12 + log (Ne/H) vs. 12 + log (O/H), for PNe (in black) in NGC 6822. In red the H ii regions. The linear fit (including only PNe data) corresponds to 12 + log (Ne/H) = (0.925 ± 0.071) × (12 + log (O/H)) − (0.176 ± 0.566), with r = 0.88. 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 = 0.0001, green squares) have been included. See text for discussion.
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