Crystallization of white dwarfs in globular clusters

White dwarf stars are the most common end point of stellar evolution. Due to their large numbers and multiple applications, white dwarfs are among the most interesting objects to study in the universe. Based on the observations provided by the Gaia Space Mission, studies of these objects showed that a small fraction of the ultra-massive white dwarfs undergo a substantial delay in their cooling times. To explain the delay, additional energy sources inside the white dwarf have been considered. Neon 22 sedimentation and crystallization are the most important sources. Considering these two extra energy sources and high metallicity, it was possible to explain the delay. In this framework, we aimed to analyze the effect of crystallization and Neon 22 sedimentation on white dwarfs, especially in ultra-massive white dwarfs. To do this, we generated a wide sample of synthetic globular clusters with different physical properties using Monte Carlo techniques and an up-to-date set of white dwarf cooling tracks. These synthetic stellar populations were analyzed using Hertzsprung-Russell diagrams, ς distributions, and a new quantity introduced in this text, the ultra-massive quotient. The extensive analysis showed that younger and metal-richer clusters present higher ultra-massive quotients and ς histograms centered on lower values. Moreover, Hertzsprung-Russell diagrams prove that a high metallicity and a carbon-oxygen core chemical composition abruptly increase the delay time undergone by the white dwarfs due to Neon 22 sedimentation. In addition, we found that only a stellar population with ultra-massive carbon-oxygen core white dwarfs counts with a significant number of white dwarfs in the ultra-massive region. These findings allow us to compare our simulations with real observed clusters (NGC 6397, NGC 6791, and 47 Tucanae). The comparison shows that the ultra-massive white dwarfs are located around the same values of ς for synthetic and observed clusters. Additionally, we could predict, thanks to the ultra-massive quotient, which clusters have the highest percentage of ultra-massive white dwarfs visible in their color-magnitude diagrams.

• Autores: Castro Idarraga, Juan Pablo
• Tipo de recurso: Trabajo de grado de pregrado
• Fecha de publicación: 2024
• Institución: Universidad de los Andes
• Repositorio: Séneca: repositorio Uniandes
• Idioma: eng
• OAI Identifier: oai:repositorio.uniandes.edu.co:1992/74727
• Acceso en línea: https://hdl.handle.net/1992/74727
• Palabra clave: White dwarfs; Astrophysics; Crystallization; Globular clusters; Física
• Rights: openAccess
• License: Attribution-ShareAlike 4.0 International