Isotopic abundances and line formation in the Orion Nebula

Measurements of the spatial variation of HC-12N-14 and HC-13N-14 line emission from the molecular cloud associated with the Orion Nebula indicate that HC-12N-14 has a high central opacity. This result seems to contradict a recent suggestion that the observed hyperfine structure of the line indicates a low opacity, which in turn would yield a C-13/C-12 abundance significantly different from other determinations. The observed hyperfine structure can be understood in the high-opacity case if radiative trapping in the lines is considered.