Kepler Monitoring of an L Dwarf II. Clouds with Multiyear Lifetimes

TL;DR

This study uses multi-wavelength observations over nearly two years to show that an L dwarf's cloud features are long-lived and stable, contrasting with the short-term weather variability seen in cooler brown dwarfs.

Contribution

It demonstrates that ultracool dwarf clouds can have multiyear lifetimes and are not necessarily linked to magnetic activity variations.

Findings

Kepler and Spitzer data show stable light curves over two years.

Cloud cover explains multi-wavelength variability without magnetic coupling.

Long-lived clouds contrast with short-term weather in cooler brown dwarfs.

Abstract

We present Kepler, Spitzer Space Telescope, Gemini-North, MMT, and Kitt Peak observations of the L1 dwarf WISEP J190648.47+401106.8. We find that the Kepler optical light curve is consistent in phase and amplitude over the nearly two years of monitoring with a peak-to-peak amplitude of 1.4%. Spitzer Infrared Array Camera 3.6 micron observations are in phase with Kepler with similar light curve shape and peak-to-peak amplitude 1.1%, but at 4.5 micron, the variability has amplitude $<$0.1%. Chromospheric H$\alpha$ emission is variable but not synced with the stable Kepler light curve. A single dark spot can reproduce the light curve but is not a unique solution. An inhomogeneous cloud deck, specifically a region of thick cloud cover, can explain the multi-wavelength data of this ultracool dwarf and need not be coupled with the asynchronous magnetic emission variations. The long life of the cloud is in contrast with weather changes seen in cooler brown dwarfs on the timescale of hours and days.