The HST Large Program on ω Centauri. V. Exploring the Ultracool Dwarf Population with Stellar Atmosphere and Evolutionary Modeling

Abstract

Brown dwarfs can serve as both clocks and chemical tracers of the evolutionary history of the Milky Way due to their continuous cooling and high sensitivity of spectra to composition. We focus on brown dwarfs in globular clusters that host some of the oldest coeval populations in the galaxy. Currently, no brown dwarfs in globular clusters have been confirmed, but they are expected to be uncovered with advanced observational facilities such as the James Webb Space Telescope (JWST). In this paper we present a new set of stellar models specifically designed to investigate low-mass stars and brown dwarfs in ω Centauri—the largest known globular cluster. The parameters of our models were derived from iterative fits to Hubble Space Telescope photometry of the main-sequence members of the cluster. Despite the complex distribution of abundances and the presence of multiple main sequences in ω Centauri, we find that the modal color–magnitude distribution can be represented by a single stellar population with parameters determined in this study. The observed luminosity function is well represented by two distinct stellar populations having solar and enhanced helium mass fractions and a common initial mass function, in agreement with previous studies. Our analysis confirms that the abundances of individual chemical elements play a key role in determining the physical properties of low-mass cluster members. We use our models to draw predictions of brown dwarf colors and magnitudes in anticipated JWST NIRCam data, confirming that the beginning of the substellar sequence should be detected in ω Centauri in forthcoming observations.

Physical properties and trigonometric distance of the peculiar dwarf WISE J181005.5−101002.3

“…Figure 17 in Zhang et al 2017b). Metal-depleted low-mass stars, despite the higher temperatures caused by the dearth of metals in their atmospheres, are less luminous than their solarmetallicity counterparts due to their smaller radii (Baraffe et al 1997;Allard et al 2012;Gonzales et al 2021;Gerasimov et al 2022). As for low-metallicity brown dwarfs below the hydrogen burning-mass limit, there is no recent theoretical work in the literature we can rely on for the predictions of their sizes and temperatures, but an approximate observational stellar-substellar boundary is drawn in Figure 1 of Zhang et al (2017a).…”

Exploring the Chemistry and Mass Function of the Globular Cluster 47 Tucanae with New Theoretical Color–Magnitude Diagrams

“…In general, the photometric colors of low-mass stars are highly sensitive to the variations in element abundances due to the dominant molecular chemistry and opacity in low-temperature atmospheres (Marley et al 2002;Gerasimov et al 2022b). Furthermore, low-mass stars are particularly valuable as chemical tracers, since molecular absorption is less affected by the nonequilibrium radiation field (Johnson 1994), while the interiors of low-mass stars are fully mixed and undergo minimal nuclear processing (Chabrier & Baraffe 1997;Gerasimov et al 2022a). Sophisticated stellar models are required to take full advantage of this potential.…”

“…Considerable progress in the photometric analysis of cool stars near the end of the main sequence (0.1 M e ) in globular clusters has been made since the advent of space-based observations with the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST). Recent results include measurement of the [O/Fe] spread in the globular clusters NGC 6752 (Dotter et al 2015), M92 (Ziliotto et al 2023), and 47 Tucanae (Milone 2023); photometric characterization of individual populations in NGC 6752 (Milone et al 2019;Gerasimov et al 2022b); and measurement of α-enhancement in ω Centauri (Gerasimov et al 2022a).…”

Beyond the Local Volume. II. Population Scaleheights and Ages of Ultracool Dwarfs in Deep HST/WFC3 Parallel Fields

“…Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. young stellar associations (Stauffer et al 1998;Martín et al 2018) and potentially in older globular clusters and the Galaxy at large (Burgasser 2004;Caiazzo et al 2017;Gerasimov et al 2022). Their fully convective interiors and cool, molecule-rich photospheres allow for sensitive measurement of metallicity (Rojas-Ayala et al 2012;Veyette et al 2017;Zhang et al 2017), ideal for studies of chemical enrichment history.…”