Proper motions and variabilities of stars near the Orion Nebula

Relative proper motions were measured and membership probabilities were determined for over 1000 stars within 15-arcmin of the center of the Orion Nebula, M42. About 900 of the stars are associated with the Orion complex. The motions are of sufficient precision to measure the internal velocity dispersions. These dispersions are nearly isotropic, and depend only weakly on magnitude and distance from the Trapezium. The corresponding velocity dispersion in one coordinate is just over 2 km/s. The distribution of stars follows closely the distribution of gas as measured by CO-13. The stars appear to be, at most, only weakly bound to the molecular cloud. A search for variable stars has found that nearly half of the member stars are either known variables or showed variation on the plates. (Author)