Cloud properties of Nearest Brown Dwarfs Revealed through Spectral Monitoring Study

starweatherWe’ve just reported the first results from our April 2013  campaign to monitor the nearby brown dwarf binary Luhman 16AB, in a paper published in the Astrophysical Journal.  Our results confirm the earlier “flux reversal” seen in FIRE spectroscopy, and allow us to make the first constraints on the cloud coverage fraction and the temperatures of the “clouds” and “holes” in the atmosphere of Luhman 16B.  We also confirm an apparent correlation between rotation period and variability amplitude, which may emerge if cloud features are related to the Rhine’s scale of brown dwarf atmospheres.

The analysis merged 45 minutes of simultaneous monitoring observations of resolved, low-resolution near-infrared spectroscopy taken with the IRTF SpeX spectrograph and combined-light red optical photometry from TRAPPIST telescope in Chile. This was the maximum time we could observe the system since it is at -53° declination and never gets more than 16º above the horizon at Mauna Kea!

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Modeling of the SpeX data frame: the top panel shows the original data frame, the middle panel shows a model of that image. Residuals are at the bottom, amplified by a factor of 10.

We carefully extracted the spectral data frame-by-frame, as the two sources are close enough to be partially blended.  The figure above illustrates how this was done.  Here, the spectrum on bottom is of Luhman 16B, which is 20% brighter at Luhman 16A at the left side (1-1.3 µm) and 20% fainter on the right side (2-2.4 µm), confirming the flux reversal we noted in Burgasser et al. (2013).

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At top, the brightness ratio of Luhman 16B/Luhman 16A as a function of wavelength, showing how this ratio declined more at shorter wavelengths. The blue line is our best-fit model. At bottom, the brightness temperatures of Luhman 16A (black) and Luhman 16B (red) as a function of wavelength.

By taking the ratio of these spectra, we were able to determine that both the relative spectral flux (Luhman 16B/Luhman 16A) and the combined light photometry declined during the observing period, pinning Luhman 16B as the variable component. Moreover, the decline was stronger at shorter wavelengths, which we interpret as Luhman 16B showing a more dusty face toward Earth as it rotated during our observation.

Because the distance to Luhman 16 is well constrained, we were able to determine the brightness temperatures of both components as a function of wavelength, and then use that information to create a simple two-layer model of the atmosphere of Luhman 16B.  Matching to the observations, we determined that the “clouds” and “cloud holes” had temperatures of 1500K and 1800K, respectively, with clouds covering 30-55% of the atmosphere.  We could also estimate that over a full cycle the cloud coverage facing Earth changes by 15-30%, making it likely that the surface is covered by large features.

This model is consistent with the idea, proposed by Daniel Apai and Adam Showman, that the Rhines scale sets the size of the largest cloud features.  The Rhines scale is the ratio of the wind speed and the rotation rate, and sets the size scale for wind-driven cloud structures are in planetary atmospheres.  This scaling predicts extremely fast winds, of order 1-3 km/s, which is consistent with changes seen in the photometric light curve over a few rotation periods.  Finally, we find further evidence of a correlation between the amplitude of variability and covering fraction of clouds for L/T brown dwarfs, based on measurements for, SIMP 0136+0933 (2.4 hr, 5.5% amplitude at 1.25 µm), Luhman 16B (4.9 hr, 13.5% amplitude) and  2MASS 2139-0220 (7.7 hr, 30% amplitude).  This trend may again be related to the Rhines scale.

Our results were presented at a press conference at the 223rd American Astronomical Society meeting in Washington, DC (minute 57 in the Windows on Other Worlds session) and picked up by the BBC and Christian Science Monitor.

The paper can be accessed at NASA ADS.


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