A stochastic and analytical model of hierarchical fragmentation: The fragmentation of gas structures into young stellar objects in the interstellar medium

TL;DR

This paper presents a stochastic, analytical model of hierarchical fragmentation in molecular clouds, explaining the formation and properties of young stellar objects and their multiplicity across different scales.

Contribution

It introduces a novel hierarchical fragmentation model that combines stochastic processes with analytical physics to describe star formation at multiple scales.

Findings

Fragmentation is self-regulated and scale-dependent.

A characteristic scale prevents further fragmentation at a few tens of AU.

Hierarchical fragmentation explains observed stellar multiplicity and cloud structures.

Abstract

Molecular clouds are the most important incubators of young stars clustered in various stellar structures whose spatial extension can vary from a few AU to several thousand AU. Although the reality of these stellar systems has been established, the physical origin of their multiplicity remains an open question. Our aim was to characterise these stellar groups at the onset of their formation by quantifying both the number of stars they contain and their mass using a hierarchical fragmentation model of the natal molecular cloud. We developed a stochastic and predictive model that reconciles the continuous multi-scale structure of a fragmenting molecular cloud with the discrete nature of the stars that are the products of this fragmentation. This model was implemented within a gravo-turbulent fragmentation framework to analytically follow the fragmentation properties along spatial scales using an isothermal and adiabatic equations of state (EOSs). Using an adiabatic EOS we determined a characteristic spatial scale where further fragmentation is prevented, around a few tens of AU. We show that fragmentation is a self-regulated process as fragments tend to become marginally unstable following a $M \propto R$ Bonnor-Ebert-like mass-size profile. Supersonic turbulent fragmentation structures the cloud down to $R \approx 0.1$ pc, and gradually turns into a less productive Jeans-type fragmentation under subsonic conditions so hierarchical fragmentation is a scale dependant process. Our work suggests that pre-stellar objects resulting from gas fragmentation, have to progressively increase their accretion rate in order to form stars. A hierarchical fragmentation scenario is compatible with both the multiplicity of stellar systems identified in Taurus and the multi-scale structure extracted within NGC 2264 molecular cloud.