JWST Imaging of the Closest Globular Clusters -- V. The White Dwarfs Cooling Sequence of M4 (2501.10070v1)

Abstract: We combine infrared (IR) observations collected by the James Webb Space Telescope with optical deep images by the Hubble Space Telescope taken approximately 20 years earlier to compute proper-motion membership for the globular cluster (GC) M 4 (NGC 6121) along its entire white dwarf (WD) cooling sequence (CS). These new IR observations allow us, for only the second time in a GC, to compare WD models with observations over a wide range of wavelengths, constraining fundamental astrophysical properties of WDs. Furthermore, we investigate the presence of WDs with IR excess along the WD CS of M 4, similar to the recent study conducted on the GC NGC 6397. We also determine the age difference between M 4 and NGC 6397 by comparing the absolute F150W2 magnitudes of the luminosity function peak at the bottom of the observed WD CS, and find that M 4 is slightly younger, by 0.8+/-0.5 Gyr.

• The paper demonstrates that JWST infrared data reveals a broader dispersion in M4’s white dwarf cooling sequence, suggesting infrared excess possibly linked to circumstellar material or substellar companions.
• It employs high-precision astrometry over an 18.66-year baseline and rigorous photometric calibration against Gaia DR3 to accurately identify faint cluster members.
• The study refines age estimates by concluding that M4 is marginally younger than NGC 6397, with a best-fit age of approximately 12.2 Gyr, thereby enhancing stellar evolution models.

JWST Observations of White Dwarfs in M4: An Analytical Perspective

The paper on the globular cluster M4, formally known as NGC 6121, embarks on a rigorous analysis using the James Webb Space Telescope (JWST) to explore the white dwarf (WD) cooling sequence. This research provides critical insights into the astrophysical characteristics of these compact objects, leveraging the exceptional capabilities of JWST to measure stellar properties in infrared wavelengths. The paper is particularly significant as it compares the findings with earlier Hubble Space Telescope (HST) data, allowing for precise proper-motion measurements which are pivotal in identifying cluster membership. Herein, we examine various facets of this comprehensive paper.

Methodological Overview

The paper utilizes infrared data from the JWST, combined with historical optical data from HST, to determine the proper-motion membership of stars within M4’s WD cooling sequence. The advantage of using IR observations lies in their reduced susceptibility to extinction, which is crucial given M4’s complex foreground absorption. This paper marks only the second instance of utilizing infrared wavelengths in a globular cluster to examine WD models against observational data, following the exploration of NGC 6397.

Key Findings

1. Cool White Dwarf Analysis: A critical outcome of this research is the infrared color-magnitude diagram (CMD) analysis, revealing a broader distribution among the faint WDs in comparison to what is predicted by photometric errors. This breadth might indicate the presence of WDs with infrared excess, potentially associated with circumstellar disks or substellar companions, echoing findings from NGC 6397.
2. Astrometry and Photometric Calibration: The paper underscores the enhanced astrometric precision offered by JWST over a substantial time baseline of 18.66 years, reinforced by specific calibration processes against Gaia DR3 catalog. This precision enabled the identification of faint stellar objects that would otherwise blend into the cosmic background.
3. Age Determination: Based on the absolute F150W2 magnitudes and the luminosity function peak, it concludes that M4 is marginally younger than NGC 6397 by an estimated 0.8 ± 0.5 Gyr. This result suggests a best-fit age of M4 at approximately 12.2 Gyr, aligning with previous analyses but offering improved precision through the incorporation of multi-wavelength data.

Practical and Theoretical Implications

The findings have far-reaching implications for our understanding of stellar evolution and cosmology. The potential presence of WDs with IR excess invites further investigation into the conditions supportive of planet formation within ancient star clusters. Moreover, the alignment (or discrepancy) between observed WD characteristics and model predictions offers a robust testbed for refining stellar evolution models.

From a practical standpoint, this research sets a precedent for upcoming studies, particularly in its methodological synthesis of archival data with modern observations, which can be replicated across different stellar environments.

Future Prospects

The paper advocates for extended observations with additional JWST filters such as F444W and MIRI 10 μm, emphasizing the necessity for high-resolution spectral data to elucidate the nature of the observed infrared excess. These future endeavors are expected to provide unambiguous evidence for, or against, the presence of substellar companions or circumstellar materials around WDs in globular clusters.

In conclusion, this paper significantly enhances our comprehension of WD populations in globular clusters while highlighting nuanced behaviors at infrared wavelengths, paving the path for comprehensive studies into the life cycles of stellar populations in ancient galactic repositories.