Spitzer observations of HH 54 and HH 7-11:: Mapping the H2 ortho-to-para ratio in shocked molecular gas

We report the results of spectroscopic mapping observations carried out toward the Herbig-Haro objects HH 7-11 and HH 54 over the 5.2-37 mu m region using the Infrared Spectrograph on the Spitzer Space Telescope. These observations have led to the detection and mapping of the S(0)-S(7) pure rotational lines of molecular hydrogen, together with emissions in fine-structure transitions of Ne+, Si+, S, and Fe+. The H-2 rotational emissions indicate the presence of warm gas with a mixture of temperatures in the range 400 - 1200 K - consistent with the expected temperature behind nondissociative shocks of velocity similar to 10-20 km s(-1) - while the fine-structure emissions originate in faster shocks of velocity similar to 35-90 km s(-1) that are dissociative and ionizing. The H-2 ortho-to-para ratio is quite variable, typically falling substantially below the equilibrium value of 3 attained at the measured gas temperatures. The nonequilibrium ortho-to-para ratios are characteristic of temperatures as low as similar to 50 K, and are a remnant of an earlier epoch, before the gas temperature was elevated by the passage of a shock. Correlations between the gas temperature and H-2 ortho-to-para ratio show that ortho-to-para ratios < 0.8 are attained only at gas temperatures below similar to 900 K; this behavior is consistent with theoretical models in which the conversion of para- to ortho-H-2 behind the shock is driven by reactive collisions with atomic hydrogen, a process that possesses a substantial activation energy barrier (E-A/k similar to 4000 K) and is therefore very inefficient at low temperature. The lowest observed ortho-to-para ratios of only similar to 0.25 suggest that the shocks in HH 54 and HH 7 are propagating into cold clouds of temperature <= 50 K in which the H-2 ortho-to-para ratio is close to equilibrium.