Statistical Properties of Brown Dwarf Companions: Implications for Different Formation Mechanisms

TL;DR

This study analyzes the demographics of brown dwarf companions around solar-type stars, revealing a gap in their period-mass distribution that suggests different formation mechanisms for brown dwarfs above and below this gap.

Contribution

It identifies a significant period-mass gap in brown dwarf companions and links their formation mechanisms to their position relative to this gap.

Findings

A short period and medium mass gap exists in brown dwarf distribution.

Brown dwarfs below the gap likely form via disk gravitational instability.

Brown dwarfs above the gap likely form through molecular cloud fragmentation.

Abstract

The mass domain where massive extrasolar planets and brown dwarfs overlap is still poorly understood due to the paucity of brown dwarfs orbiting close to solar-type stars, the so-called brown dwarf desert. In this paper we collect all of available data about close brown dwarfs around solar type stars and their host stars from literature and study the demographics of the brown dwarf desert. The data clearly show a short period and a medium mass gap in the brown dwarf period-mass distribution diagram ($ 35<m\sin i<55 M_{\rm Jup}$ and $P<100$ days), representing the "driest land" in the brown dwarf desert. Observation biases are highly unlikely to cause this gap due to its short period and medium mass, of which brown dwarfs can be easily detected by previous RV surveys. Brown dwarfs above and below this gap have significantly different eccentricity distribution, which not only confirms that this gap is real, but also implies that they may have different origins. Our further statistical study indicates that brown dwarfs below this gap may primarily form in the protoplanetary disk through disk gravitational instability, while brown dwarfs above this gap may dominantly form like a stellar binary through molecular cloud fragmentation. Our discoveries have offered important insights about brown dwarf formation mechanisms and their possible relationships with planet and star formation.