Herbig-Haro flows: Probes of early stellar evolution

Outflow activity is associated with all stages of early stellar evolution, from deeply embedded protostellar objects to visible young stars. Herbig-Ha ro (HH) objects are the optical manifestations of this powerful mass loss. Analysis of HH flows, and in particular of the subset of highly collimated HH jets, provides indirect but important insights into the nature of the ac cretion and mass-loss processes that govern the formation of stars. The rec ent recognition that HH flows may attain parsec-scale dimensions opens up t he possibility of partially reconstructing the mass-ejection history of the newly born driving sources and, therefore, their mass-accretion history. F urthermore, HH flows are astrophysical laboratories for the analysis of sho ck structures, of hydrodynamics in collimated flows, and of their interacti on with the surrounding environment. HH flows may be an important source of turbulence in molecular clouds. Recent technological developments have ena bled detailed observations of outflows from young stars at near-infrared, m id-infrared, submillimeter, millimeter, and centimeter wavelengths, providi ng a comprehensive picture of the outflow phenomenon of young stars.