Infrared Space Observatory Observations of Molecular Hydrogen in HH 54:Measurement of a Nonequilibrium Ratio of Ortho- to Para-H2
Infrared Space Observatory Observations of Molecular Hydrogen in HH 54:Measurement of a Nonequilibrium Ratio of Ortho- to Para-H2
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红外空间观测站对 HH 54 中氢分子的观测：邻位氢与对位氢的非平衡比的测量
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发表时间：
1998
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通讯作者：
M. Harwit
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作者：
D. Neufeld;G. Melnick;M. Harwit
我们用红外空间观测站上的短波长谱仪探测到了S(1)、S(2)、S(3)、S(4)和S(5)分子氢向流出源HH-54的纯旋转线。观测到的H_2谱线比表明存在热分子气体，其H_2密度至少为105 cm~(-3)，温度为~650K，其中邻位和对位H_2的比例仅为1.2±0.4，明显小于在该温度下气体中预期的平衡比3。这些观测表明，测量到的邻位/对位氢的比率是气体热历史中较早阶段的遗留下来的，当时气体已经在≲90K的温度下达到平衡。根据预期的平衡时间刻度，我们认为在HH54中观察到的邻位/对位氢的非平衡比率是一个计时器，它为发射气体的温度设定了一个保守的上限~5000年。对HH 54测得的S(2)/S(1)和S(3)/S(1)H_2谱线比与Timmermann最近提出的慢C型激波后H_2向邻位H_2转化的理论模型是一致的，但前提是H_2与H_2的临震比为≲_(0.2)。
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.