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Origin of Infrared Excess in a Subset of NGC 6397 White Dwarfs

Determine the physical mechanism responsible for the excess flux observed in JWST/NIRCam F322W2 for approximately 25% of NGC 6397 white dwarfs with mF150W2 > 25.5, which produces up to ~0.5 mag redward offsets in F150W2−F322W2 color relative to hydrogen-atmosphere white dwarf isochrones, and establish a clear explanation for this excess.

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Background

The authors present JWST/NIRCam observations of NGC 6397 and identify an apparent split at the faint end of the white dwarf cooling sequence in the F150W2−F322W2 versus F322W2 color–magnitude diagram. A subset of white dwarfs displays significant infrared excess in the F322W2 filter, appearing up to 0.5 mag redder than expected relative to DA white dwarf cooling models.

They explore several possible explanations—including WD+brown dwarf binaries, circumstellar or post-AGB dust, helium-dominated atmospheres, helium-core white dwarfs, and potential associations with multiple stellar populations—but conclude none provides a clear, consistent account of the observed excess. They explicitly state that they have not found a clear explanation, indicating that the physical origin of the excess remains unresolved.

References

We find hints of a distinct population of WDs showing excess flux in F322W, comprising roughly 25% of sources mF150W2 > 25.5. While this putative feature deserves further confirmation, this excess is limited to F322W2, and broadens toward fainter magnitudes. We attempted several explanations and scenarios for this excess, including a large population of WD+BD binaries, reddening from circumstellar debris disks or post- AGB material, contributions from helium-dominated atmosphere and helium-core WDs, and a potential tie to the two populations present in the stellar Main Sequence of NGC 6397. We have not found any clear explanation for this excess.

— JWST Imaging of the Closest Globular Clusters -- I. Possible Infrared Excess Among White Dwarfs in NGC 6397 (2405.01631 - Bedin et al., 2 May 2024) in Section 5, Conclusions (main findings bullet 5)