Weather at the L/T Transition: A Large J-Band Survey for Variability of Cool Brown Dwarfs

摘要

An outstanding issue in the understanding of ultracool dwarf atmospheres is the abrupt shift from red (J-K ~2) to blue (J-K ~0) near-infrared colors, accompanied by a J-band brightening, at the boundary between L and T spectral types, marking the transition from cloudy to cloud-free atmospheres. While current 1-dimensional cloud models generally fail to capture the main features of the transition, most notably the narrow temperature range over which it occurs, it has been hypothesized that a disruption of the cloud layer as it sits progressively lower in the photosphere could contribute to a more abrupt decrease in dust opacity in this regime. If present, such patchy cloud coverage should give rise to photometric variability on rotational timescales. Motivated in part by the recent discovery of ~50 mmag periodic variability of the T2 dwarf SIMP0136 by Artigau et al. 2009, we have undertaken the most comprehensive near-infrared variability survey of cool brown dwarfs to date, designed to test for heterogeneous cloud coverage in a sample of brown dwarfs spanning mid-L to late-T spectral types. Our J-band search has targeted 56 objects with high cadence, high precision, photometric sequences, and is complemented by follow-up observations in H and K_s bands to characterize the nature of the variations. Here we describe our large survey, and showcase the first, preliminary results. Our data set hints at a higher incidence of variability within the the L/T transition regime.