Infrared Space Observatory observations toward the reflection nebula NGC 7023: A nonequilibrium ortho-to-para-H-2 ratio

We have observed the S(0), S(1), S(2), S(3), S(4), and S(5) rotational line s of molecular hydrogen (H-2) toward the peak of the photodissociation regi on (PDR) associated with the reflection nebula NGC 7023. The observed H-2 l ine ratios show that they arise in warm gas with kinetic temperatures simil ar to 300-700 K. However the data cannot be fitted by an ortho-to-para-H, r atio of 3. An ortho-to-para (OTP) ratio in the range similar to 1.5-2 is ne cessary to explain our observations. This is the first detection of a noneq uilibrium OTP ratio measured from the H-2 pure rotational lines in a PDR. T he existence of a dynamical PDR is discussed as the most likely explanation for this low OTP ratio.