Jets Associated with Young Stars

Abstract

It is almost half a century since the discovery by Herbig (1951) and Haro (1952) of the enigmatic nebulosities that bear their name. It was clear from very early on that HH objects were associated in some way with star formation and, in fact, it was originally suggested that they were actual sites were stars were made (Herbig, 1970). To this day, popular texts, even such erudite volumes as the Cambridge Atlas of Astronomy, still make statements like “… we also find in this class stars called Herbig-Haro objects, which are stars in the process of being formed”! of course we now know this idea to be incorrect and as early as 1958 (Osterbrock 1958) it was proposed that HH objects might be the result of an energetic outflow from a young star. Observational support for this idea, however, was lacking until Schwartz (1975) noted the resemblance between the spectra of HH objects and those of knots in supernova remnants. This led him to propose that HH objects were due to radiatively cooled shocks driven by supersonic winds from young stars. Work by Raymond (1976, 1979), Dopita (1978) and others confirmed this suggestion and they found good agreement between theoretical shock models and observed line intensities. Estimated shock velocities, derived from fitting various line ratios, ranged from around 50kms⁻¹ up to about 300kms⁻¹ depending upon the degree of excitation of the HH object.