Observations of near infrared emission-lines are becoming available and can be a powerful tool to improve our knowledge on planetary nebulae properties. For wavelengths in the range 1 to 5 μm, the emission-lines correspond to atomic transitions of high ionized species of heavy elements. In particular, the [Si VI] 1.96μm and [Si VII] 2.48μm lines have already been detected (Ashley and Hyland, 1988).
Detailed calculations are reported of the intensities of the near infrared forbidden lines of neutral carbon atoms at λ 985.0 nm, 982.3 nm and 872.7 nm emitted from dense clouds subjected to intense radiation fields. The metastable levels that produce the lines are excited by radiative recombination of the C+ ions produced by photoionization. Impacts of electrons with C atoms in the heated edge zones of the clouds contribute an insignificant part to the excitation. The lines observed in M42 and NGC 2024 can be interpreted as arising in gas with densities in excess of 105 cm−3 and radiation fields with intensities between 103 and 106 times the average interstellar field intensity. Radiative recombination of C+ ions may also be an important source of the emission lines detected in the planetary nebulae NGC 6270 and NGC 7027.
Infrared Emission-Lines and the Stellar Temperature