Spitzer observations of HH54 and HH7-11: mapping the H2 ortho-to-para ratio in shocked molecular gas

Abstract

We report the results of spectroscopic mapping observations carried out toward the Herbig-Haro objects HH7-11 and HH54 over the 5.2 - 37 micron region using the Infrared Spectrograph of 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 H2 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 ~ 10 - 20 km/s, while the fine structure emissions originate in faster shocks of velocity 35 - 90 km/s that are dissociative and ionizing. Maps of the H2 line ratios reveal little spatial variation in the typical admixture of gas temperatures in the mapped regions, but show that the H2 ortho-to-para ratio is quite variable, typically falling substantially below the equilibrium value of 3 attained at the measured gas temperatures. The non-equilibrium ortho-to-para ratios are characteristic of temperatures as low as ~ 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 H2 ortho-to-para ratio show that ortho-to-para ratios < 0.8 are attained only at gas temperatures below ~ 900 K; this behavior is consistent with theoretical models in which the conversion of para- to ortho-H2 behind the shock is driven by reactive collisions with atomic hydrogen, a process which possesses a substantial activation energy barrier (E_A/k ~ 4000 K) and is therefore very inefficient at low temperature.