Infrared Space Observatory Observations of Molecular Hydrogen in HH 54:Measurement of a Nonequilibrium Ratio of Ortho- to Para-H_2

Details Infrared Space Observatory Observations of Molecular Hydrogen in HH 54:Measurement of a Nonequilibrium Ratio of Ortho- to Para-H_2 Infrared Space Observatory Observations of Molecular Hydrogen in HH 54:Measurement of a Nonequilibrium Ratio of Ortho- to Para-H_2

Oct 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...506l..75n&link_type=abstract

The Astrophysical Journal, Volume 506, Issue 1, pp. L75-L78.

Astronomy and Astrophysics

Astronomy

40

Infrared: Ism: Lines And Bands, Ism: Molecules, Molecular Processes

Scientific paper

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 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 ~650 K in which the ratio of ortho- to para-H_2 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-H_2 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-H_2 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) 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 ratio of ortho- to para-H_2 was <~0.2. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) with the participation of ISAS and NASA.

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