CCH in prestellar cores
  We study the abundance of CCH in prestellar cores both because of its role in
the chemistry and because it is a potential probe of the magnetic field. We
also consider the non-LTE behaviour of the N=1-0 and N=2-1 transitions of CCH
and improve current estimates of the spectroscopic constants of CCH. We used
the IRAM 30m radiotelescope to map the N=1-0 and N=2-1 transitions of CCH
towards the prestellar cores L1498 and CB246. Towards CB246, we also mapped the
1.3 mm dust emission, the J=1-0 transition of N2H+ and the J=2-1 transition of
C18O. We used a Monte Carlo radiative transfer program to analyse the CCH
observations of L1498. We derived the distribution of CCH column densities and
compared with the H2 column densities inferred from dust emission. We find that
while non-LTE intensity ratios of different components of the N=1-0 and N=2-1
lines are present, they are of minor importance and do not impede CCH column
density determinations based upon LTE analysis. Moreover, the comparison of our
Monte-Carlo calculations with observations suggest that the non-LTE deviations
can be qualitatively understood. For L1498, our observations in conjunction
with the Monte Carlo code imply a CCH depletion hole of radius 9 x 10^{16} cm
similar to that found for other C-containing species. We briefly discuss the
significance of the observed CCH abundance distribution. Finally, we used our
observations to provide improved estimates for the rest frequencies of all six
components of the CCH(1-0) line and seven components of CCH(2-1). Based on
these results, we compute improved spectroscopic constants for CCH. We also
give a brief discussion of the prospects for measuring magnetic field strengths
using CCH.
