Theory of bipolar outflows from high-mass young stellar objects

There is a growing number of observational indicators for the presence of bipolar outflows in massive, young stellar objects that are still accreting mass as part of their formation process. In particular, there is evidence that the outflows from these objects can attain higher velocities and kinetic luminosities than their lower-mass counterparts. Furthermore, the higher-mass objects appear to smoothly continue the correlation found in T Tauri stars between outflow and accretion signatures, and in several cases there are direct clues to the existence of a circumstellar disk from optical and infrared imaging and spectroscopy as well as from millimeter-wavelength interferometry. These results suggest that the disk-outflow connection found in low-mass pre-main-sequence stars extends to more massive objects, and that a similar physical mechanism may drive the outflows in both cases. We examine the observational basis for this hypothesis and consider how the commonly invoked centrifugally driven wind models of bipolar outflows in low-mass stars would be affected by the various physical processes (such as photoionization, photoevaporation, radiation pressure, and stellar wind ram pressure) that operate in higher-mass stars. We then list some of the interesting questions that one could hope to address as this young field of research continues to develop.