The Brown-dwarf Desert Persists as a Mass-ratio Desert around Low-mass Stars

TL;DR

This study uses microlensing data to confirm a persistent mass-ratio desert around low-mass stars, indicating a fundamental separation between planetary and sub-stellar companions likely formed through different processes.

Contribution

It identifies a statistically significant companion mass-ratio desert at 0.02 to 0.05, extending the understanding of the brown dwarf desert across stellar types using microlensing surveys.

Findings

A companion mass-ratio desert at 0.02 to 0.05.

Occurrence rate density of giant planets below the desert.

Persistence of the mass-ratio desert across stellar types.

Abstract

Sun-like stars are known to host a paucity of brown dwarf companions at close separations. Direct imaging surveys of intermediate-mass stars have suggested that the brown dwarf desert may be fundamentally a feature in the mass ratio. Microlensing surveys provide a unique opportunity to investigate the nature of this desert around low mass stars, as microlensing hosts have typical masses of 0.05-0.8 $M_\odot$. Here, we perform a statistical analysis of homogeneously selected binary-lens microlensing events in the literature, and identify a companion mass-ratio desert at $0.02\lesssim q\lesssim0.05$ and projected separations around 1-5 au. We derive a statistically significant truncation to the giant-planet mass-ratio distribution at $q\simeq0.02$, above which the occurrence rate density drops by approximately an order of magnitude. Due to the possibility that the small number of detected companions in this desert orbit white dwarfs, this mass-ratio desert may be closer to being completely dry around main-sequence stars. Below this desert, we derive a giant-planet ($q>4\times10^{-4}$) occurrence rate density of $5.8\pm0.8$% per decade of projected separation. Our analysis furthers the hypothesis that the brown dwarf desert is fundamentally a feature in the mass ratio, separating distinct populations of planetary and non-planetary companions that are likely formed via core accretion and gravitational instability, respectively. The persistence of this desert across stellar types suggests that both gas-giant planets and sub-stellar companions form in a scale-invariant fashion, with planets growing to a maximum of approximately 2% their host masses.