Shaping the Brown Dwarf Desert: Predicting the Primordial Brown Dwarf Binary Distributions from Turbulent Fragmentation

TL;DR

This paper demonstrates that turbulent core fragmentation can explain the observed distributions of brown dwarf binaries, including the brown dwarf desert and the tight binding of low-mass systems, suggesting a unified formation mechanism for stars and brown dwarfs.

Contribution

It provides an analytical and simulation-based model showing that turbulent fragmentation accounts for brown dwarf binary properties and the brown dwarf desert, unifying their formation with stars.

Findings

The brown dwarf desert naturally arises from angular momentum scaling in turbulent cores.

Very low-mass binaries are more tightly bound than stellar binaries.

The model reproduces the observed fraction of wide brown dwarf binaries.

Abstract

The formation of brown dwarfs (BDs) poses a key challenge to star formation theory. The observed dearth of nearby ($\leq 5$ AU) brown dwarf companions to solar-mass stars, known as the brown dwarf desert, as well as the tendency for low-mass binary systems to be more tightly-bound than stellar binaries, have been cited as evidence for distinct formation mechanisms for brown dwarfs and stars. In this paper, we explore the implications of the minimal hypothesis that brown dwarfs in binary systems originate via the same fundamental fragmentation mechanism as stars, within isolated, turbulent giant molecular cloud cores. We demonstrate analytically that the scaling of specific angular momentum with turbulent core mass naturally gives rise to the brown dwarf desert, as well as wide brown-dwarf binary systems. Further, we show that the turbulent core fragmentation model also naturally predicts that very low-mass (VLM) binary and BD/BD systems are more tightly-bound than stellar systems. In addition, in order to capture the stochastic variation intrinsic to turbulence, we generate $10^4$ model turbulent cores with synthetic turbulent velocity fields to show that the turbulent fragmentation model accommodates a small fraction of binary brown dwarfs with wide separations, similar to observations. Indeed, the picture which emerges from the turbulent fragmentation model is that a single fragmentation mechanism may largely shape both stellar and brown dwarf binary distributions during formation.