Astronomy and Astrophysics, volume 449, 621-629 (2006/4-2)
The importance of the ortho:para H2 ratio for the deuteration of molecules during pre-protostellar collapse.
FLOWER D.R., PINEAU DES FORETS G. and WALMSLEY C.M.
Abstract (from CDS):
We have studied the evolution of molecular gas during the early stages of protostellar collapse, when the freeze-out of ``heavy'' species on to grains occurs. In addition to studying the freeze-out of ``heavy'' species on to grains, we wished to compute the variation of the population densities of the different nuclear spin states of `tracer' molecular ions, such as H2D+ and D2H+, which are currently observed only in their ortho and para forms, respectively. Chemical processes which determine the relative populations of the nuclear spin states of molecules and molecular ions were included explicitly. Nuclear spin-changing reactions have received much less attention in the literature than those leading to deuteration; but, in fact, the former processes are as significant as the latter and often involve the same reactants. A ``free-fall'' model of gravitational collapse was adopted. We found that the ortho:para ratios of some species, e.g. H2D+, vary considerably as the density increases. Because the dynamical timescale is much shorter than some of the chemical timescales, there can be large departures of the predictions of the free-fall model from the steady-state solution at the same density and temperature. In the case of H2, it seems unlikely that the steady state value of the ortho:para ratio is attained before protostellar collapse from the progenitor molecular cloud commences. Values of the ortho:para H2 ratio much higher than in steady state, which would prevail in ``young'' molecular clouds, are found to be inconsistent with high levels of deuteration of the gas. The internal energy of ortho-H2 acts as a reservoir of chemical energy which inhibits the deuteration of H3+ and hence of other species, such as N2H+ and NH3. The principal conclusion is that the degree of deuteration of molecular ions and molecules is sensitive to the ortho:para H2 ratio and hence to the chemical and thermal history of the precursor molecular cloud.