The coeval and co-evolving members of globular clusters serve as stellar astrophysics laboratories, as otherwise inaccessible parameters such as age and composition are often restricted within the cluster and may be inferred from its colour-magnitude diagrams. Until recently, the typically large distances of globular clusters confined such studies to their most massive members. Now, the promise of highly sensitive ground- and space-based facilities will extend the reach of these studies into the substellar regime. We are investigating the elemental abundances of stars in one of the closest and most well-studied globular clusters, ω Centauri, using a new grid of synthetic spectra and colour-magnitude diagrams produced with a custom combination of ATLAS and PHOENIX 15 model atmospheres and the MESA evolutionary code (within the MIST framework). To our knowledge, these new models yield the best agreement with HST photometry, and emphasize the importance of atmosphere-interior coupling at low effective temperatures. We find that α-elements enhancement in the abundances of ultracool dwarfs plays a key role in determining their observable characteristics. Our models provide greater consistency in the colour-magnitude diagram between the most massive and least massive stars on the main sequence, although we find that the best-fit metallicity for the ultracool dwarfs is somewhat higher. Comprehensive treatment of molecules and clouds by the Settling formalism of the PHOENIX code enables us to assess the effects of gravity and composition on the formation of cloud layers. This allows us to extend model isochrones beyond the limit of existing photometric data. We make quantitative predictions for the colours and magnitudes of ω Centauri substellar population, which will inform observing strategies for future studies with JWST, TMT, GMT and ELT.

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