IN SEARCH OF WEATHER ON BROWN DWARFS: THE J(HK)-BAND MONITORING PROGRAM 1. THE ROLE OF DUST IN BROWN DWARF ATMOSPHERES:

One of the most puzzling behaviors exhibited by cool brown dwarfs is their rapid evolution from very red (J-K=2) to very blue (J-K=0) near infrared colors at the boundary between L and T spectral types, marking the transition from cloudy to cloud-free atmospheres. It has been suggested that the rapidity of this evolution, accompanied by a J-band brightening, is caused by the fragmentation of photospheric cloud decks, allowing flux from deeper, warmer layers to escape. This hypothesis has testable consequences: such patchy cloud coverage should give rise to rotationally modulated variability. We describe our upcoming J-band monitoring program for photometric variability on ~50 T-dwarfs. Our goal is to achieve a continuous time series of high-precision differential J-band photometry for each target over a large fraction of a rotational period. This survey will be conducted over the coming year, using the Wide-field InfraRed Camera (WIRC) on the du Pont 2.5-m telescope at Las Campanas Observatory, with observations commencing in August 2009. As clouds begin to clear at the boundary between L and T spectral types—to so-called L/T transition—there is a very rapid evolution from very red (J-K=2) to very blue (J-K=0) near infrared colors. This rapid evolution is accompanied by a counter-intuitive brightening in the J-band with decreasing effective temperature (figure 2)[3,4]