# On the Formation and Properties of Fluid Shocks and Collisionless Shock Waves in Astrophysical Plasmas

## TL;DR

This paper investigates the formation and properties of fluid and collisionless shocks in astrophysical plasmas, identifying critical densities and conditions that determine shock nature and their observational implications.

## Contribution

It introduces a method to distinguish collisionless from collisional shock formation based on plasma parameters and explores their astrophysical consequences.

## Key findings

- Critical density threshold for collisionless shock formation.
- Conditions under which downstream becomes field free.
- Parameter space for strong shock formation in classical and degenerate plasmas.

## Abstract

When two plasmas collide, their interaction can be mediated by collisionless plasma instabilities or binary collisions between particles of each shell. By comparing the maximum growth rate of the collisionless instabilities with the collision frequency between particles of the shells, we determine the critical density separating the collisionless formation from the collisional formation of the resulting shock waves. This critical density is also the density beyond which the shock downstream is field free, as plasma instabilities do not have time to develop electromagnetic patterns. We further determine the conditions on the shells initial density and velocity for the downstream to be collisional. If these quantities fulfill the determined conditions, the collisionality of the downstream also prevents the shock from accelerating particles or generating strong magnetic fields. We compare the speed of sound with the relative speed of collision between the two shells, thus determining the portion of the parameters space where strong shock formation is possible for both classical and degenerate plasmas. Finally, we discuss the observational consequences in several astrophysical settings.