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 H _ { 2 } line ratios indicate the presence of warm molecular gas with an H _ { 2 } density of at least 10 ^ { 5 } cm ^ { -3 } and a temperature \sim 650 K in which the ortho- to para-H _ { 2 } ratio is only 1.2 \pm 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-H _ { 2 } ratio is the legacy of an earlier stage in the thermal history of the gas when the gas had reached equilibrium at a temperature \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 90 K. Based upon the expected timescale for equilibration , we argue that the non-equilibrium ortho- to para-H _ { 2 } ratio observed in HH 54 serves as a chronometer that places a conservative upper limit of \sim 5000 yr on the period for which the emitting gas has been warm .
The S ( 2 ) /S ( 1 ) and S ( 3 ) /S ( 1 ) H _ { 2 } line ratios measured toward HH 54 are consistent with recent theoretical models of Timmermann for the conversion of para- to ortho-H _ { 2 } behind slow , ‘ C ’ -type shocks , but only if the preshock ortho- to para-H _ { 2 } ratio was \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 0.2 .