Extended baseline photometry of rapidly changing weather patterns on the brown dwarf binary Luhman-16

Abstract

Luhman-16 (WISE J1049-5319) was recently discovered to be a nearby (∼2 pc) brown dwarf binary that exhibits a high degree of photometric variability (Δm ∼ 0.1 mag). This is thought to be due to the evolution of "cloud" features on the photosphere, but Luhman-16 has been found to show unusually rapid changes, possibly resulting from fast-evolving "weather." This target is of particular interest because it consists of a co-evolutionary pair of brown dwarfs spanning the transition between L and T spectral types (L7.5 and T0.5), which are expected to be associated with changes in cloud surface coverage. Being comparatively bright (I ∼ 15.5 mag), the target is well suited for observation with the new Las Cumbres Observatory Global Telescope Network (LCOGT) of 1 m telescopes. We present long-time baseline photometric observations from two of LCOGT's southern hemisphere sites, which were used in tandem to monitor Luhman-16 for up to 13.25 hr at a time (more than twice the rotation period), for a total of 41.2 days in the SDSS-i′ and Pan-STARRS-Z filters. We use this data set to characterize the changing rotational modulation, which may be explained by the evolution of cloud features at varying latitudes on the surfaces of the two brown dwarfs.