NEUTRAL CARBON FAR-RED FORBIDDEN LINE EMISSION FROM PLANETARY-NEBULAE

The temperature-sensitive neutral carbon forbidden lines at 8727, 9824 and 9850 Angstrom have been measured simultaneously for the first time from a planetary nebula. The nebulae NGC 2346, NGC 2440, NGC 3132 and IC 4406 were observed. Accurate rest wavelengths of these lines are obtained. The observed line ratios I[lambda 9824 + lambda 9850]/I[lambda 8727] are consistent with collisional excitation by electron impacts. It is demonstrated that radiative recombination and stellar continuum fluorescence are unimportant in exciting the observed [C I] lines, with the possible exception of NGC 2440 where a contribution from the former process cannot be ruled out. For NGC 2346, NGC 3132 and IC 4406, the observed [C I] line ratios yield electron temperatures between 7400 and 8000 K, about 1800 to 2800 K lower than those deduced from the [N II], [S III] and [O III] Line ratios that we also measured. Electron densities are derived from the observed [N I], [S II] and [Cl III] doublet ratios.