The Onset of Convective Coupling and Freezing in the White Dwarfs of 47 Tucanae

TL;DR

This study measures white dwarf cooling in 47 Tucanae using Hubble data, highlighting the effects of convective coupling and crystallisation on their luminosity and matching observed data with models.

Contribution

It provides the first detailed analysis of convective coupling and crystallisation effects on white dwarf cooling in a globular cluster, validated by observational data.

Findings

Convective coupling occurs at similar times as crystallisation in 47 Tuc white dwarfs.

Latent heat from crystallisation has a minor impact on cooling rates.

Convective coupling significantly increases white dwarf brightness, aligning models with observations.

Abstract

Using images from the Hubble Space Telescope Advanced Camera for Surveys, we measure the rate of cooling of white dwarfs in the globular cluster 47 Tucanae and compare it to modelled cooling curves. We examine the effects of the outer convective envelope reaching the nearly isothermal degenerate core and the release of latent heat during core crystallisation on the white dwarf cooling rates. For white dwarfs typical of 47 Tuc, the onset of these effects occur at similar times. The latent heat released during crystallisation is a small heat source. In contrast, the heat reservoir of the degenerate core is substantially larger. When the convective envelope reaches the nearly isothermal interior of the white dwarf, the star becomes brighter than it would be in the absence of this effect. Our modelled cooling curves that include this convective coupling closely match the observed luminosity function of the white dwarfs in 47 Tuc.