Physical Processes in Gaseous Nebulae. XVIII. The Chemical Composition of the Planetary Nebulae.

Bowen 3nd Wyse's conclusion that the chemical composition of the planetary nebulae is essentially the same as that of the sun is substantiated by a more detailed treatment of the same problem in which an attempt has been made to estimate the relative abundances of the lighter elements. For hydrogen, helium, and carbon, which are represented only by permitted lines of the recombination spectrum, we have estimated mean relative abundances of 1000, 100~ and 0.6. The abundance of hydrogen follows from the intensity of the continuous spectrum at the limit of the BaLmer series. An estimate of the abundance of ionized helium is provided by the observed tine intensities and the theory given by Goldberg. The singlets are stronger than they should be, according to Goldberg's theory; and we may interpret the dis- crepançies between observations and theory by supposing that collisional excitations and de-excitations of the atoms in the high 23S and 2'S metastable levels play a role in fixing the population of the high- energy levels. The abundances of the C iii and C iv ions have been evaluated from the recombination lines of C n and C m with the aid of approximate wave functions calculated by Slater's rules and of b-factors estimated from the hydrogenic case. The 0 in abundance in NGC 7009, as estimated from the On recombination lines, appears to agree with that found from the green nebular lines, thus removing the discrepancy suggested by Wyse. S We have estimated the abundances of the other ions from their forbidden lines and, with the exception of 0111, from approximate coffisional cross-sections. To calculate the numbers of atoms of any kind, we must make some estimate of their distribution among various stages of ionization. This step, which has tc~ be made empirically, seems to introduce the greatest share of the uncertainty in the final results. The average abundances of nitrogen, oxygen, fluorine, neon, sulphur, chlorine, and argon are found to be 0.2, 0.25, 0.0001, 0.01, 0.036, 0.002, and 0.0015, respectively-all on the basis of hydrogen as 1000. Compari- sons with the compositions of the solar atmosphere and that of r Scorpii are discussed. It was not possible to estimate the contribution of the metals. The possible effects of stratification and of the bright-line radiation of the central star are briefly discussed.