1998ApJ...506L..75N

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 on board the Infrared Space Observatory. The observed H₂ line ratios indicate the presence of warm molecular gas with an H₂ density of at least 10⁵ cm^–3 and a temperature ∼650 K in which the ratio of ortho- to para-H₂ 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 ratio of ortho- to para-H₂ 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 nonequilibrium ratio of ortho- to para-H₂ observed in HH 54 serves as a chronometer that places a conservative upper limit of ∼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 ratio of ortho- to para-H₂ was ≲0.2.