Atmosphere structure of a brown dwarf from HST and Spitzer observations

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An outline of the paper: Vertical atmospheric structure in a variable brown dwarf: pressure-dependent phase shifts in simultaneous HST-Spitzer light curves, by Buenzli et al.

It is thought that the transition from L-type to T-type brown dwarfs, at temperatures around 1400 K,  is characterised by the disappearance of silicate clouds that dominate the late L-style atmospheres. Two brown dwarfs near the L-T transition show strong variability on the timescale of their rotation (a few hours), and this has led to one of the most exciting hypothesis of recent years in the field, the “patchy cloud” scenario. According to this scenario, what we see is a cloud coverage that breaks into patches near the L-T transition, causing flux variations as the breaks in the cloud cover rotate in and out of view.

Buenzli et al. have used HST and Spitzer to monitor the flux of the late-T brown dwarfs 2MASSJ22282889-4310262 (and we sometimes complain about unwieldy exoplanet names!) over 12 hours. They find a clear 2% variability of the flux along the rotation period of 1.4 hours.

Lightcurve of 2MASS2228 with Hubble and Spitzer space telescopes, showing out-of-phase variations in flux of about 1% at different infrared wavelengths.

Most interestingly, they found that while the amplitude of the variation in different wavelength ranges remain similar, the phase of the variation is strongly offset. The phase shift seems to evolve smoothly as wavelengths probe deeper layers in the atmosphere (the wavelength coverage is 1.2-1.7 microns for HST and 4-5 microns for Spitzer, which according to models should span the 1-10 bar range in the atmosphere of the brown dwarf).

Phase shift in the light curve of the brown dwarf 2MASSJ2228, as a function of the pressure probed by the observations according to atmosphere models.

 

Thanks to the different wavelengths, the authors can test whether the variations result from changes in opacity (patchy clouds) or in temperature (atmospheric circulation).

Their conclusion is that the patchy-cloud explanation doesn’t work on its own, since in that case one would expect the amplitude of the variation to change with wavelength rather than the phase. What works best is a mixture of the two effect – in other words a full “weather” situations with hot and cold currents as well as forming and dissipating cloud structures.

We may need this kind of data for many more objects before clear patterns start emerging. This is slightly disturbing when we think that in hot-Jupiter studies we can only dream of having the kind of extensive constraints available for these brown dwarfs.

Feature Image: Jon Lomberg

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About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.