Limits on the infrared photometric monitoring of brown dwarfs

Recent monitoring programmes of ultra-cool field M and L dwarfs (low-mass stars or brown dwarfs) have uncovered low-amplitude photometric I-band variations that may be associated with an inhomogeneous distribution of photospheric condensates. Further evidence hints that this distribution may evolve on very short time-scales, specifically of the order of a rotation period or less. In an attempt to study this behaviour in more detail, we have carried out a pilot programme to monitor three L dwarfs in the near-infrared where these objects are significantly brighter than at shorter wavelengths. We present a robust data analysis method for improving the precision and reliability of infrared photometry. No significant variability was detected in either the J or K' bands in 2M1439 and SDSS1203 above a peak-to-peak amplitude of 0.04 mag (0.08 mag for 2M1112). The main limiting factor in achieving lower detection limits is suspected to be second-order extinction effects in the atmosphere of the Earth, on account of the very different colours of the target and reference stars. Suggestions are given for overcoming such effects, which should improve the sensitivity and reliability of infrared variability searches.