ANOTHER ONE BUSTS THE DUST

Brown dwarfs are substellar objects with core temperatures insufficient for sustained hydrogen fusion. Their physical properties such as mass, temperature, and radius are similar to those of gas giant planets, so studying brown dwarfs may also benefit exoplanet studies.

There is a population of ’red’ L dwarfs, which have redder J − K colors in the near-infrared than normal objects. Red L dwarfs include young, low-gravity objects, which are systematically red, and red field-gravity objects. The observed reddening in L dwarfs is not well explained by current atmosphere models.

We present an analysis of red L dwarfs using our model which includes small, sub-micron size dust particles in the upper atmosphere in addition to the cloud decks common to all L dwarfs which consist of larger particles. We hypothesize that the red NIR colors of some L dwarfs could be explained by a “dust haze" of small particles. We developed a model which combines Mie theory and Hansen particle size distributions to reproduce the extinction due to the proposed dust haze. We apply our analysis to 20 young L dwarfs and 36 red field L dwarfs. We constrain the properties of the dust haze including particle size distribution and column density using Markov-Chain Monte Carlo methods. We find that sub-micron range silicate grains reproduce the observed reddening. We also find that Hansen particle size distributions reproduce the shape of the observed reddening better than power law particle size distributions. Current brown dwarf atmosphere models include large grain (∼ 10 μm) dust clouds but not ISM-size small dust grains. Our results provide a proof of concept and motivate a combination of large and small dust grains in brown dwarf atmosphere models.

Finally, we discuss our future prospects and possible application of the dust haze analysis for atmospheric models of brown dwarfs and exoplanets, and variability in brown dwarfs.