The Physics of Crystallization from Globular Cluster White Dwarf Stars in NGC 6397

TL;DR

This study uses Hubble data of NGC 6397 to empirically confirm the first-order phase transition during white dwarf crystallization, aligning with theoretical predictions and providing insights into stellar interior physics.

Contribution

First empirical evidence supporting the first-order nature of white dwarf crystallization and demonstration of how observational data can constrain interior stellar parameters.

Findings

Data consistent with ion crystallization theory

Latent heat release observed during phase transition

Constraints on interior composition and phase separation

Abstract

We explore the physics of crystallization in the deep interiors of white dwarf stars using the color-magnitude diagram and luminosity function constructed from proper motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with the theory of crystallization of the ions in the interior of white dwarf stars and provide the first empirical evidence that the phase transition is first order: latent heat is released in the process of crystallization as predicted by van Horn (1968). We outline how this data can be used to observationally constrain the value of Gamma = E_{Coulomb}/E_{thermal} near the onset of crystallization, the central carbon/oxygen abundance, and the importance of phase separation.