Apparent Stability in Self-Gravitating Turbulence and the Evolution of Molecular Clouds

TL;DR

This paper models self-gravitating molecular clouds as turbulent eddies in a dynamical system, explaining why they appear stable despite being metastable and dynamically unstable.

Contribution

It introduces a phase-space model showing that clouds near equilibrium can persist due to slow evolution, reconciling observations of stability with underlying instability.

Findings

Equilibrium states are saddle points in phase space, stable in force balance but unstable in energy.

Trajectories tend to slow near equilibrium, causing clouds to be observed in these states.

Relaxation to force balance is faster than the growth of instability, explaining observed stability.

Abstract

Recent observations of hydrostatic structure and virial equilibrium in supersonically turbulent, self-gravitating molecular clouds imply a stability that contrasts with the transcience of turbulent structure. To investigate this contradiction, we model a molecular cloud as a turbulent eddy and study its evolution as a dynamical system. In a two-dimensional phase space of structure and energy, we find that the dynamical equilibrium is a saddle point, stable in the direction aligned with force balance, but unstable in the direction of energy balance because of the combination of the turbulent dissipation and the negative heat capacity of self-gravitation. Near the saddle point, evolutionary trajectories follow a characteristic pattern that first approaches the equilibrium before departing in the direction of instability. Since the phase-space speed is proportional to the virial and energy imbalance, trajectories slow near the equilibrium resulting in a local overdensity of clouds. Also, near equilibrium, the relaxation to force balance is faster than the growth rate of the instability in energy. Consequently, more clouds are observed in near equilibrium states with hydrostatic structure even though the equilibrium is metastable. This resolves the apparent contradiction of equilibrium structure observed in dynamically unstable, self-gravitating turbulence.