Roaring Storms in the Planetary-Mass Companion VHS 1256-1257 b: Hubble Space Telescope Multi-epoch Monitoring Reveals Vigorous Evolution in an Ultra-cool Atmosphere

TL;DR

This study presents multi-epoch Hubble observations of the planetary-mass companion VHS 1256-1257 b, revealing highly variable, rapidly evolving atmospheric features with the highest amplitude variability observed in a substellar object, shedding light on atmospheric dynamics.

Contribution

First multi-epoch spectral monitoring of VHS 1256-1257 b demonstrating rapid atmospheric variability and high amplitude fluctuations, advancing understanding of brown dwarf atmospheric dynamics.

Findings

Peak-to-valley flux difference of 33±2%, reaching 38% in J band.

Light curve best modeled by multiple sine waves and linear trend.

Atmospheric variability consistent with heterogeneous clouds or thermal anomalies.

Abstract

Photometric and spectral variability of brown dwarfs probes heterogeneous temperature and cloud distribution and traces the atmospheric circulation patterns. We present a new 42-hr Hubble Space Telescope (HST) Wide Field Camera 3 G141 spectral time series of VHS 1256$-$1257 b, a late L-type planetary-mass companion that has been shown to have one of the highest variability amplitudes among substellar objects. The light curve is rapidly evolving and best-fit by a combination of three sine waves with different periods and a linear trend. The amplitudes of the sine waves and the linear slope vary with wavelength, and the corresponding spectral variability patterns match the predictions by models invoking either heterogeneous clouds or thermal profile anomalies. Combining these observations with previous HST monitoring data, we find that the peak-to-valley flux difference is $33\pm2$% with an even higher amplitude reaching 38% in the $J$ band, the highest amplitude ever observed in a substellar object. The observed light curve can be explained by maps that are composed of zonal waves, spots, or a mixture of the two. Distinguishing the origin of rapid light curve evolution requires additional long-term monitoring. Our findings underscore the essential role of atmospheric dynamics in shaping brown dwarf atmospheres and highlight VHS 1256$-$1257 b as one of the most favorable targets for studying atmospheres, clouds, and atmospheric circulation of planets and brown dwarfs.