Photometric variability in the ultracool dwarf BRI 0021-0214:: Possible evidence for dust clouds

We report CCD photometric monitoring of the nonemission ultracool dwarf BRI 0021-0214 (M9.5) obtained during 10 nights in 1995 November and four nights in 1996 August with CCD cameras on 1 m class telescopes at the observatories of the Canary Islands. We present differential photometry of BRI 0021-0214, and we report significant variability in the I-band light curve obtained in 1995. A periodogram analysis finds a strong peak at a period of 0.84 days. This modulation appears to be transient, because it is present in the 1995 data but not in the 1996 data. We also find a possible period of 0.20 days, which appears to be present in both the 1995 and 1996 data sets. However, we do not find any periodicity close to the rotation period expected from the spectroscopic rotational broadening (less than or equal to0.14 days). BRI 0021-0214 is a very inactive object, with extremely low levels of Ha and X-ray emission. Thus, it is unlikely that magnetically induced cool spots can account for the photometric variability. The photometric variability of BRI 0021-0214 could be explained by the presence of an active meteorology that leads to inhomogeneous clouds on the surface. The lack of photometric modulation at the expected rotational period suggests that the pattern of surface features may be more complicated than previously anticipated.