The Observational Study of Herbig-Haro Shock Waves

Abstract

We review the basic shock properties and the origin and the geometry of Herbig-Haro (H-H) shock waves. We first discuss different aspects of “normal” H-H objects which are connected with working surfaces (including internal working surfaces) of jets from young stellar objects. The emphasis is on unsolved problems of the H-H shock waves and not on the problems of the jet.

We study the line flux ratios of high excitation H-H objects (high velocity shocks) and low excitation HH objects (low velocity shocks) and carry out a comparison with theoretical predictions in both cases. We emphasize an unexplained deficit of higher ions (especially OIII and SIII, but also various other ions) in high excitation objects. This lets the line flux ratios of HH objects appear as if their shock velocities are almost never above 100 km s⁻¹, while other shock diagnostics (position-velocity diagrams, integrated line profiles, distributions of fluxes along the axis of the bow shock, etc.) definitely indicate higher shock velocities.

Some aspects of the spectrum interpretation of the very low velocity shocks (like HH7) are explained quite well by the theory. A basic unsolved problem is, however, the explanation of the CI lines whose flux is up to a factor 10 times stronger than predicted for any model. Obviously we are very far from correctly predicting the ionization of C in shock models.

In the last chapter we discuss, as one example of a very unusual HH-object, HH255 (Burnham’s nebula). Detailed line fluxes in the immediate environment of T Tauri (the source of HH255) have shown that HH255 has a shock wave spectrum and is definitely an HH object. In the very narrow region between 3” and 4” S of T Tauri we find a sharp peak of the velocity dispersion, the centroid velocity, and N_e. In the same region there is an almost discontinous increase in ionization. Between 4” and 10” S (corresponding to 600–1600 a.u.) of T Tauri (the source of HH255) the ionization remains high but the centroid velocity is zero (with respect to T Tauri) and the velocity dispersion is very small. This result is completely surprising for a shock wave which according to the flux ratios must have ~90 km s⁻¹⁻¹ shock velocity. Why should a cooling region of a shock have a centroid velocity of ~0 km s⁻¹ over a large range of distance from the stellar source? At present the geometry of the HH255 is enigmatic.