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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 ii and iii are apparently about 4 dex overabundant. In contrast, evidence for the presence of other rare earths is difficult to find in the spectrum. It appears that Ce iii, Pr iii, Nd ii and iii, and Eu ii are detected with inferred overabundances ranging between 1.5 and 5 dex, but for most of these elements, the abundance of the non-detected ionization state is significantly lower than that inferred for the detected state. HD 66318 thus seems to exhibit strong discrepancies between abundances deduced for different ionization states of rare earths, a phenomenon so far found only in somewhat cooler stars.  Even within one ionization state it has not been found possible to fit most observed lines with a single value of abundance. For example, when we fit medium strength lines of Fe ii, the calculated strong lines of this ion are deeper than observed, and the calculated weak lines are less deep than observed. This situation is probably due to strong vertical abundance stratification of most of the elements studied. Finally, HD 66318 also shows a new form of core-wing anomaly in H α in which the observed line profile falls below the computed one in the core. These characteristics clearly suggest that the atmospheric structure of HD 66318 is not closely similar to that of a normal main sequence A star of similar parameters.

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