Studies of extremely young clusters. VII. Spectroscopic observations of faint stars in the Orion nebula.

Spectroscopic observations, including MK spectral classifications and radial velocities, are given for 61 stars in the Orion Nebula in the magnitude range where V is between 11.0 and 14.5. Whereas most of the observations are made at a dispersion of 115 A/mm in the region from 3600 to 5400 A, a few are made with a dispersion of 34 A/mm in the wavelength range 5850 to 6720 A. These data, along with existing photometric and proper motion measurements, are used to determine the cluster membership of the observed stars. The observations suggest that the YY Ori phenomenon is restricted to stars with ultraviolet excesses and that the occurrence of an ultraviolet excess is correlated with veiling of the absorption spectrum longward of the Balmer limit by continuous emission, both phenomena varying in intensity with time. A number of faint, late-type stars having high rotational velocities are found. It is found that the mean radial velocity of the stars in the Orion Nebula cluster is 32 km/s and is independent of spectral type. A real dispersion in the radial velocities of the faint stars is found which is present in the proper motions as well.