Galactic abundances of neon, argon, and chlorine derived from planetary nebulae

Abstract

Mean neon and argon to oxygen ratios are found from planetary nebulae to be NeIO = 0.225 + 0.01 and ArlO = 7.0 + 0.5 x 10- . The mean ArICl ratio is 21.3 + 2.3, which results in ClIO = 3.3 + 0.5 x 10- . The errors quoted are internal, and arise mostly from observational scatter. The accuracy of these nebular abundances is now limited by the calculated atomic data. The NeIO ratio is probably the most accurate, and the CliAr, Cl/O the least. The external error imposed by the Cl cross sections is probably less than + 25%. If the three extreme halo planetaries are excluded, these ratios are constant among nebulae within the observational errors, so that the mean values have galactic significance. No variation in NeIO with galactic position is seen, except that the halo planetary Ha 4-1 is a factor of 10 deficient in Ne compared with 0. Comparison with other observations shows that these planetary abundance ratios are essentially solar. The observed ratios are in general agreement with the predictions of explosive nucleosynthesis; however, the measured Cl is a factor of 3-5 more abundant than the predicted values. The neon, argon, and chlorine abundances found in planetaries are apparently typical of the interstellar medium out of which the stars were formed. Since solar values for these elements are apparently not as well determined, the abundances presented here could be used to improve the compilations of "solar" or "local galactic" abundances. Subject headings: nebulae: abundances - nebulae: planetary