Anatomy of the Herbig--Haro object HH7 bow shock

We perform a detailed shock diagnosis of the Herbig—Haro object HH7, a well-defined bow shock from a protostellar outflow. We first present molecular hydrogen images in the 2-1 S(1) and 1-0 S(1) K-band emission lines. We then introduce revised models for magneto-hydrodynamic bow shocks that incorporate a limited C and O chemistry and account for the shock thickness. We employ these models to interpret the new images as well as ISO data, the line profile, H2 position—velocity diagram, optical images and the proper motion. This yields a C-shock model that satisfies the constraints, confirming that ambipolar diffusion is the linchpin in the shock physics. The best model is a slow-moving paraboloidal bow of speed 55 km s−1, with a pre-shock density of 8 × 103 cm−3 and an H2/H number ratio of just 0.25. The bow moves at an angle of ∼30° to the line of sight and at a position angle of ∼95° in the plane of the sky rather than along the outflow axis of ∼123°. The bow model also predicts the observed low line emission from H2O, without the need for gas-phase depletion. Predictions for imaging and spectroscopy at far-infrared wavelengths, employing the 63-µm [O i] line, are presented.