ISO observations toward the reflection nebula NGC 7023: A nonequilibrium ortho- to para-H2 ratio

Details ISO observations toward the reflection nebula NGC 7023: A nonequilibrium ortho- to para-H2 ratio ISO observations toward the reflection nebula NGC 7023: A nonequilibrium ortho- to para-H2 ratio

1999-04-12

arxiv.org/abs/astro-ph/9904136v1

Astronomy and Astrophysics

Astrophysics

4 pages, 3 figures

Scientific paper

10.1086/312063

We have observed the S(0), S(1), S(2), S(3), S(4) and S(5) rotational lines of molecular hydrogen (H2) towards the peak of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. The observed H2 line ratios show that they arise in warm gas with kinetic temperatures ~300 - 700 K. However, the data cannot be fitted by an ortho- to para- (OTP) ratio of 3. An OTP ratio in the range ~1.5 - 2 is necessary to explain our observations. This is the first detection of a non-equilibrium OTP ratio measured from the H2 pure-rotational lines in a PDR. The existence of a dynamical PDR is discussed as the most likely explanation for this low OTP ratio.

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