SpitzerObservations of HH 54 and HH 7–11: Mapping the H2Ortho‐to‐Para Ratio in Shocked Molecular Gas

We report the results of spectroscopic mapping observations carried outtoward the Herbig-Haro objects HH7-11 and HH54 over the 5.2 - 37 micron regionusing the Infrared Spectrograph of the Spitzer Space Telescope. Theseobservations have led to the detection and mapping of the S(0) - S(7) purerotational lines of molecular hydrogen, together with emissions in finestructure transitions of Ne+, Si+, S, and Fe+. The H2 rotational emissionsindicate the presence of warm gas with a mixture of temperatures in the range400 - 1200 K, consistent with the expected temperature behind nondissociativeshocks of velocity ~ 10 - 20 km/s, while the fine structure emissions originatein 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 typicaladmixture of gas temperatures in the mapped regions, but show that the H2ortho-to-para ratio is quite variable, typically falling substantially belowthe equilibrium value of 3 attained at the measured gas temperatures. Thenon-equilibrium ortho-to-para ratios are characteristic of temperatures as lowas ~ 50 K, and are a remnant of an earlier epoch, before the gas temperaturewas elevated by the passage of a shock. Correlations between the gastemperature and H2 ortho-to-para ratio show that ortho-to-para ratios < 0.8 areattained only at gas temperatures below ~ 900 K; this behavior is consistentwith theoretical models in which the conversion of para- to ortho-H2 behind theshock is driven by reactive collisions with atomic hydrogen, a process whichpossesses a substantial activation energy barrier (E_A/k ~ 4000 K) and istherefore very inefficient at low temperature.Comment: 45 pages, including 16 figures. Accepted for publication in Ap