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

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 $0.02\lesssim q\lesssim 0.05$ and projected separations around 1-5 au. We derive a statistically significant truncation to the giant-planet mass-ratio distribution at $q\simeq 0.02$, 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 ($q>4\times 10^{-4}$) occurrence rate density of $5.8\pm 0.8$% 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.

Accepted to ApJ Letters. 11 pages, 3 figures. Comments welcome

References in corpus (23)

• The Exoplanet Mass-Ratio Function from the MOA-II Survey: Discovery of a Break and Likely Peak at a Neptune Mass
• The 100 pc White Dwarf Sample in the SDSS Footprint
• The M-dwarfs in Multiples (MinMs) survey - I. Stellar multiplicity among low-mass stars within 15 pc
• A Jovian analogue orbiting a white dwarf star
• Systematic KMTNet Planetary Anomaly Search, Paper I: OGLE-2019-BLG-1053Lb, A Buried Terrestrial Planet
• Discovery of Seven Companions to Intermediate Mass Stars with Extreme Mass Ratios in the Scorpius-Centaurus Association
• PopSyCLE: A New Population Synthesis Code for Compact Object Microlensing Events
• Systematic KMTNet Planetary Anomaly Search. V. Complete Sample of 2018 Prime-Field
• Systematic KMTNet Planetary Anomaly Search. IV. Complete Sample of 2019 Prime-Field
• OGLE-2016-BLG-0613LABb: A Microlensing Planet in a Binary System
• Microlensing events indicate that super-Earth exoplanets are common in Jupiter-like orbits
• Systematic KMTNet Planetary Anomaly Search, Paper VII: Complete Sample of $q<10^{-4}$ Planets from the First Four-Year Survey
• The Masses of a Sample of Radial-Velocity Exoplanets with Astrometric Measurements
• The IAU Working Definition of an Exoplanet
• Systematic KMTNet Planetary Anomaly Search. VIII. Complete Sample of 2019 Subprime Field Planets
• Exploring the Brown Dwarf Desert with Precision Radial Velocities and Gaia DR3 Astrometric Orbits
• An Earth-Mass Planet and a Brown Dwarf in Orbit Around a White Dwarf
• Three microlensing planets with no caustic-crossing features
• OGLE-2016-BLG-0263L\lowercase{b}: Microlensing Detection of a Very Low-mass Binary Companion Through a Repeating Event Channel
• A low-mass companion desert among intermediate-mass visual binaries: The scaled-up counterpart to the brown dwarf desert
• Brown-dwarf companions in microlensing binaries detected during the 2016--2018 seasons
• Systematic KMTNet Planetary Anomaly Search. XII. Complete Sample of 2017 Subprime Field Planets
• Probable brown dwarf companions detected in binary microlensing events during the 2018-2020 seasons of the KMTNet survey