NICMOS Kernel-Phase Interferometry II: Demographics of Nearby Brown Dwarfs

TL;DR

This study uses kernel-phase interferometry on archival HST data to analyze the demographics of binary brown dwarfs, revealing tighter, more equal-mass systems and refining their separation and mass-ratio distributions.

Contribution

It introduces the application of kernel-phase interferometry to brown dwarf surveys, providing new insights into their binary demographics at small separations.

Findings

Companion fraction of 11% after bias correction.

Separation distribution centered at ~2.2 au, smaller than previous studies.

Strong preference for equal-mass systems with a power-law index of ~4.

Abstract

Star formation theories have struggled to reproduce binary brown dwarf population demographics (frequency, separation, mass-ratio). Kernel-phase interferometry is sensitive to companions at separations inaccessible to classical imaging, enabling tests of formation at new physical scales below the hydrogen burning limit. We analyze the detections and sensitivity limits from our previous kernel-phase analysis of archival HST/NICMOS surveys of field brown dwarfs. After estimating physical properties of the 105 late M to T dwarfs using Gaia distances and evolutionary models, we use a Bayesian framework to compare these results to a model companion population defined by log-normal separation and power-law mass-ratio distributions. When correcting for Malmquist bias, we find a companion fraction of $F=0.11^{+0.04}_{-0.03}$ and a separation distribution centered at $\rho=2.2^{+1.2}_{-1.0}$ au, smaller and tighter than seen in previous studies. We also find a mass-ratio power-law index which strongly favors equal-mass systems: $\gamma=4.0^{+1.7}_{-1.5}-11^{+4}_{-3}$ depending on the assumed age of the field population ($0.9-3.1$ Gyr). We attribute the change in values to our use of kernel-phase interferometry which enables us to resolve the peak of the semimajor axis distribution with significant sensitivity to low-mass companions. We confirm the previously-seen trends of decreasing binary fraction with decreasing mass and a strong preference for tight and equal-mass systems in the field-age sub-stellar regime; only $0.9^{+1.1}_{-0.6}$ % of systems are wider than 20 au and $<1.0^{+1.4}_{-0.6}$% of systems have a mass-ratio $q<0.6$. We attribute this to turbulent fragmentation setting the initial conditions followed by a brief period of dynamical evolution, removing the widest and lowest-mass companions, before the birth cluster dissolves.