Discovery of a 14.5 kG magnetic field in the NGC 2516 star HD 66318

We have been searching for magnetic Ap stars in open clusters, in order to clarify the time evolution of magnetic fields in middle main sequence stars from the ZAMS to the TAMS. We have discovered that the star HD 66318 in the open cluster NGC 2516 has an extraordinarily large magnetic field: the measured mean longitudinal component $B_{\rm l} \approx 4.5$ kG, and the mean field modulus $B_{\rm s} \approx 14.5$ kG. This star thus has one of the largest fields so far discovered in a non-degenerate star, and the largest field known in a current Ap star cluster member. We estimate that HD 66318 has completed about $16 \pm 5$% of its main sequence life. It thus appears to contradict the hypothesis of Hubrig et al. that magnetic fields are only found in stars that have completed at least 30% of their main sequence lifetimes. There is no indication that the spectrum or brightness of the star is variable, and the spectral lines are very sharp. The star probably has a very long rotation period (years). We have modelled some parts of the observed spectrum, assuming that the chemical composition is uniform both horizontally and vertically, and using a simple multipolar expansion for the field structure; although our model does not reproduce exactly the observed spectrum, it is clear that the atmospheric chemical composition of the star is very peculiar, with Ti, Cr and Fe overabundant by between 1.5 and 2.5 dex. Both La i