Studies of Extremely Young Clusters.VI. Spectroscopic Observations of the Ultraviolet-Excess Stars in the Orion Nebula Cluster and NGC 2264

Abstract

Spectroscopic observations of 25 ultraviolet-excess stars in the Orion Nebula cluster and NGC 2264 are presented. These objects are found to be late-type gravitationally contracting stars with masses of 0.2-0.5 M0, radii of 2-6 R0, and absolute bolometric magnittldes of +2.5 to +5.8, surrounded by zones in which line and continuous emission originate. In nine of these stars, absorption lines of hydrogen and (sometimes) Ca ii having radial velocities of +150 to +400 km 1 are observed, indicating that infall of material is occurring. The rate of infall is variable and amounts to <10-" M0 per year. The majority of the ultra-violet-excess stars that do not show the redward-displaced absorption lines appear to be similar systems in which the infall rate was lower at the time of observation, or in which infall occurs in a plane perpendicular to the line of sight. Two RW Aur-like stars may be slightly older systems in which infall has ceased. The nature of the ultraviolet excess and blue continuum is not clear, but these features may result from hydrogen emission in the infalling material and/or at the shock front where this material strikes the surface of the star. The blue continuum makes the stars appear too blue in the color-magnitude diagram, and this effect accounts for the tendency of these stars to lie closer to the main sequence than predicted by theory. The ages of the ultraviolet-excess stars appear to be smaller than the ages estimated from the point at which the cluster sequences depart from the main sequence, indicating that a spread in the time of star formation probably exists in these clusters.