Cooling and Instabilities in Colliding Flows

TL;DR

This study uses hydrodynamic simulations to analyze how radiative cooling affects shock stability and instabilities in colliding protostellar flows, revealing oscillatory behaviors and the dominance of the nonlinear thin shell instability.

Contribution

It demonstrates the impact of temperature-dependent cooling on shock stability and identifies the conditions under which radiative shock instability and NTSI occur in colliding flows.

Findings

Radiative shock instability causes shock front oscillations.

No evidence of thermal instability-induced density clumping.

NTSI dominates when cooling length is small.

Abstract

Collisional self-interactions occurring in protostellar jets give rise to strong shocks, the structure of which can be affected by radiative cooling within the flow. To study such colliding flows, we use the AstroBEAR AMR code to conduct hydrodynamic simulations in both one and three dimensions with a power law cooling function. The characteristic length and time scales for cooling are temperature dependent and thus may vary as shocked gas cools. When the cooling length decreases sufficiently rapidly the system becomes unstable to the radiative shock instability, which produces oscillations in the position of the shock front; these oscillations can be seen in both the one and three dimensional cases. Our simulations show no evidence of the density clumping characteristic of a thermal instability, even when the cooling function meets the expected criteria. In the three-dimensional case, the nonlinear thin shell instability (NTSI) is found to dominate when the cooling length is sufficiently small. When the flows are subjected to the radiative shock instability, oscillations in the size of the cooling region allow NTSI to occur at larger cooling lengths, though larger cooling lengths delay the onset of NTSI by increasing the oscillation period.