Evidence for metallicity spreads in three massive M 31 globular clusters

Aims. We quantify the intrinsic width of the red giant branches of three massive globular clusters in M 31 in a search for metallicity spreads within these objects. Methods. We present HST/ACS observations of three massive clusters in M 31, G78, G213, and G280. A thorough description of the photometry extraction and calibration is presented. After derivation of the color-magnitude diagrams, we quantify the intrinsic width of the red giant branch of each cluster. Results. This width translates into a metallicity dispersion that indicates a complex star formation history for this type of system. For G78, σ[Fe/H] = 0.86 ± 0.37; for G213, 0.89 ± 0.20; and for G280, 1.03 ± 0.26. We find that the metallicity dispersion of the clusters does not scale with mean metallicity. We also find no trend with the cluster mass. We discuss some possible formation scenarios that would explain our results.