CLOUDS search for variability in brown dwarf atmospheres. Infrared spectroscopic time series of L/T transition brown dwarfs

August 2008 • 2008A&A...487..277G

Authors • Goldman, B. • Cushing, M. C. • Marley, M. S. • Artigau, É. • Baliyan, K. S. • Béjar, V. J. S. • Caballero, J. A. • Chanover, N. • Connelley, M. • Doyon, R. • Forveille, T. • Ganesh, S. • Gelino, C. R. • Hammel, H. B. • Holtzman, J. • Joshi, S. • Joshi, U. C. • Leggett, S. K. • Liu, M. C. • Martín, E. L. • Mohan, V. • Nadeau, D. • Sagar, R. • Stephens, D.

Abstract • Context: L-type ultra-cool dwarfs and brown dwarfs have cloudy atmospheres that could host weather-like phenomena. The detection of photometric or spectral variability would provide insight into unresolved atmospheric heterogeneities, such as holes in a global cloud deck. Indeed, a number of ultra-cool dwarfs have been reported to vary. Additional time-resolved spectral observations of brown dwarfs offer the opportunity for further constraining and characterising atmospheric variability.
Aims: It has been proposed that growth of heterogeneities in the global cloud deck may account for the L- to T-type transition when brown dwarf photospheres evolve from cloudy to clear conditions. Such a mechanism is compatible with variability. We searched for variability in the spectra of five L6 to T6 brown dwarfs to test this hypothesis.
Methods: We obtained spectroscopic time series using the near-infrared spectrographs ISAAC on VLT-ANTU, over 0.99-1.13 μm, and SpeX on the Infrared Telescope Facility for two of our targets in the J, H, and K bands. We searched for statistically variable lines and for a correlation between those.
Results: High spectral-frequency variations are seen in some objects, but these detections are marginal and need to be confirmed. We find no evidence of large-amplitude variations in spectral morphology and we place firm upper limits of 2 to 3% on broad-band variability, depending on the targets and wavelengths, on the time scale of a few hours. In contrast to the rest of the sample, the T2 transition brown dwarf SDSS J1254-0122 shows numerous variable features, but a secure variability diagnosis would require further observations.
Conclusions: Assuming that any variability arises from the rotation of patterns of large-scale clear and cloudy regions across the surface, we find that the typical physical scale of cloud-cover disruption should be smaller than 5-8% of the disk area for four of our targets, using simplistic heterogeneous atmospheric models. The possible variations seen in SDSS J1254-0122 are not strong enough to allow us to confirm the cloud-breaking hypothesis.

Based on observations obtained at the European Observatory, Paranal, Chile, under programme 71.C-0559.

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