The Birth of High Mass Stars: Accretion and/or Mergers?

The observational consequences of the merger scenario for massive star formation are explored and contrasted with the gradual accumulation of mass by accretion. In high density proto-star clusters, envelopes and disks provide a viscous medium which can dissipate the kinetic energy of passing stars, greatly enhancing the probability of capture. Protostellar mergers may produce high luminosity infrared flares lasting years to centuries followed by a luminosity decline on the Kelvin-Helmholtz time-scale of the merger product. Mergers may be surrounded by thick tori of expanding debris, impulsive wide-angle outflows, and shock induced maser and radio continuum emission. Collision products are expected to have fast stellar rotation and a large multiplicity fraction. Close encounters or mergers will produce circumstellar debris disks with an orientation that differs form that of a pre-existing disk. Thus, massive stars growing by a series of mergers may produce eruptive outflows with random orientations; the walls of the resulting outflow cavities may be observable as filaments of dense gas and dust pointing away from the massive star. The extremely rare merger of two