Characterizing the Brown Dwarf Formation Channels from the Initial Mass Function and Binary-star Dynamics

TL;DR

This paper investigates brown dwarf formation channels by analyzing the initial mass function and binary-star dynamics, proposing an empirical residual mass function, and updating models to better match observations of substellar populations.

Contribution

It introduces the concept of peripheral fragmentation as an additional brown dwarf formation channel and refines binary formation models to align with observational data.

Findings

Residual mass function aligns with observed brown dwarf distributions

Updated binary model matches observed binary fractions and mass ratios

Peripheral fragmentation improves brown dwarf formation modeling

Abstract

The stellar initial mass function (IMF) is a key property of stellar populations. There is growing evidence that the classical star-formation mechanism by the direct cloud fragmentation process has difficulties reproducing the observed abundance and binary properties of brown dwarfs and very-low-mass stars. In particular, recent analytical derivations of the stellar IMF exhibit a deficit of brown dwarfs compared to observational data. Here we derive the residual mass function of brown dwarfs as an empirical measure of the brown dwarf deficiency in recent star-formation models with respect to observations and show that it is compatible with the substellar part of the Thies-Kroupa IMF and the mass function obtained by numerical simulations. We conclude that the existing models may be further improved by including a substellar correction term that accounts for additional formation channels like disk or filament fragmentation. The term "peripheral fragmentation" is introduced here for such additional formation channels. In addition, we present an updated analytical model of stellar and substellar binarity. The resulting binary fraction and the dynamically evolved companion mass-ratio distribution are in good agreement with observational data on stellar and very-low-mass binaries in the Galactic field, in clusters, and in dynamically unprocessed groups of stars if all stars form as binaries with stellar companions. Cautionary notes are given on the proper analysis of mass functions and the companion mass-ratio distribution and the interpretation of the results. The existence of accretion disks around young brown dwarfs does not imply that these form just like stars in direct fragmentation.