Knowledge Bank

The ortho:para ratio of H$2$ 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 H$\mathrm{3+}$ + HD $\to$ H$2$D$+$ + H$2$ + 220 K (and H$2$D$+$ + 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-H$2$ (o-H$2$) reacts with H$2$D$+$ (or D$2$H$+$, D$\mathrm{3+}$), as ortho-H$2$ possesses $\sim$170Kofinternalrotationalenergyinitsgroundstate.Recentmodelingworkhasdemonstratedtheimportanceofo-H$_2$ in cold, dense, highly depleted cores using a chemical network that includes all nuclear spin modifications of H$\mathrm{3+}$, H$2$, and their isotopologues, but the initial o-H$2$ 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-H$2$ 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 $\sim$170reactionsiscombinedwithaphysicalmodelofadensecloud,includingtime-dependentphysicalconditions.IncludedinthenetworkarethenuclearspinmodificationsofH$_2$,H$_2^+$,andH$_3^+$,aswellasnuclearspindependentratecoefficientsforthethermalizationreactionsH$_2$ + H$+$ and H$\mathrm{3+}$ + H$2$. By modeling the time-dependent chemistry, we find that the ortho:para ratio of H$2$ requires 10$7$-10$8$ 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 H$\mathrm{3+}$ + H$2$ reactions, and the relative abundances of H$\mathrm{3+}$ and H$+$, but is largely insensitive to the total density and temperature. Even at steady state, the o-H$2$ fraction is calculated to be $>$0.5\%at10K,whichisseveralordersofmagnitudeaboveitsvalueatthermodynamicequilibrium.Theprospectsforusingobservationsoftheortho:pararatioofH$_3^+$ as a probe of the H$2$ ortho:para ratio will be discussed.