Dichotomy in the Dynamical Status of Massive Cores in Orion

To study the evolution of high-mass cores, we have searched for evidence of collapse motions in a large sample of starless cores in the Orion molecular cloud. We used the Caltech Submillimeter Observatory telescope to obtain spectra of the optically thin (HCO ) and optically thick (HCO ) high-density tracer molecules in 27 cores with masses 11 M . The redand blue-asymmetries seen in the line profiles of the optically thick line , with respect to the optically thin line indicate that 2/3 of these cores are not static. We detect evidence for infall (inward motions) in 9 cores and outward motions for 10 cores, suggesting a dichotomy in the kinematic state of the nonstatic cores in this sample. Our results provide an important observational constraint on the fraction of collapsing (inward motions) versus noncollapsing (reexpanding) cores for comparison with model simulations. Subject headings: ISM: clouds — ISM: individual (Orion) — ISM: kinematics and dynamics — ISM: molecules — stars: formation