Statistical properties of brown dwarf companions: implications for different formation mechanisms

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 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 < 55M(Jup) and P < 100 d), 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 radial velocity 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 disc through disc 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.