Poster Abstracts

Name/Affiliation:  Caroline Morley (University of California Santa Cruz)

Title:  Clouds and Variability in Cool Brown Dwarfs

There is growing evidence that heterogeneous clouds cover the photospheres of brown dwarfs of many spectral types, causing photometric variability as cloudier hemispheres rotate in and out of view. In the warmer L dwarfs, these clouds are thick dusty layers of iron and silicates. At the L/T transition, these clouds form holes or dissipate; in this transitional effective temperature regime the largest variability is observed, a suggestive hint that variability is caused by the breakup of these thick cloud layers. In the cooler T dwarfs, the layers of iron and silicate clouds are below the photosphere; within the photosphere secondary condensates made of alkali salts and sulfides solidify, reddening the colors of this group of objects, which are otherwise quite blue in the near-infrared. In the coolest brown dwarfs, the Y dwarfs, more volatile species will begin to condense; the first volatile to condense is water, below effective temperatures of ~400 K. We present a new grid of model atmospheres for objects from 200-450 K including water ice clouds; we find that they become optically thick in Y dwarfs cooler than 350 K and strongly influence mid-infrared spectra, with some potentially observable spectral features in the near- and mid-infrared. While the most dramatically variable brown dwarfs are found at the L/T transition, later T and Y dwarfs exhibit variability as well. To understand this variability, an initial approach is to disentangle the effects caused by heterogeneous clouds and hot spots. We present models where we predict the spectral dependence of variability caused by each of these effects, and find that these two processes have quite different spectral dependence. Broad-wavelength spectral observational campaigns should be able to disentangle these processes and give insight into the 3D temperature and cloud structures of brown dwarfs.