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Exploring the Ultracool Dwarf Population of ω Centauri with Stellar Atmosphere and Evolutionary Modelling

Presentation #331.04 in the session Brown Dwarfs.

Published onJun 29, 2022
Exploring the Ultracool Dwarf Population of ω Centauri with Stellar Atmosphere and Evolutionary Modelling

At least 1/6 of the local stellar population consists of brown dwarfs – substellar objects with masses insufficient for hydrogen burning in the core. As opposed to hydrogen-burning stars that maintain nearly constant temperature throughout their lifetimes, brown dwarfs cool continuously and their spectra evolve significantly with decreasing effective temperature. Brown dwarfs may therefore be used as clocks in studies of the Milky Way’s formation and evolution. The current sample of known brown dwarfs mostly consists of objects with near-solar metallicities that are not representative of the early metal-poor stages of Galactic evolution. On the other hand, globular clusters in the Milky Way are expected to provide an abundant reservoir of metal-poor brown dwarfs with ages tracing the earliest stages of Milky Way formation. Furthermore, the physical properties of brown dwarfs in globular clusters may be constrained from the properties of stellar cluster members. Due to their intrinsically faint magnitudes, no confirmed brown dwarfs in globular clusters have been detected so far. However, this situation is expected to change with the next generation of observational facilities most notably including James Webb Space Telescope (JWST). In the advent of first brown dwarf observations in globular clusters, we calculate new evolutionary and atmosphere models for the substellar population of the largest globular cluster in the Milky Way, ω Centauri. We fit our models to existing Hubble Space Telescope observations of the faint end of the cluster’s Main Sequence, and use these models to determine the expected colors and magnitudes of cluster brown dwarfs at wavelengths sampled by JWST. In agreement with previous studies, we find that the cluster is well-represented by two model populations with solar and enhanced helium mass fractions, drawn from the same two-component broken power law initial mass function. We calculate best-fit metal abundances for the modal trend of the cluster colour-magnitude diagram and obtain an upper limit on the fractional representation of the helium-rich population in ω Centauri. The magnitudes of the brightest brown dwarfs in the cluster are found to fall well within the sensitivity limit of JWST, with the detectable brown dwarf number density reaching as much as 50% of the Main Sequence star density within a 1 hr exposure.

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