the low ion levels assumed. Other shocks transform via a steadily growing neutral precursor to a diminishing jump. For neutralised shocks, this takes the form of an extended long-lived ramp. Molecular hydrogen emission signatures are presented. After the jump speed has dropped to under 25 km/s, a non-dissociative jump section can dominate the spectra for a long period. This produces a high-excitation spectrum. Once the jump has further weakened, to < 8 km/s, the fully developed ion front is responsible for brisk progress towards a constant C-type excitation. The time scale for emission-line variations is ~(6 /n_i) yr, where n_i is the pre-shock ion number density.