Rotational Evolution of Very Low Mass Objects and Brown Dwarfs

The regulation of angular momentum is one of the key processes for our understanding of stellar evolution. The rotational evolution of solar-mass stars is mainly determined by the magnetic interaction with their circumstellar disk and angular momentum loss through stellar winds, and In contrast to solar-mass stars, very low mass (VLM) objects and brown dwarfs are believed to be fully convective. This may lead to major differences of rotation and activity, since fully convective objects may not host a solar-type dynamo. Here, we report on our observational efforts to understand the rotational evolution of VLM objects. By means of photometric monitoring, we determined 62 rotation periods for targets in three clusters, which form an age sequence from 3 to 125Myr. We find that VLM objects rotate faster than their solar-mass siblings in all evolutionary stages. Their rotational evolution seems to be determined by hydrostatic contraction and exponential angular momentum loss. The photometric amplitudes of the light curves are much lower than for solar-mass stars. This may be explained as a consequence of smaller spot coverage, more symmetric spot distributions, or lower contrast between spots and their environment. Most of these results can be explained with a change of the magnetic field properties with decreasing mass. VLM objects possibly possess only small-scale, turbulent magnetic fields.