JWST Imaging of the Closest Globular Clusters -- II. Discovery of Brown Dwarfs in NGC 6397 and Measurement of Age from the Brown Dwarf Cooling Sequence, using SANDee - a New Grid of Model Isochrones across the Hydrogen-Burning Limit (2405.01634v2)

Abstract: Globular clusters contain vast repositories of metal-poor stars that represent some of the oldest stellar generations in the Universe. The archaeological footprint of early Galactic evolution may be retained in the measurable properties of globular clusters, such as their ages, mass functions and chemical abundances. Until recently, all photometric studies of globular clusters were restricted to stellar members. Now, the sensitivity of JWST can extend this analysis to the substellar regime. If detected in sufficient numbers, brown dwarf members can provide tight constraints on the properties of their parent population. We present SANDee - a new grid of stellar models that accurately represent the color-magnitude diagrams of globular clusters across the hydrogen-burning limit at a wide range of metallicities. Using JWST NIRCam photometry and the new models, we identify three brown dwarfs in the globular cluster NGC 6397 with effective temperatures of 1300-1800 K, confirmed by both proper motion and model fitting. We use the observed luminosities of discovered brown dwarfs to obtain the first age estimate of a globular cluster from its substellar cooling sequence: 13.4 +/- 3.3 Gyr. We also derive the local mass function of the cluster across the hydrogen-burning limit and find it to be top-heavy, suggesting extensive dynamical evolution. We expect that the constraints on both age and mass function of NGC 6397 derived in this work can be greatly improved by a second epoch of NIRCam imaging in the same field.

• The paper identifies three brown dwarfs within the globular cluster NGC 6397 using JWST NIRCam photometry and the newly developed SANDee model grid.
• The study measures the age of NGC 6397 at 13.4 3.3 Gyr by analyzing the substellar cooling sequence of the discovered brown dwarfs.
• Analysis of the present-day mass function near the hydrogen-burning limit reveals a top-heavy distribution, indicating extensive dynamical evolution within the cluster.

Discovery of Brown Dwarfs in Globular Cluster NGC 6397 Using JWST Imaging

This paper presents significant advancements in our understanding of globular clusters through the discovery of brown dwarfs in NGC 6397 using the James Webb Space Telescope (JWST). By leveraging the enhanced sensitivity of the JWST, the paper extends photometric analysis into the substellar regime, significantly enriching our understanding of such ancient celestial systems.

Key Findings

• Identification of Brown Dwarfs: The paper reports the identification of three brown dwarfs within the globular cluster NGC 6397. This was achieved using JWST's NIRCam photometry and a newly developed grid of model isochrones across the hydrogen-burning limit (SANDee). Brown dwarfs were confirmed through proper motion studies and matched with theoretical models, revealing effective temperatures ranging from approximately 1300 K to 1800 K.
• Substellar Cooling Sequence and Cluster Age Measurement: The paper represents a pioneering effort to infer the age of a globular cluster from its substellar cooling sequence. By analyzing the luminosities of the detected brown dwarfs, an age estimate of 13.4 ± 3.3 Gyr for NGC 6397 was obtained. This finding has important implications for the dating of globular clusters, which are typically estimated using main sequence turnoff and post-main sequence stars.
• Mass Function Analysis: The present-day local mass function of the cluster near the hydrogen-burning limit was determined to be top-heavy. This suggests extensive dynamical evolution within the cluster, providing insights into its formation and persistence over billions of years.

Theoretical and Practical Implications

• Extending the Study of Stellar Populations: This work emphasizes the potential of JWST in advancing studies of globular clusters, particularly by uncovering substellar populations. The ability to detect and analyze brown dwarfs opens new avenues for assessing the primordial mass functions and age estimates of ancient stellar assemblies.
• Evolutionary Models: The introduction of SANDee models allows for an improved understanding of color-magnitude diagrams across the hydrogen-burning limit, accounting for varied metallicities and chemical compositions. This is crucial for accurate representation and predictive modeling of stellar evolution in metal-poor environments.
• Photometric Analysis: The sensitivity improvements in JWST pave the way for more detailed photometric studies, offering insights into previously inaccessible spectral data. This can lead to refined astrophysical models and potentially resolve discrepancies in stellar atmosphere predictions, particularly for low-temperature stars.

Speculation on Future Developments

The paper suggests a promising future for astrophysics, especially in the context of using advanced telescopes like JWST. Further epochs of observation and larger samples of brown dwarfs could refine age estimates and enhance our understanding of the initial mass function. Improved completeness in photometric data and the extension of models to include varied elemental abundances can also mitigate current systematic limitations in cluster analysis.

Conclusion

This paper substantially contributes to the field of astrophysics by introducing a novel approach to analyzing globular clusters through the paper of substellar objects. The robust methodology and results present a significant leap forward in characterizing these ancient structures, offering invaluable parameters necessary for unraveling the history and evolution of the Milky Way and its stellar constituents. As future observations gather more data, we anticipate a deeper comprehension of the fundamental processes governing the formation and development of globular clusters.