α-element/O abundance ratios for our sample of PNe. Stars indicate abundances computed using Te([Ar iv]) for ions with IP > 39 eV, while red circles represent abundances computed using Te([O iii]) (see discussion in text). The solar symbol indicates the solar abundances taken from the compilation by Lodders (2010). The blue square in the log(S/O) plot is the value obtained for the Orion nebula from CELs. The y-axis ranges over 1.2 dex.
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