The Deuterium Fractionation Timescale in Dense Cloud Cores: A Parameter Space Exploration

Abstract

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 N2H + frac 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 N2H + frac 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.

The Deuterium Fraction in Massive Starless Cores and Dynamical Implications

“…Deuterated species are likely to trace colder conditions (e.g., Caselli et al 1999;Pagani et al 2013;Kong et al 2015). Note that Crapsi et al (2005) measured T ex to be about 4.5K in a number of low-mass cores, only slightly larger than our derived values.…”

“…We have developed a chemical model (Kong et al 2015) to describe the time evolution of , in starless cores, allows estimation of core age and thus constrains the dynamical history of its collapse, e.g., the collapse rate relative to free-fall.…”

Search for grain growth toward the center of L1544

“…However, pre-stellar cores may span a range of lifetimes which significantly exceeds the free-fall time (Brünken et al 2014;Kong et al 2015;Könyves et al 2015, see e.g.). If there are processes sustaining the cloud against gravitational collapse (e.g.…”

Deuteration in infrared dark clouds

“…We compared the Kong et al (2015) main fiducial model with our observations, in order to evaluate the degree of evolution of our sample. We had to convert the model's N2H + and N2D + densities to column densities.…”