TuRMoiL of Survival: A Unified Survival Criterion for Cloud-Wind Interactions

TL;DR

This paper introduces a new, physically motivated survival criterion for clouds in astrophysical cloud-wind interactions, validated through hydrodynamic simulations, and clarifies discrepancies among previous criteria.

Contribution

It proposes a unified survival criterion based on mixing and cooling timescales, improving physical consistency and reconciling prior conflicting criteria.

Findings

The new criterion aligns with empirical formulas but is grounded in physical principles.

Hydrodynamic simulations validate the criterion across various conditions.

Discrepancies among previous criteria are due to different simulation setups and cooling assumptions.

Abstract

Cloud-wind interactions play an important role in long-lived multiphase flows in many astrophysical contexts. When this interaction is primarily mediated by hydrodynamics and radiative cooling, the survival of clouds can be phrased in terms of the comparison between a timescale that dictates the evolution of the cloud-wind interaction, (the dynamical time-scale $\tau_{\rm dyn}$) and the relevant cooling timescale $\tau_{\rm cool}$. Previously proposed survival criteria, which can disagree by large factors about the size of the smallest surviving clouds, differ in both their choice of $\tau_{\rm cool}$ and (to a lesser extent) $\tau_{\rm dyn}$. Here we present a new criterion which agrees with a previously proposed empirical formulae but is based on simple physical principles. The key insight is that clouds can grow if they are able to mix and cool gas from the hot wind faster than it advects by the cloud. Whereas prior criteria associate $\tau_{\rm dyn}$ with the cloud crushing timescale, our new criterion links it to the characteristic cloud-crossing timescale of a hot-phase fluid element, making it more physically consistent with shear-layer studies. We develop this insight into a predictive expression and validate it with hydrodynamic ENZO-E simulations of ${\sim}10^4\, {\rm K}$, pressure-confined clouds in hot supersonic winds, exploring, in particular, high wind/cloud density contrasts, where disagreements are most pronounced. Finally, we illustrate how discrepancies among previous criteria primarily emerged due to different choices of simulation conditions and cooling properties, and discuss how they can be reconciled.