The abundances of nitrogen-containing molecules during pre-protostellar collapse

Aims. We have studied the chemistry of nitrogen-bearing species during the initial stages of protostellar collapse, with a view to explaining the observed longevity of N2H+ and NH3 and the high levels of deuteration of these species. Methods. We followed the chemical evolution of a medium comprising gas and dust as it underwent free-fall gravitational collapse. Chemical processes which determine the relative populations of the nuclear spin states of molecules and molecular ions were included explicitly, as were reactions which lead ultimately to the deuteration of the nitrogen-containing species N2H+ and NH3. The freeze-out of heavy molecules on to dust grains was taken into account. Results. We found that the timescale required for the nitrogen-containing species to attain their steady-state values was much larger than the free-fall time and even comparable with the probable lifetime of the precursor molecular cloud. However, it transpires that the chemical evolution of the gas during gravitational collapse is insensitive to its initial composition. If we suppose that the grain-sticking probabilities of atomic nitrogen and atomic oxygen are both less than unity ( S less than or similar to 0.3), we find that the observed differential freeze-out of nitrogen-and carbon-bearing species can be reproduced by the model of free-fall collapse when a sufficiently large grain radius (a(g) approximate to 0.50 mu m) is adopted. Furthermore, the results of our collapse model are consistent with the high levels of deuteration of N2H+ and NH3 which have been observed in L1544, for example, providing that 0.5 less than or similar to a(g) less than or similar to 1.0 mu m. We note that the ortho: para H2D+ ratio, and fractional abundance of ortho-H2D+, which is the observed form of H2D+, should be largest where ND3 is most abundant.