15min:

KYLE N. CRABTREE, Department of Chemistry, University of Illinois, Urbana, IL 61801; BENJAMIN J. MCCALL, Departments of Chemistry, Astronomy, and Physics, University of Illinois, Urbana, IL 61801.

The ortho:para ratio of H₂ is a critical parameter for deuterium fractionation in cold, dense quiescent cores. The dominant reservoir for interstellar deuterium is in the inert molecule HD, but the exothermic reaction H₃⁺ + HD \to H₂D⁺ + H₂ + 220 K (and H₂D⁺ + HD, etc.) can yield highly reactive species capable of distributing deuterium to other molecules. The barrier to the reverse reaction, however, can be overcome even at temperatures below 10 K when ortho-H₂ (o-H₂) reacts with H₂D⁺ (or D₂H⁺, D₃⁺), as ortho-H₂ possesses 170 K of internal rotational energy in its ground state. Recent modeling work has demonstrated the importance of o-H₂ in cold, dense, highly depleted cores using a chemical network that includes all nuclear spin modifications of H₃⁺, H₂, and their isotopologues, but the initial o-H₂ fraction is taken as a parameter in the model. Observationally or computationally constraining this quantity would aid in understanding deuterium fractionation in dense cores.

To learn about the initial o-H₂ fraction in a cold core, we have modeled the chemistry of non-depleted dense interstellar clouds from which cold cores are thought to form. A simplified gas-phase chemical network consisting of 28 species and 170 reactions is combined with a physical model of a dense cloud, including time-dependent physical conditions. Included in the network are the nuclear spin modifications of H₂, H₂⁺, and H₃⁺, as well as nuclear spin dependent rate coefficients for the thermalization reactions H₂ + H⁺ and H₃⁺ + H₂. By modeling the time-dependent chemistry, we find that the ortho:para ratio of H₂ requires 10⁷-10⁸ years to reach steady state under ``standard'' dense cloud conditions, which is at least on the order of the cloud lifetime. The timescale depends on the ionization rate, the rate coefficients of the various H₃⁺ + H₂ reactions, and the relative abundances of H₃⁺ and H⁺, but is largely insensitive to the total density and temperature. Even at steady state, the o-H₂ fraction is calculated to be >0.5% at 10 K, which is several orders of magnitude above its value at thermodynamic equilibrium. The prospects for using observations of the ortho:para ratio of H₃⁺ as a probe of the H₂ ortho:para ratio will be discussed.