Rotational velocities and radii of pre-main-sequence stars in the Orion Nebula cluster

We have obtained high-dispersion spectra for a sample of 256 pre-main-sequence stars in the Orion Nebula cluster to measure their projected rotational velocities and investigate the rotational evolution and physical properties of young low-mass stars. Half the stars were chosen because they had known photometric periods and the other half were selected as a control sample of objects without known periods from the same portion of the H-R diagram. More than 90% of the spectra yielded v sin i measurements, although about 1/3 are upper limits. We find strong evidence confirming the long-held assumption that the periodic light variations of T Tauri stars are caused by rotation of spots on their surfaces. We find no statistically significant difference between the v sin i distributions of the periodic and control samples, indicating that there is no strong bias in studying the rotational properties of young stars by using periodic variables. Likewise, the classical and weak T Tauri stars exhibit v sin i distributions that are statistically the same. For stars with known period and v sin i, the mean value of sin i is significantly lower than expected for a random distribution of stellar rotation axes. This could be caused by errors in one or more of the quantities that contribute to the sin i calculation or to a real physical effect. We investigate the possible causes and find that [sin i] has the expected value if we increase the effective temperatures of our stars by 400-600 K. Finally, we have calculated minimum radii (R sin i) for stars with both v sin i and period, as well as average radii for objects grouped by their location in the H-R diagram. We find evidence at the 3 sigma level that the radii of the stars on similar mass tracks are decreasing as the stars move closer to the zero-age main sequence.