Volume 566, June 2014
|Number of page(s)||16|
|Published online||20 June 2014|
The brown dwarf atmosphere monitoring (BAM) project
1 Astrophysics Group, School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
2 School of Earth & Space Exploration, Arizona State University, Tempe AZ 8528, USA
Received: 6 November 2013
Accepted: 17 April 2014
Using the SofI instrument on the 3.5 m New Technology Telescope, we have conducted an extensive near-infrared monitoring survey of an unbiased sample of 69 brown dwarfs spanning the L0 to T8 spectral range, with at least one example of each spectral type. Each target was observed for a 2−4 h period in the Js-band, and the median photometric precision of the data is ~0.7%. A total of 14 brown dwarfs were identified as variables with min-to-max amplitudes ranging from 1.7% to 10.8% over the observed duration. All variables satisfy a statistical significance threshold with a p-value ≤5% based on comparison with a median reference star light curve. Approximately half of the variables show pure sinusoidal amplitude variations similar to 2MASSJ2139+0220, and the remainder show multi-component variability in their light curves similar to SIMPJ0136+0933. It has been suggested that the L–T transition should be a region of a higher degree of variability if patchy clouds are present, and this survey was designed to test the patchy cloud model with photometric monitoring of both the L–T transition and non-transition brown dwarfs. The measured frequency of variables is 13+10−4% across the L7−T4 spectral range, indistinguishable from the frequency of variables of the earlier spectral types (30+11−8%), the later spectral types (13+10−4%), or the combination of all non-transition region brown dwarfs (22+7−5%). The variables are not concentrated in the transition, in a specific colour, or in binary systems. Of the brown dwarfs previously monitored for variability, only ~60% maintained the state of variability (variable or constant), with the remaining switching states. The 14 variables include 9 newly identified variables that will provide important systems for follow-up multi-wavelength monitoring to further investigate brown dwarf atmosphere physics.
Key words: techniques: photometric / brown dwarfs / stars: low-mass / stars: atmospheres
Based on observations made with ESO Telescopes at La Silla Observatory under programme ID 188.C-0493.
Tables 1, 2, and 4 are available in electronic form at http://www.aanda.org
Light curves (target and references) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/566/A111
© ESO, 2014
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