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I present the results of a program to spectrophotometrically monitor the L1.5dwarf 2MASSW J1145572+231730 to identify the cause of photospheric variabilityin ultra cool dwarfs. Plausible candidates are magnetically-induced star spotsand inhomogeneous photospheric dust clouds. Based on the atmospheric models andsynthetic spectra of Allard et al. (2001), the expected signatures of thesephenomena in the 0.5-2.5 um wavelength region are presented and discussed. Nearinfrared spectra of 2M1145 were obtained along with a nearby reference starobserved simultaneously in the spectrograph slit. No convincing variabilityover a 54 hour interval is found in any one of several colour indices designedto be most sensitive to dust- and spot-related variability. Nonetheless, asignificant correlation between the variability of two colour indices is found.This is slightly more consistent with the dust-related variability model thanthe cool spot one considered. Based on the theoretically predicted signaturesand the median errors in the colour indices (0.03-0.05 magnitudes), upperlimits are placed on the coverage of possible spots and clouds. Assuming the Ldwarf to be best modelled by a dusty atmosphere at 1900K, coherent clear cloudsare limited to a coverage of 10-15% of the projected surface area and 200Kcooler spots to a 20% coverage. A larger coverage of many small featuresvarying incoherently cannot be ruled out with this method. A lower effectivetemperature restricts coherent clear clouds to be much smaller; a highertemperature allows both clouds and spots to be larger. These upper limits areconsistent with the two separate variability detections in the I-band reportedby Bailer-Jones & Mundt (2001).Comment: Astronomy & Astrophysics, in press. (Minor typos corrected.

Dust clouds or magnetic spots? Exploring the atmospheres of L dwarfs with time-resolved spectrophotometry