Some properties of transiting brown dwarfs

Kurzfassung

The formation and migration of brown dwarfs around main sequence stars remain a mystery, and the discovery of close-in (a<0.1 AU) systems, which are often transiting, poses new challenges for understanding star and planet formation, regardless of whether brown dwarfs are more similar to stars or to planets. The histogram of orbital separations can reveal how binary systems form and evolve. Here we concentrate on binary systems consisting of a main sequence F,G, or K star and a brown dwarf. We update the orbital separation - occurrence rate relationship with the latest measurements which provides the same information as the aforementioned histogram. We give an estimate of the brown dwarf occurrence rate from the K2 survey by comparing the number of reported brown dwarfs and hot Jupiters. We also collate estimates from other surveys, and fit a power law separation-occurrence rate to the combined results, which resembles the occurrence rate - separation distribution of planets rather than stars. We also highlight the relative dearth of transiting brown dwarfs from ground-based surveys. We propose two possible explanations for this; (i) that brown dwarfs have smaller radii than hot Jupiters, making them harder to detect, and (ii) that brown dwarfs may orbit faster-rotating stars than hot Jupiters, making radial velocity confirmation difficult.