On the excitation of the infrared knots in the HH99 outflow

We present near-infrared (IR) spectra (0.98-2.5 mum) from the group of Herbig-Haro (HH) objects comprising HH99: a series of knots forming a bow, HH99B, and a separate knot, HH99A. Observations of H-2, [Fe II] and [C I] are used to constrain shock-model parameters and determine the origin of the emission. Previous work has shown that it is likely that the atomic and ionic emission arises in regions of higher ionization than the molecular emission. On the basis of observations, it has often been suggested that the [Fe II] and [C I] emission could arise, for example, at the apex of a bow shock, with the H-2 emission produced in the wings. Accordingly, we have combined models of C-component and J-type shocks in order to reproduce the observed H-2, [Fe II] and [C I] line spectra. We can account for the H-2 emission towards the HH99B complex by means of J-type shocks with magnetic precursors. We derive shock velocities of about 30 km s(-1) and ages of order 100 yr; the pre-shock gas has a density n(H) approximate to 10(4) cm(-3) and the magnetic field B approximate to 100 muG. The J-type shocks required to reproduce the intensities of the observed [Fe II] and [C I] lines have velocities of 50 km s(-1). It is necessary to assume that Fe has been previously eroded from grains, probably by the earlier passage of a C-type shock wave. Thus, our analysis supports the view that molecular outflows are episodic phenomena whose observed emission arises in shock waves.