Roaring Storms in the Planetary-mass Companion VHS 1256-1257 b: Hubble Space Telescope Multiepoch Monitoring Reveals Vigorous Evolution in an Ultracool Atmosphere

Abstract

The photometric and spectral variability of brown dwarfs probes heterogeneous temperature and cloud distributions 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 the 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% ± 2% 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 the atmospheres, clouds, and atmospheric circulation of planets and brown dwarfs.

The JWST Early-release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 μm Spectrum of the Planetary-mass Companion VHS 1256–1257 b

“…However, over the ∼4 hr timescale of our ERS observations, we expect a relatively low level of variability, based on estimates from the earlier HST and Spitzer observations. To estimate the range of potential variability we expect during the NIRSpec observation, we drew 10,000 sample 2 hr observations from the three-sinusoid model from Zhou et al (2022) and estimated the intrinsic variability occurring during each simulated observation as the maximum minus the minimum value from the model during that time span. We used a very fine time sampling and did not realistically simulate noise or the actual cadence of our JWST observations.…”

“…After VHS 1256 b was chosen as an ERS target, Bowler et al (2020) discovered that it has the largest known variability amplitude of any L dwarf. Later, Zhou et al (2022) determined that VHS 1256 b has the largest known variability amplitude of any brown dwarf (38% peak to peak at the J band, separated by 2 yr). This variability is potentially caused by heterogeneous temperature and cloud distributions that may be particularly prevalent in the atmospheres of young, low-mass planets (Apai et al 2013(Apai et al , 2017Vos et al 2019;Zhou et al 2020;Apai et al 2021;Vos et al 2022).…”

JWST/NIRSpec Observations of the Planetary Mass Companion TWA 27B*

“…The differences in photometry can be plausibly explained by variability. In particular, the wide range of offsets for VHS 1256b is not surprising given that it is known to exhibit large wavelength-dependent variability (Bowler et al 2020;Zhou et al 2020Zhou et al , 2022a. The synthetic photometry in Vega magnitudes from the NIRSpec data is presented in Table 1.…”

Redder than Red: Discovery of an Exceptionally Red L/T Transition Dwarf

“…2MASS 21392676+0220226 (2MASS J2139+0220) is a T1.5 dwarf (Burgasser et al 2006) that is well-known for its largeamplitude infrared variability. Radigan et al (2012) monitored 2MASS J2139+0220 and found J-band variability with a peakto-peak amplitude of ∼26%, which until recent observations of VHS 1256−1257B (Zhou et al 2022) was the highest amplitude variability found for any brown dwarf. Since the Radigan et al (2012) study, this object has been the subject of numerous variability investigations (Apai et al 2013;Khandrika et al 2013;Karalidi et al 2015), with Yang et al (2016) finding variability of 11%-12% in Spitzer/IRAC ch1 and ch2 photometry.…”