Variability in SSTc2d J163134.1-240100, a brown dwarf with quasi-spherical mass loss

We report on a search for variability in the young brown dwarf SST1624 ($\sim$M7 spectral type, $M\sim 0.05\,M_{\odot}$), previously found to feature an expanding gaseous shell and to undergo quasi-spherical mass loss. We find no variability on timescales of 1-6,hours. Specifically, on these timescales, we rule out the presence of a period with amplitude $>1\%$. A photometric period in that range would have been evidence for either pulsation powered by Deuterium burning or rotation near breakup. However, we see a 3% decrease in the K-band magnitude between two consecutive observing nights (a 10,$\sigma$ result). There is also clear evidence for variations in the WISE lightcurves at 3.6 and 4.5$\,\mu m$ on timescales of days, with a tentative period of about 6,d (with a plausible range between 3 and 7,d). The best explanation for the variations over days is rotational modulation due to spots. These results disfavour centrifugal winds driven by fast rotation as mechanism for the mass loss, which, in turn, makes the alternative scenario – a thermal pulse due to Deuterium burning – more plausible.