2009A&A...505.1199P

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. We used the IRAM 30m 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 C¹⁸O. 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 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 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]/[H₂] of (1.0±0.3)x10^–8 in L1498 and (0.9±0.3)x10^–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 9x10¹⁶ 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.