Jets from young stars

Abstract

Recent optical observations have shown that many young stars are capable of generating highly-collimated bipolar flows. Such flows are evident from narrow emission-line jets extending over 0.01–1 pc and having opening angles of a few degrees. Their measured radial velocities reach values of up to 400 km/s. For most known jets the “driving stars” are T Tauri stars or IR-sources of low to moderate luminosity (1-100 Lo). The typical velocities, Mach numbers, particle densities, and mass fluxes of these jets are estimated to be 200–400 km/s, 10–40, 15–150 cm−3, and 10−10-10−7 Mo/yr, respectively. The estimated mass fluxes and velocities are consistent with our present knowledge of T Tauri star wind properties. The emission-line spectra of the jets are the same as observed in Herbig-Haro (HH) objects. Furthermore, the brightest parts of some jets (hot spots) are known as HH objects, having often been discovered many years before their associated jet. Thus, both phenomena are highly related and in both cases the emission lines are very probably formed in the cooling regions of shock waves with velocities of 50–100 km/s. There are a variety of mechanisms, which can in principal excite internal (oblique) shockwaves in these jets. Examples of likely excitation mechanisms are fluid dynamical instabilities or pressure gradients in the ambient medium.