The deuterium fraction [ N _ { 2 } D ^ { + } ] / [ N _ { 2 } H ^ { + } ] , may provide information about the ages of dense , cold gas structures , important to compare with 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 H _ { 3 } O ^ { + } and its deuterated forms have been added , significantly improving the consistency with comprehensive chemical networks .
Deuterium chemistry and spin states of H _ { 2 } and H _ { 3 } ^ { + } isotopologues are included in this primarily gas-phase chemical model .
We investigate dependence of deuterium chemistry on model parameters : density ( n _ { 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 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 } ^ { N _ { 2 } H ^ { + } } \gtrsim 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 } \gtrsim 10 .
For fiducial model parameters , achieving D _ { frac } ^ { N _ { 2 } H ^ { + } } \gtrsim 0.1 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 the support of starless cores and thus the regulation of star formation .