YOUNG BROWN DWARFS AT HIGH CADENCE: WARMSPITZERTIME SERIES MONITORING OF VERY LOW MASS σ ORIONIS CLUSTER MEMBERS

Abstract

The continuous temporal coverage and high photometric precision afforded by space observatories has opened up new opportunities for the study of variability processes in young stellar cluster members. Of particular interest is the phenomenon of deuterium-burning pulsation in brown dwarfs and very-low-mass stars, whose existence on 1-4 hours timescales has been proposed but not yet borne out by observations. To investigate short-timescale variability in young, low-mass objects, we carried out high-precision, high-cadence time series monitoring with the Warm Spitzer mission on 14 low mass stars and brown dwarfs in the ~3 Myr Sigma Orionis cluster. The flux in many of our raw light curves is strongly correlated with sub-pixel position and can vary systematically as much as 10%. We present a new approach to disentangle true stellar variability from this "pixel-phase effect," which is more pronounced in Warm Spitzer observations as compared to the cryogenic mission. The light curves after correction reveal that most of the sample is devoid of variability down to the few-millimagnitude level, on the minute to day timescales probed. However, one exceptional brown dwarf displays erratic brightness changes at the 10-15% level, suggestive of variable obscuration by dusty material. The uninterrupted 24-hour datastream and sub-1% photometric precision enables limits on pulsation in the near-infrared. If this phenomenon is present in our light curves, then its amplitude must lie below 2-3 millimagnitudes. In addition, we present three field eclipsing binaries and one pulsator for which optical ground-based data is also available.