A wide field imager attached to the MPG/ESO 2.2 m telescope on La Silla has been used to monitor the Orion Nebula Cluster on 45 nights between 25 Dec 1998 and 28 Feb 1999. Ninety-two images were obtained during this period through an intermediate band filter centered at 815.9 nm. More than 1500 sources with I magnitudes between 12.5 and 20 were monitored. We find that essentially every star brighter than 16th mag (where the precision is <0.01 mag) is a variable, with about half having a peak-to-peak variation of ~0.2 mag or more. A clear correlation is found between the level of variability and infrared excess emission, in the sense that stars with evidence for circumstellar disks have larger amplitudes of variation. A search for periodic variables was carried out and 369 such stars were discovered, most or all of which are rotating, spotted T Tauri stars. Periodic variables are most commonly found among the low amplitude variables. 46% of the stars with magnitudes between 12.5 and 16 and standard deviation, sigma < 0.1 mag, were found to be periodic, whereas only 24% of the stars in the same magnitude range with $\sigma$ > 0.1 yielded periods.
Our work confirms the existence of a bimodal period distribution, with peaks near 2 and 8 days, for stars with M > 0.25 M_sun and a unimodal distribution peaked near 2 days, for lower mass stars. We show that a statistically significant correlation exists between infrared excess emission and rotation in the sense that slower rotators are more likely to show evidence of circumstellar disks. Slowly rotating stars, with angular velocities, omega < 1 radian/d, corresponding to rotation periods longer than 6.28 d, have a mean infrared excess emission, Delta(I-K) = 0.55 +/- 0.05, indicative of the presence of inner disks, while rapid rotators, with omega > 2 radians/d, corresponding to rotation periods shorter than 3.14 d, have a much smaller mean of 0.17 +/- 0.05. This supports the hypothesis that disks are involved in regulating stellar rotation during the pre-main sequence phase.
We explore a simple and commonly adopted model of rotational evolution in which stars conserve angular velocity while locked to a disk and conserve angular momentum once released. If these assumptions are valid, and if the locking period is 8 days, we find that more than half of the stars in the ONC are no longer locked to disks and that an exponential decay model with a disk-locking half-life of about 0.5-1 My fits the observations well. Assuming that the mean ages of the higher and lower mass stars are the same, the faster rotation of the lower mass stars can be understood as either a consequence of a shorter disk-locking time or a shorter initial disk-locking period, or both.
Astronomy & Astrophysics, 2002, A&A, 316, 513-532
[online publication]