Comparing the White Dwarf Cooling Sequences in 47 Tuc and NGC 6397

Abstract

Using deep Hubble Space Telescope imaging, color-magnitude diagrams are constructed for the globular clusters 47 Tuc and NGC 6397. As expected, because of its lower metal abundance, the main sequence of NGC 6397 lies well to the blue of that of 47 Tuc. A comparison of the white dwarf cooling sequences of the two clusters, however, demonstrates that these sequences are indistinguishable over most of their loci - a consequence of the settling out of heavy elements in the dense white dwarf atmosphere and the near equality of their masses. Lower quality data on M4 continues this trend to a third cluster whose metallicity is intermediate between these two. While the path of the white dwarfs in the color-magnitude diagram is nearly identical in 47 Tuc and NGC 6397, the numbers of white dwarfs along the path are not. This results from the relatively rapid relaxation in NGC 6397 compared to 47 Tuc and provides a cautionary note that simply counting objects in star clusters in random locations as a method of testing stellar evolutionary theory is likely dangerous unless dynamical considerations are included.

7 pages, 4 figures, accepted for publication in the Astrophysical Journal

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Cited by in corpus (17)

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• A stellar-mass black hole population in the globular cluster NGC 6101?
• White Dwarf Subsystems in Core-Collapsed Globular Clusters
• A 1.05 $M_{\odot }$ Companion to PSR J2222-0137: The Coolest Known White Dwarf?
• A comparative analysis of the observed white dwarf cooling sequence from globular clusters
• The white dwarf population of NGC 6397
• Deep HST Imaging in 47 Tucanae: A Global Dynamical Model
• Slowly cooling white dwarfs in M13 from stable hydrogen burning
• A Measurement of Diffusion in 47 Tucanae
• Horizontal branch morphology: A new photometric parametrization
• The HST Large Programme on NGC 6752. III. Detection of the Peak of the White Dwarf Luminosity Function
• Far-Ultraviolet Observation of the Globular Cluster NGC 6397
• Probing Mass Segregation in NGC 6397
• Impact of distance determinations on Galactic structure. II. Old tracers
• Evolution of long-lived globular cluster stars IV. Initial helium content and white-dwarf properties
• The Onset of Convective Coupling and Freezing in the White Dwarfs of 47 Tucanae