Oxygen gradient obtained from ORLs for our sample of PNe (blue dots) and from deep spectrophotometric data of H ii regions (red triangles, data taken from Esteban et al. 2005, 2013). Lines are least-squares fits to the data. The black dashed line represents the results of a chemical evolution model by Romano et al. (2010). Clearly, for PNe, the O gradient is flatter and, at the solar Galactocentric distance the O abundance is larger for PNe than for H ii regions (see text).
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