Nitrogen chemistry and depletion in starless cores
  We investigated the chemistry of nitrogen--containing species, principally
isotopomers of CN, HCN, and HNC, in a sample of pre-protostellar cores. We used
the IRAM 30 m telescope to measure the emission in rotational and hyperfine
transitions of CN, HCN, 13CN, H13CN, HN13C, and HC15N, in L 1544, L 183, Oph D,
L 1517B, L 310. The observations were made along axial cuts through the dust
emission peak, at a number of regularly--spaced offset positions. The
observations were reduced and analyzed to obtain the column densities, using
the measurements of the less abundant isotopic variants in order to minimize
the consequences of finite optical depths in the lines. The observations were
compared with the predictions of a free--fall gravitational collapse model,
which incorporates a non-equilibrium treatment of the relevant chemistry. We
found that CN, HCN, and HNC remain present in the gas phase at densities well
above that at which CO depletes on to grains. The CN:HCN and the HNC:HCN
abundance ratios are larger than unity in all the objects of our sample.
Furthermore, there is no observational evidence for large variations of these
ratios with increasing offset from the dust emission peak and hence with
density. Whilst the differential freeze--out of CN and CO can be understood in
terms of the current chemistry, the behaviour of the CN:HCN ratio is more
difficult to explain. Models suggest that most nitrogen is not in the gas phase
but may be locked in ices. Unambiguous conclusions require measurements of the
rate coefficients of the key neutral--neutral reactions at low temperatures.
