Bipolar molecular outflows from young stars and protostars

A violent outflow of high-velocity gas is one of the first manifestations of the formation of a new star. Such outflows emerge bipolarly from the young object and involve amounts of energy similar to those involved in accretion processes. The youngest (proto-)stellar low-mass objects known to date (the Class 0 protostars) present a particularly efficient outflow activity, indicating that outflow and infall motions happen simultaneously and are closely linked since the very first stages of the star formation processes. This article reviews the wealth of information being provided by large millimeter-wave telescopes and interferometers on the small-scale structure of molecular outflows, as well as the most recent theories about their origin. The observations of highly collimated CO outflows, extremely high velocity (EHV) flows, and molecular ''bullets'' are examined in detail, since they provide key information on the origin and propagation of outflows. The peculiar chemistry operating in the associated shocked molecular regions is discussed, highlighting the recent high-sensitivity observations of low-luminosity sources. The classification schemes and the properties of the driving sources of bipolar outflows are summarized with special attention devoted to the recently identified Class 0 protostars. All these issues are crucial for building a unified theory on the mass-loss phenomena in young stars.