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CLOUDS search for variability in brown dwarf atmospheres. Infrared spectroscopic time series of L/T transition brown dwarfs
Context: L-type ultra-cool dwarfs and brown dwarfs have cloudyatmospheres that could host weather-like phenomena. The detection ofphotometric or spectral variability would provide insight intounresolved atmospheric heterogeneities, such as holes in a global clouddeck. Indeed, a number of ultra-cool dwarfs have been reported to vary.Additional time-resolved spectral observations of brown dwarfs offer theopportunity for further constraining and characterising atmosphericvariability. Aims: It has been proposed that growth ofheterogeneities in the global cloud deck may account for the L- toT-type transition when brown dwarf photospheres evolve from cloudy toclear conditions. Such a mechanism is compatible with variability. Wesearched for variability in the spectra of five L6 to T6 brown dwarfs totest this hypothesis. Methods: We obtained spectroscopic timeseries using the near-infrared spectrographs ISAAC on VLT-ANTU, over0.99-1.13 ?m, and SpeX on the Infrared Telescope Facility for two ofour targets in the J, H, and K bands. We searched for statisticallyvariable lines and for a correlation between those. Results: Highspectral-frequency variations are seen in some objects, but thesedetections are marginal and need to be confirmed. We find no evidence oflarge-amplitude variations in spectral morphology and we place firmupper limits of 2 to 3% on broad-band variability, depending on thetargets and wavelengths, on the time scale of a few hours. In contrastto the rest of the sample, the T2 transition brown dwarf SDSS J1254-0122shows numerous variable features, but a secure variability diagnosiswould require further observations. Conclusions: Assuming that anyvariability arises from the rotation of patterns of large-scale clearand cloudy regions across the surface, we find that the typical physicalscale of cloud-cover disruption should be smaller than 5-8% of the diskarea for four of our targets, using simplistic heterogeneous atmosphericmodels. The possible variations seen in SDSS J1254-0122 are not strongenough to allow us to confirm the cloud-breaking hypothesis.Based on observations obtained at the European Observatory, Paranal,Chile, under programme 71.C-0559.
| The Identification of IRAS Point Sources - Part One - a 304-DEGREE Field Centred on the South Galactic Pole
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1986MNRAS.223..279W&db_key=AST
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