The Ratio of Ortho- to Para-H2 in Photodissociation Regions

TL;DR

This paper clarifies the distinction between the ortho-to-para ratio of H2 molecules and the vibrationally excited ortho-to-para ratios in photodissociation regions, showing that observed ratios are consistent with an equilibrium ratio of 3.

Contribution

The study resolves confusion in the literature by demonstrating that FUV-pumped vibrational ratios are not the total molecular ratios, supported by models and ISO observations.

Findings

Vibrationally excited ortho-to-para ratios are typically around 1.7 due to optical depth effects.

Most previous measurements are consistent with a total ortho-to-para ratio of 3.

Models and observations support the equilibrium ratio of 3 for temperatures above 200 K.

Abstract

We discuss the ratio of ortho- to para-H2 in photodissociation regions (PDRs). We draw attention to an apparent confusion in the literature between the ortho-to-para ratio of molecules in FUV-pumped vibrationally excited states, and the H2 ortho-to-para abundance ratio. These ratios are not the same because the process of FUV-pumping of fluorescent H2 emission in PDRs occurs via optically thick absorption lines. Thus, gas with an equilibrium ratio of ortho- to para-H2 equal to 3 will yield FUV-pumped vibrationally excited ortho-to-para ratios smaller than 3, because the ortho-H2 pumping rates are preferentially reduced by optical depth effects. Indeed, if the ortho and para pumping lines are on the ``square root'' part of the curve-of-growth, then the expected ratio of ortho and para vibrational line strengths is the square root of 3, ~ 1.7, close to the typically observed value. Thus, contrary to what has sometimes been stated in the literature, most previous measurements of the ratio of ortho- to para-H2 in vibrationally excited states are entirely consistent with a total ortho-to-para ratio of 3, the equilibrium value for temperatures greater than 200 K. We present an analysis and several detailed models which illustrate the relationship between the total ratios of ortho- to para-H2 and the vibrationally excited ortho-to-para ratios in PDRs. Recent Infrared Space Observatory (ISO) measurements of pure rotational and vibrational H2 emissions from the PDR in the star-forming region S140 provide strong observational support for our conclusions.