[ITAL]Infrared Space Observatory[/ITAL] Observations toward the Reflection Nebula NGC 7023: A Nonequilibrium Ortho–to–Para-H[TINF]2[/TINF] Ratio

Abstract

We have observed the S(0), S(1), S(2), S(3), S(4), and S(5) rotational lines of molecular hydrogen (H2) toward the peak of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. The observed H2line 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-H2ratio of 3. An ortho-to-para (OTP) ratio in the range ∼1.5-2 is necessary to explain our observations. This is the first detection of a nonequilibrium OTP ratio measured from the H2pure rotational lines in a PDR. The existence of a dynamical PDR is discussed as the most likely explanation for this low OTP ratio.