| Abstract
| - Aims. We study the abundance of C 2H 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 C 2H and improve current estimates of the spectroscopic constants of C 2H. Methods. We used the IRAM 30 m radiotelescope to map the N = 1-0 and N = 2-1 transitions of C 2H towards the prestellar cores L1498 and CB246. Towards CB246, we also mapped the 1.3 mm dust emission, the J = 1-0 transition of N 2H + and the J = 2-1 transition of C 18O. We used a Monte Carlo radiative transfer program to analyse the C 2H observations of L1498. We derived the distribution of C 2H column densities and compared with the H 2 column densities inferred from dust emission. Results. 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 C 2H 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 extinctions less than 20 visual magnitudes, we derive toward these two cores (assuming LTE) a relative abundance [ C 2H] /[ H 2] of (1.0 ±0.3) $\times$10 -8 in L1498 and (0.9 ±0.3) $\times$10 -8 in CB246 in reasonable agreement with our Monte-Carlo estimates. For L1498, our observations in conjunction with the Monte Carlo code imply a C 2H depletion hole of radius 9 $\times$10 16 cm similar to that found for other C-containing species. We briefly discuss the significance of the observed C 2H abundance distribution. Finally, we used our observations to provide improved estimates for the rest frequencies of all six components of the C 2H (1-0) line and seven components of C 2H (2-1). Based on these results, we compute improved spectroscopic constants for C 2H. We also give a brief discussion of the prospects for measuring magnetic field strengths using C 2H.
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