C2H in prestellar cores

Aims. We study the abundance of C2H 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 C2H and improve current estimates of the spectroscopic constants of C2H. Methods. We used the IRAM 30 m radiotelescope to map the N = 1-0 and N = 2-1 transitions of C2H 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 (CO)-O-18. We used a Monte Carlo radiative transfer program to analyse the C2H observations of L1498. We derived the distribution of C2H 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 C2H 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 [ C2H]/[ H-2] of (1.0 +/- 0.3) x 10(-8) in L1498 and (0.9 +/- 0.3) x 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 C2H 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 C2H abundance distribution. Finally, we used our observations to provide improved estimates for the rest frequencies of all six components of the C2H (1-0) line and seven components of C2H (2-1). Based on these results, we compute improved spectroscopic constants for C2H. We also give a brief discussion of the prospects for measuring magnetic field strengths using C2H.