Astrophys. J., 804, 98 (2015/May-2)
The deuterium fractionation timescale in dense cloud cores: a parameter space exploration.
KONG S., CASELLI P., TAN J.C., WAKELAM V. and SIPILA O.
Abstract (from CDS):
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 (ηH), temperature, cosmic ray ionization rate, and gas-phase depletion factor of heavy elements (fD). 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 DfracN2_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 FD ≳ 10. For fiducial model parameters, achieving DfracN2_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.
astrochemistry - ISM: clouds - stars: formation
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