2015ApJ...807..181C

We present spectroscopy of nine planetary nebulae (PNe) in the outskirts of M31, all but one obtained with the 10.4 m GTC telescope. These sources extend our previous study of the oxygen abundance gradient of M31 to galactocentric radii as large as 100 kpc. None of the targets are bona fide members of a classical, metal-poor, and ancient halo. Two of the outermost PNe have solar oxygen abundances, as well as radial velocities consistent with the kinematics of the extended disk of M31. The other PNe have a slightly lower oxygen content ([O/H]) and in some cases large deviations from the disk kinematics. These PNe support the current view that the external regions of M31 are the result of a complex interaction and merger process, with evidence for a widespread population of solar-metallicity stars produced in a starburst that occurred ∼2 Gyr ago.