Hyperfine transitions of 13CN from pre-protostellar sources
  Recent quantum mechanical calculations of rate coefficients for collisional
transfer of population between the hyperfine states of 13CN enable their
population densities to be determined. We have computed the relative
populations of the hyperfine states of the N = 0, 1, 2 rotational states for
kinetic temperatures 5 $\le$ T $\le$ 20 K and molecular hydrogen densities 1
$\le$ n(H2) $\le$10 10 cm --3. Spontaneous and induced radiative transitions
were taken into account. Our calculations show that, if the lines are optically
thin, the populations of the hyperfine states, F, within a given rotational
manifold are proportional to their statistical weights, (2F + 1) -- i.e. in
local thermodynamic equilibrium -- over the entire range of densities. We have
re-analysed IRAM 30 m telescope observations of 13CN hyperfine transitions (N =
1 $\rightarrow$ 0) in four starless cores. A comparison of these observations
with our calculations confirms that the hyperfine states are statistically
populated in these sources.
