Abstract
Since the time of their discovery, HH-objects have been the subject of extensive observational studies and intense theoretical work. At present the general consensus is in the sense that they represent shock waves of about 100 km i running in a dense, nearly neutral medium with density % 100 . This review summarizes current models for the general scenario of the origin of these objects which result in the formation of shock waves with characteristics similar to those observed in HH-objects. Some models regard the objects as the result of an eruptive event from a T Tauri star most likely an FU Ori phenomenon. Other models interpret the objects in terms of wind-accelerated clumps of material interacting either with a stellar wind or with the ambient medium. There is contradictory observational evidence favoring both alternatives. In particular, direct correlations between the total spatial velocity of individual subcondensations in HH2 and their physical parameters such as electron density, the surface brightness and size, favor the latter interpretation.
