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We obtain a magnitude-limited sample of Andromeda (M 31) disc PNe withchemical abundance estimated through the direct detection of the [OIII] 4363$\mathring{\mathrm A}$ line. This leads to $205$ and $200$ PNe with oxygen andargon abundances respectively. We find that high- and low-extinction M 31 discPNe have statistically distinct argon and oxygen abundance distributions. Inthe radial range $2-30$ kpc, the older low-extinction disc PNe are metal-pooreron average with a slightly positive radial oxygen abundance gradient ($0.006\pm 0.003$ dex/kpc) and slightly negative for argon ($-0.005 \pm 0.003$dex/kpc), while the younger high-extinction disc PNe are metal-richer onaverage with steeper radial abundance gradients for both oxygen ($-0.013 \pm0.006$ dex/kpc) and argon ($-0.018 \pm 0.006$ dex/kpc), similar to thegradients computed for the M 31 HII regions. The M 31 disc abundance gradientsare consistent with values computed from major merger simulations, with themajority of the low-extinction PNe being the older pre-merger disc stars in thethicker disc, and the majority of the high-extinction PNe being younger starsin the thin disc, formed during and after the merger event. The chemicalabundance of the M 31 thicker disc has been radially homogenized because of themajor merger. Accounting for disc scale-lengths, the positive radial oxygenabundance gradient of the M 31 thicker disc is in sharp contrast to thenegative one of the MW thick disc. However, the thin discs of the MW and M 31have remarkably similar negative oxygen abundance gradients.

The survey of planetary nebulae in Andromeda (M31). IV. Radial oxygen and argon abundance gradients of the thin and thicker disc