[H97b] 21395 , the SIMBAD biblio

Rotational studies at a variety of ages and masses 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 significant lack of known rotational periods in that mass range. We aim to extend previous studies well down into the substellar regime, providing for the first time information on rotational periods for a large sample of young VLM stars and BDs. This extensive rotational period study of YSOs in the 1Myr old Orion Nebula Cluster (ONC) is based on a deep photometric monitoring campaign using the Wide Field Imager (WFI) camera on the ESO/MPG 2.2m telescope on La Silla, Chile. Time series data was obtained with about 95 data points spread over 19 nights. Accurate I-band photometry of 2908 stars was obtained within a magnitude range of 13 to 21mag, i.e. extending three magnitudes deeper than previous studies in the ONC. Two different power spectral analysis techniques were used to search for periodic variability. In addition, the χ² variability test was used for the detection of irregular variables. We found 487 periodic variables with estimated masses between 0.5M_☉ and 0.015M_☉, 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. In addition to the periodic variables, 808 objects show non-periodic brightness variations. We study the dependence of the period distribution on mass and variability level and compare this with known objects in the ONC with masses up to 1.5M_☉ and with the ∼2Myr old cluster NGC 2264. We find that substellar objects rotate on average faster than the VLM stars. In addition, our rotational data suggest a dependence of the rotational periods on position within the field, which can be explained by a possible age spread in the ONC with a somewhat younger central region. The results of a comparison between the period distributions of the ONC and NGC 2264 favours this hypothesis. In addition, periodic variables with larger peak-to-peak amplitudes rotate on average slower than those with small peak-to-peak amplitude variations, which can possibly be explained by different magnetic field topologies.