Values of the density indicator [S ii] λ6731/6717 as a function of [O ii] λ3726/3729. Points with error bars: observed values in the sample of PNe by Wang et al. (2004). Continuous lines: lines of equal electron density at a temperature of 104 K from ne = 20 cm-3 (upper right) to ne = 105 cm-3 (lower left). Red: using the [S ii] transition probabilities by Tayal & Zatsarinny (2010). Blue: using the [S ii] transition probabilities from Mendoza & Zeippen (1982).
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