log(N/O) versus 12 + log(O/H) for PNe (black dots), and H ii regions (squares). The open circle corresponds to PN20, reclassified as a H ii region. In the left panel, stellar evolution models for different metallicities by Karakas (2010, K10; Z = 0.0001, blue diamonds; Z = 0.004, magenta triangles) and Fishlock et al. (2014, F14; Z = 0.001, brown stars) have been included. In each metallicity sequence initial masses (at the ZAMS) of the progenitor stars are labeled for some models. In the right panel stellar evolution models for Z = 0.001 (red diamonds) by Ventura et al. (2013, 2014b, V1314) and Z = 0.004 (blue triangles) by Ventura et al. (2014a, V14) have been included. Again initial masses (at the ZAMS) of the progenitor stars are labeled for some models.
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