Rotation Studies of Young Very Low Mass Stars and Brown Dwarfs

Rotation studies over as large a range of ages and masses as possible are important for constraining the angular momentum evolution of young stellar objects (YSO). Of particular interest are the very low mass (VLM) stars and brown dwarfs (BDs), because of the relatively small number of known rotational periods (Prot) in that mass range. This thesis for the first time provides important information on rotational properties of a large sample of young VLM stars and BDs. The extensive rotational period study of YSOs in the 1 Myr old Orion Nebula Cluster (ONC) presented here is based on a deep photometric monitoring campaign. As a result, 487 periodic variables (PVs) were found (377 of which are new) with estimated masses between 0.5-0.015 M_odot, 124 of which are BD candidates. This is by far the most extensive and complete rotational period data set for young VLM stars and BDs to date. The dependence of the period distribution on mass and level of rotational modulation was investigated and a comparison with higher mass PVs in the ONC and with the PVs in the approx. 2 Myr old open cluster NGC 2264 was carried out. Substellar objects were found to rotate on average faster than the VLM stars, a trend which was already seen for higher mass stars. In addition, the results presented here suggest a dependence of the Prot on position within the ONC, with the objects located near the cluster center showing on average larger Prot. This result can be explained by a possible age spread in the ONC with a somewhat younger central region. An interesting correlation between Prot and modulation amplitude was found in which PVs with larger amplitudes rotate on average slower than those with smaller amplitudes, which can in principle be explained by different magnetic field topologies. In addition, 732 periodic variables in the ONC with known JHK colours were analysed in order to investigate whether rotation periods correlate with the presence of circumstellar accretion disks in particular at very small masses. A highly significant rotation-disk correlation was found among the low and very low mass periodic variables (0.075<M