Warm Molecular Hydrogen and Ionized Neon in the HH 2 Outflow^*

We report on spectro-imaging observations of the Herbig-Haro 2 outflow with the ISOCAM camera on board the Infrared Space Observatory. The [Ne II] 12.81 μm and [Ne III] 15.55 μm lines are detected only toward the jet working surface (HH 2H), consistent with the high excitation of this knot in the optical range, while H₂ pure rotational emission is found all over the shocked region HH 2. The low-energy transition S(2) traces warm gas (T ~ 400 K) peaked toward knots E and F and extended ejecta (T ~ 250-380 K) with masses of a few times 10^-3 M_☉ in the high-velocity CO outflow extending between the powering source and HH 2. Such emission could arise from low-velocity C-type shocks (v ≃ 10-15 km s^-1). The higher transitions S(3)-S(7) trace the emission of hot shocked gas (T = 1000-1400 K) from individual optical knots in the HH 2 region. The ortho-to-para (OTP) ratio exhibits large spatial variations between 1.2 (knot E) and 2.5 (knot H), well below its value at LTE. The emission of the S(3)-S(7) lines is well accounted for by planar C-shock models with a typical velocity V_s = 20-30 km s^-1 propagating into a medium of density n_i = 10⁴-10⁵ cm^-3 with an initial OTP ratio close to 1 in the preshock gas. In the leading edge of the jet, where the geometry of the emission allows a simple modeling, a good agreement is found with velocities derived from the optical proper motions measured in the ionized gas.