Thermal Effects of Collisional Line Excitation in Gaseous Nebulae.

The cooling effects of ions of oxygen, neon, nitrogen, and other light elements may be estimated from the observed intensities of all the relevant forbidden lines. The electron temperatures in nebulae such as IC 418, with relatively weak forbidden lines (as compared with the Baimer lines), must be substantially greater than in nebulae with relatively strong forbidden lines. Thus the electron temperatures in highexcitation planetaries are not necessarily higher than those in low-excitation planetaries but may even be relatively low. The electron temperature caunot rise substantially above 25,000 or 30,000 K in a planetary nebula because of the dissipation of energy in the excitation of the hydrogen lines, unless the hydrogen is substantially all ionized.