On the nature of a secondary main-sequence turn-off in the rich LMC cluster NGC 1868

Evidence for a second main-sequence turn-off in a deep colour-magnitude diagram (CMD) of NGC 1868 is presented. The data were obtained with Hubble Space Telescope (HST) WFPC2 and reach down to m(555) similar or equal to 25 mag. Besides the usual tau similar or equal to 0.8 Gyr turn-off found in previous analyses, another possible turn-off is seen at m(555) similar or equal to 21 mag (M (V) similar or equal to 2.5), which is consistent with an age of tau similar or equal to 3 Gyr. This CMD feature stands out clearly especially when contaminating field Large Magellanic Cloud (LMC) stars are statistically removed. The background subtracted CMD also visibly displays a red subgiant branch extending about 1.5 mag below the younger turn-off and the clump of red giants. The significance of the secondary turn-off in NGC 1868 was confirmed with Monte Carlo simulations and bootstrapping techniques. Star counts in selected regions in the cluster CMD indicate a mass ratio of old population/young population in the range 5 less than or similar toM(old) M-young less than or similar to 12 per cent, depending on the mass-function slope. The existence of such a subpopulation in NGC 1868 is significant even in the presence of uncertainties in background subtraction. The possibility that the secondary turn-off is associated with the field star population was examined by searching for similar features in CMDs of field stars. Statistically significant excesses of stars redwards of the main sequence were found in all such fields in the range 20 less than or similar tom(555)less than or similar to 22 mag. These however are much broader features that do not resemble the main-sequence termination of a single population. We also discuss other alternative explanations for the feature at m(555)similar or equal to 21 mag, such as unresolved binarism, peculiar stars or CMD discontinuities associated with the Bohm-Vitense gap.