Hydrogen cyanide and isocyanide in prestellar cores
  We studied the abundance of HCN, H13CN, and HN13C in a sample of prestellar
cores, in order to search for species associated with high density gas. We used
the IRAM 30m radiotelescope to observe along the major and the minor axes of
L1498, L1521E, and TMC 2, three cores chosen on the basis of their CO depletion
properties. We mapped the J=1-0 transition of HCN, H13CN, and HN13C towards the
source sample plus the J=1-0 transition of N2H+ and the J=2-1 transition of
C18O in TMC 2. We used two different radiative transfer codes, making use of
recent collisional rate calculations, in order to determine more accurately the
excitation temperature, leading to a more exact evaluation of the column
densities and abundances. We find that the optical depths of both H13CN(1-0)
and HN13C(1-0) are non-negligible, allowing us to estimate excitation
temperatures for these transitions in many positions in the three sources. The
observed excitation temperatures are consistent with recent computations of the
collisional rates for these species and they correlate with hydrogen column
density inferred from dust emission. We conclude that HCN and HNC are
relatively abundant in the high density zone, n(H2) about 10^5 cm-3, where CO
is depleted. The relative abundance [HNC]/[HCN] differs from unity by at most
30 per cent consistent with chemical expectations. The three hyperfine
satellites of HCN(1-0) are optically thick in the regions mapped, but the
profiles become increasingly skewed to the blue (L1498 and TMC 2) or red
(L1521E) with increasing optical depth suggesting absorption by foreground
layers.
