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On the nature of a secondary main sequence turn-off in the rich LMC cluster NGC 1868 | | B. Santiago
; L. Kerber
; R. Castro
; R. de Grijs
; | | Date: |
8 Jul 2002 | | Journal: | Mon.Not.Roy.Astron.Soc. 336 (2002) 139 | | Subject: | astro-ph | | Affiliation: | Univ. Federal do Rio Grande do Sul, Porto Alegre, Brasil, Institute of Astronomy, Cambridge, UK | | Abstract: | Evidence for a second main-sequence turn-off in a deep colour-magnitude diagram of NGC 1868 is presented. The data were obtained with HST/WFPC2 and reach down to m_{555} ~ 25. Besides the usual t ~ 0.8 Gyr turn-off found in previous analyses, another possible turn-off is seen at m_{555} ~ 21 (M_V ~ 2.5), which is consistent with an age of t ~ 3 Gyrs. This CMD feature stands out clearly especially when contaminating field 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% <= M_{old} / M_{young} <= 12%, 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 <= m_{555} <= 22. 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} ~ 21, such as unresolved binarism, peculiar stars or CMD discontinuities associated with the Bohm-Vitense gap. | | Source: | arXiv, astro-ph/0207172 | | Services: | Forum | Review | PDF | Favorites | |
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