Abstract.We have carried out calculations of ionization equilibrium and deuterium fractionation for conditions appropriate to a completely depleted, low mass pre-protostellar core, where heavy elements such as C, N, and O have vanished from the gas phase and are incorporated in ice mantles frozen on dust grain surfaces. We put particular emphasis on the interpretation of recent observations of H 2 D + towards the centre of the prestellar core L 1544 (Caselli et al. 2003) and also compute the ambipolar diffusion timescale. We consider explicitly the ortho and para forms of H 2 , H + 3 , and H 2 D + . Our results show that the ionization degree under such conditions depends sensitively on the grain size distribution or, more precisely, on the mean grain surface area per hydrogen nucleus. Depending upon this parameter and upon density, the major ion may be H + , H + 3 , or D + 3 . We show that the abundance of ortho-H 2 D + observed towards L 1544 can be explained satisfactorily in terms of a complete depletion model and that this species is, as a consequence, an important tracer of the kinematics of prestellar cores.

We have computed the rate of radiative energy loss from a gas containing H 2 which is collisionally excited by H, He and H 2 . For this purpose, we have used the results of recent quantum mechanical calculations of the cross-sections for rovibrational transitions between all the energy levels up to approximately 20 000 K above the ground state. The temperature dependence of the rate coef®cients for collisional de-excitation is found to be well represented by a simple functional form. The cooling function has been evaluated in steady state on a grid covering a wide range of values of the gas density and temperature, the atomic to molecular hydrogen density ratio, and the ortho-to para-H 2 ratio. A fortran program is provided for the purpose of rapid numerical interpolation to any desired set of values of these parameters. The properties of the cooling function are discussed, as are the time-scales required for the attainment of steady state.

No 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 H 2 D + and D 2 H + , 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. H 2 D + , 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 H 2 , 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 H 2 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-H 2 acts as a reservoir of chemical energy which inhibits the deuteration of H + 3 and hence of other species, such as N 2 H + and NH 3 . Conclusions. The principal conclusion is that the degree of deuteration of molecular ions and molecules is sensitive to the ortho:para H 2 ratio and hence to the chemical and thermal history of the precursor molecular cloud.

We study the production of SiO in the gas phase of molecular outflows, through the sputtering of Si-bearing material in refractory grain cores, which are taken to be olivine. We calculate also the rotational line spectrum of the SiO. The sputtering is driven by neutral particle impact on charged grains, in steady-state C-type shock waves, at the speed of ambipolar diffusion. The emission of the SiO molecule is calculated by means of an LVG code. A grid of models, with shock speeds in the range 20 < v s < 50 km s −1 and preshock gas densities 10 4 < n H < 10 6 cm −3 , has been generated. We compare our results with those of an earlier study (Schilke et al. 1997). Improvements in the treatment of the coupling between the charged grains and the neutral fluid lead to narrower shock waves and lower fractions of Si ( < ∼ 10%) being released into the gas phase. Erosion of grain cores is significant ( > ∼ 1%) only for C-type shock speeds v s > 25 km s −1 , given the adopted properties of olivine. More realistic assumptions concerning the initial fractional abundance of O 2 lead to SiO formation being delayed, so that it occurs in the cool, dense postshock flow. Good agreement is obtained with recent observations of SiO line intensities in the L1157 and L1448 molecular outflows. The inferred temperature, opacity, and SiO column density in the emission region differ significantly from those estimated by means of LVG "slab" models. The fractional abundance of SiO is deduced and found to be in the range 4 × 10 −8 < ∼ n(SiO)/n H < ∼ 3 × 10 −7 . Observed line profiles are wider than predicted and imply multiple, unresolved shock regions within the beam.