2015ApJ...804...98K

The deuterium fraction, [N₂D⁺]/[N₂H⁺], may provide information about the ages of dense, cold gas structures, which are important for comparing dynamical models of cloud core formation and evolution. Here we introduce a complete chemical network with species containing up to three atoms, with the exception of the oxygen chemistry, where reactions involving H₃O⁺ and its deuterated forms have been added, significantly improving the consistency with comprehensive chemical networks. Deuterium chemistry and spin states of H₂ and H₃⁺ isotopologues are included in this primarily gas-phase chemical model. We investigate the dependence of deuterium chemistry on these model parameters: density (η_H), temperature, cosmic ray ionization rate, and gas-phase depletion factor of heavy elements (f_D). We also explore the effects of time-dependent freeze-out of gas-phase species and the dynamical evolution of density at various rates relative to free-fall collapse. For a broad range of model parameters, the timescales to reach large values of D_frac^N2_H⁺^ ≳ 0.1, observed in some low- and high-mass starless cores, are relatively long compared to the local free-fall timescale. These conclusions are unaffected by introducing time-dependent freeze-out and considering models with evolving density, unless the initial F_D ≳ 10. For fiducial model parameters, achieving D_frac^N2_H requires collapse to be proceeding at rates at least several times slower than that of free-fall collapse, perhaps indicating a dynamically important role for magnetic fields in supporting starless cores and thus the regulation of star formation.