El. Martin et al., Photometric variability in the ultracool dwarf BRI 0021-0214: Possible evidence for dust clouds, ASTROPHYS J, 557(2), 2001, pp. 822-830
We report CCD photometric monitoring of the nonemission ultracool dwarf BRI
0021-0214 (M9.5) obtained during 10 nights in 1995 November and four night
s in 1996 August with CCD cameras on 1 m class telescopes at the observator
ies of the Canary Islands. We present differential photometry of BRI 0021-0
214, and we report significant variability in the I-band light curve obtain
ed 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. How
ever, we do not find any periodicity close to the rotation period expected
from the spectroscopic rotational broadening (less than or equal to0.14 day
s). BRI 0021-0214 is a very inactive object, with extremely low levels of H
a and X-ray emission. Thus, it is unlikely that magnetically induced cool s
pots can account for the photometric variability. The photometric variabili
ty of BRI 0021-0214 could be explained by the presence of an active meteoro
logy that leads to inhomogeneous clouds on the surface. The lack of photome
tric modulation at the expected rotational period suggests that the pattern
of surface features may be more complicated than previously anticipated.