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

Abstract

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 m555 -~ 25 mag. Besides the usual τ 0.8 Gyr turn-off found in previous analyses, another possible turn-off is seen at m555 - ~ 21 mag (MV - ~ 2.5), which is consistent with an age of τ - ~ 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 inNGC1868was 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 < ~ Mold/Myoung < ~ 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 < ~ m555 < ~ 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 m555 - ~ 21 mag, such as unresolved binarism, peculiar stars or CMD discontinuities associated with the Böhm–Vitense gap.