The importance of the ortho:para H₂ ratio for the deuteration of molecules during pre-protostellar collapse

¹ Physics Department, The University, Durham DH1 3LE, UK e-mail: david.flower@durham.ac.uk
² Institut d'Astrophysique Spatiale (IAS), Bâtiment 121, 91405 Orsay, France
³ Université Paris-Sud 11 and CNRS (UMR 8617)
⁴ INAF, Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

Received: 23 September 2005
Accepted: 7 December 2005

Abstract

Context.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.Aims.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.Methods.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.Results.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 H and hence of other species, such as N₂H⁺ and NH₃. Conclusions.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.