Fragmentation of Shocked Flows: Gravity, Turbulence and Cooling

TL;DR

This paper investigates how large-scale flows, turbulence, and thermal processes contribute to rapid fragmentation in molecular clouds, facilitating early star formation through a combination of dynamical and thermal instabilities.

Contribution

It identifies the regimes where dynamical and thermal instabilities dominate, elucidating their roles in cloud fragmentation and star formation.

Findings

Dynamical instabilities generate turbulence disrupting large-scale flows.

Thermal fragmentation amplifies density perturbations leading to dense star-forming cores.

Global gravity influences only the largest scales, promoting star group formation.

Abstract

The observed rapid onset of star formation in molecular clouds requires rapid formation of dense fragments which can collapse individually before being overtaken by global gravitationally-driven flows. Many previous investigations have suggested that supersonic turbulence produces the necessary fragmentation, without addressing however the source of this turbulence. Motivated by our previous (numerical) work on the flow-driven formation of molecular clouds, we investigate the expected timescales of the dynamical and thermal instabilities leading to the rapid fragmentation of gas swept up by large-scale flows, and compare them with global gravitational collapse timescales. We identify parameter regimes in gas density, temperature and spatial scale within which a given instability will dominate. We find that dynamical instabilities disrupt large-scale coherent flows via generation of turbulence, while strong thermal fragmentation amplifies the resulting low-amplitude density perturbations, thus leading to small-scale, high-density fragments as seeds for {\em local} gravity to act upon. Global gravity dominates only on the largest scales; large-scale gravitationally-driven flows promote the formation of groups and clusters of stars formed by turbulence, thermal fragmentation, and rapid cooling.