ISO observations of molecular hydrogen in HH 54: measurement of a non-equilibrium ortho- to para-H2 ratio

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We have detected the S(1), S(2), S(3), S(4) and S(5) pure rotational lines of molecular hydrogen toward the outflow source HH 54, using the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). The observed H2 line ratios indicate the presence of warm molecular gas with an H2 density of at least 100,000 per cubic centimeter and a temperature 650 K in which the ortho- to para-H2 ratio is only 1.2 +/- 0.4, significantly smaller than the equilibrium ratio of 3 expected in gas at that temperature. These observations imply that the measured ortho- to para-H2 ratio is the legacy of an earlier stage in the thermal history of the gas when the gas had reached equilibrium at a temperature < 90 K. Based upon the expected timescale for equilibration, we argue that the non-equilibrium ortho- to para-H2 ratio observed in HH 54 serves as a chronometer that places a conservative upper limit 5000 yr on the period for which the emitting gas has been warm. The S(2)/S(1) and S(3)/S(1) H2 line ratios measured toward HH 54 are consistent with recent theoretical models of Timmermann for the conversion of para- to ortho-H2 behind slow, `C'-type shocks, but only if the preshock ortho- to para-H2 ratio was < 0.2.