The impact of thermodynamics on gravitational collapse: filament formation and magnetic field amplification

TL;DR

This study explores how different thermodynamic responses of interstellar gas influence gravitational collapse, filament formation, and magnetic field amplification during star formation, highlighting distinct outcomes for varying equations of state.

Contribution

It systematically analyzes the effects of polytropic equations of state on collapse dynamics, filamentary structures, and magnetic field growth in high-redshift and present-day star formation.

Findings

Gamma > 1 leads to virialized, turbulent cores with tangled magnetic fields.

Gamma < 1 results in rapid collapse, filamentary structures, and coherent magnetic fields.

Magnetic energy grows exponentially in both regimes during the kinematic phase.

Abstract

Stars form by the gravitational collapse of interstellar gas. The thermodynamic response of the gas can be characterized by an effective equation of state. It determines how gas heats up or cools as it gets compressed, and hence plays a key role in regulating the process of stellar birth on virtually all scales, ranging from individual star clusters up to the galaxy as a whole. We present a systematic study of the impact of thermodynamics on gravitational collapse in the context of high-redshift star formation, but argue that our findings are also relevant for present-day star formation in molecular clouds. We consider a polytropic equation of state, P = k rho^Gamma, with both sub-isothermal exponents Gamma < 1 and super-isothermal exponents Gamma > 1. We find significant differences between these two cases. For Gamma > 1, pressure gradients slow down the contraction and lead to the formation of a virialized, turbulent core. Weak magnetic fields are strongly tangled and efficiently amplified via the small-scale turbulent dynamo on timescales corresponding to the eddy-turnover time at the viscous scale. For Gamma < 1, on the other hand, pressure support is not sufficient for the formation of such a core. Gravitational contraction proceeds much more rapidly and the flow develops very strong shocks, creating a network of intersecting sheets and extended filaments. The resulting magnetic field lines are very coherent and exhibit a considerable degree of order. Nevertheless, even under these conditions we still find exponential growth of the magnetic energy density in the kinematic regime.