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5.4. Neon, Sulfur and Argon

Neon, sulfur, and argon are products of the late stages of massive star evolution. 20Ne results from carbon burning, while S and Ar are products of O burning. As they are all considered part of the alpha-element group, their abundances are expected to track O/H closely.

Neon abundances in extragalactic H II regions are derived primarily from optical measurements of [Ne III], although spacecraft measurements of the IR [Ne II] and [Ne III] fine-structure lines are becoming available. A representative sample of Ne/O values for H II regions with measured Te in spiral and irregular galaxies is shown in Figure 19. The scatter increases for the H II regions with higher O/H because of the more uncertain electron temperatures. It is apparent that the Ne abundance tracks O quite closely, in agreement with results from planetary nebulae (Henry 1989).

Figure 19

Figure 19. Ne/O abundance ratios in spiral and irregular galaxies. Symbols are the same as in Figure 18.

Figure 20 shows data for sulfur and argon. For log O/H < -3.5, S/O and Ar/O are essentially constant with O/H, and fall within the range predicted by WW93. For log O/H > -3.5, however, there is evidence for declining S/O and Ar/O as O/H increases. The cause of the decline is not clear. It is possible that the ionization corrections for unseen S+3 have been underestimated in the more metal-rich H II regions. More observational study, especially IR spectroscopy, of metal-rich H II regions is needed, to rule out ionization or excitation effects.

Figure 20

Figure 20. Top: S/O abundance ratios in spiral and irregular galaxies. Bottom: Ar/O abundance ratios in spiral and irregular galaxies. Symbols are the same as in Figure 18.

Because S and Ar are produced close to the stellar core, the yields of S and Ar may be sensitive to conditions immediately prior to and during the supernova explosion, such as explosive processing or fall-back onto the compact remnant (WW93). If real, however, the declining S/O and Ar/O cannot be accounted for by simple variations in the stellar mass function and hydrostatic nucleosynthesis (Garnett 1989); some variation in massive star and/or supernova nucleosynthesis at high metallicities (perhaps due to strong stellar mass loss) may be needed.

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