  We present near infrared (IR) spectra (0.98--2.5 mu m) 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 H2, [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 H2
emission produced in the wings. Accordingly, we have combined models of
C-component and J-type shocks in order to reproduce the observed H2, [Fe II]
and [C I] line spectra. We can account for the H2 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 of approximately 10^4 cm-3 and the magnetic field is approximately 100
mu G. 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.
