Unexplained high ortho-to-para ratios in highly excited H₂ in the Orion Bar

Explain why the observed ortho-to-para ratios of highly vibrationally and rotationally excited H₂ in the Orion Bar approach the thermal value of 3, whereas the Meudon PDR models predict values near $\sqrt{3}$ after UV pumping and fluorescence.

Background

The IGRINS observations of the Orion Bar show ortho-to-para ratios greater than $\sqrt{3}$ and often close to 3 in highly excited H₂ levels. The Meudon PDR models instead predict ratios near $\sqrt{3}$, as expected when radiative fluorescence dominates and collisional ortho-to-para conversion is inefficient during the cascade.

The authors tested collisional de-excitation, conversion on dust surfaces, and the ortho-to-para ratio of nascent H₂, but none of these processes resolved the discrepancy under the modeled Orion Bar conditions. Consequently, the physical process responsible for the observed high ratios remains unidentified.