Upper panel: N/O abundance ratio plotted against He abundance for our PN sample. The top-right box marks the criteria given by Peimbert (1990) to define Type I PNe. In green we included the data of the Galactic PNe sample by Henry et al. (2004). Predictions of stellar evolution models by Karakas (2010) for Z = 0.02 and different initial masses (indicated in units of M⊙) are also included. The solar symbol represents the solar values. Lower panel: relationship between N/O abundance ratio and the total N abundance; the fit to our data is shown as a dashed line (see text). The predictions of stellar evolution models by Karakas (2010) for stars with different initial masses and for two different metallicities are included.
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