Infrared and optical study of sulfur and neon abundances in planetary nebulae

Forbidden emission lines in the optical, one-micron, and mid-infrared spectral regions have been observed in a large sample of planetary nebulae, in order to measure abundances of all the predominant ions of sulfur and neon. The fine-structure lines [SIV] 10.5 μm and [NeII] 12.8 μm were measured with a high-resolution infrared grating spectrometer, and [NeIII] 3969 A, Hβ 4861 A, [SII] 6717 + 6731 A, [SIII] 9531 A, and P delta 10049 A were observed with a photoelectric scanner. The hydrogen emission was measured primarily from P delta, which is less affected by interstellar extinction or line-blending than Hβ or Hα. The abundances derived here are more accurate than those previously available, since all of the main ions are measured directly, and the observed lines are relatively insensitive to uncertainties in the electron density, electron temperature, and extinction. This study has shown that sulfur abundances do not vary by more than 40% among planetary nebulae of widely varying positions and ages, nor between planetary nebulae and the present interstellar medium. This result is in contrast to evidence found by many authors that the abundance of oxygen in HII regions and planetary nebulae varies with galactic position, and that iron abundances in stars of the galactic disk also vary with galactic radius. Recent nucleosynthesis calculations suggest that sulfur may be produced by stars more massive than those which synthesize both oxygen and iron. It is possible, therefore, that sulfur was enriched more rapidly than oxygen or iron in the evolution of the Galaxy, by an early generation of massive stars