Bipolar Molecular Outflows

Abstract

Stars form by the gravitational collapse of dense gas in molecular clouds, so one would naively expect that infall motions dominate the gas kinematics in star-forming regions. Observations, however, show this not to be the case, and that outflow motions — not infall — is what prevails around young stellar objects (YSOs). This apparent contradiction was fully realized about 20 years ago, when the first systematic molecular observations of star-forming regions were done, and bipolar molecular outflows were found towards almost every region of star formation (e.g, Snell et al. 1980, Bally & Lada 1983). The discovery of outflows has deeply changed our picture of how stars are born; their observational study has dominated the field of star formation research, and their theoretical understanding has posed a serious challenge to theorists of stellar birth. After 20 years of intense outflow research, enormous progress has been done, but some basic questions still remain unanswered. In this review we attempt to briefly describe what we know about molecular outflows, and also show the challenges that lie ahead in this exciting field of star formation.