Clues to Substellar Formation: Rotation and the Low-Mass End of the Initial Mass Function

We have monitored S Ori 45, a young, low-mass (20 Mjup) brown dwarf of the σ Orionis cluster (∼3 Myr, 352 pc), using optical and near-infrared filters. S Ori 45 (spectral type M8.5) is found to be multi-periodic with a dominant modulation at 2.5–3.5 h, and a short modulation at about 46 min. We ascribe the longer of these modulations to a rotation period. After comparing these results with observations of more massive cluster brown dwarfs and field brown dwarfs, we conclude that substellar objects present rotational and angular momentum evolution.