Abstract: White dwarfs are the final state of most stars once nuclear burning in the core has finished, leaving a remnant that evolves through a straightforward cooling process that is largely thought to be well-understood. The electron-degenerate core of a white dwarf is an extreme environment that provides a testbed of physics in regimes not achievable with ground-based experiments. In this dissertation, observations of white dwarfs are used to test white dwarf cooling theory and look for evidence of novel physical phenomena. The cooling and kinematics of ultramassive white dwarfs in the solar neighbourhood are analysed using Gaia EDR3 observations to reinvestigate an anomalous cooling delay previously reported based on earlier Gaia DR2 observations, which challenged the conventional understanding of white dwarf cooling. The cooling of white dwarfs in the globular cluster 47 Tucanae is also analysed in detail by comparing cooling models to archival HST data to both test the implementation of element diffusion in stellar evolution simulations and determine the values of parameters important for modelling white dwarf cooling like the white dwarf mass and envelope thickness. A thorough understanding of these properties enables the cooling of white dwarfs to be used to indirectly search for evidence of novel particles such as axions. The emission of axions produced in the core of a white dwarf via axion bremsstrahlung would provide an additional energy loss mechanism and thus affect the cooling rate. A new bound on the axion-electron coupling of $g_{aee} \leq 0.81 \times 10^{-13}$ is derived from the cooling of white dwarfs in 47 Tucanae. This improves upon previous constraints by nearly a factor of two and excludes the range of values favoured by the hints of axions suggested by galactic white dwarf luminosity functions. Axions could also be produced in the core of a very hot, magnetic white dwarf like ZTF J1901+1458 via the Fe-57 transition and then convert to photons in the magnetosphere, and it is shown that X-ray observations of ZTF J1901+1458 by NuSTAR to search for the corresponding signal could constrain the product of an effective axion-nucleon coupling and axion-photon coupling at a level more stringent than both current and future planned ground-based observations.
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2024-08-01T13:00:002024-08-01T16:00:00White Dwarfs as Probes of Novel Physical PhenomenaEvent Information:
Abstract:White dwarfs are the final state of most stars once nuclear burning in the core has finished, leaving a remnant that evolves through a straightforward cooling process that is largely thought to be well-understood. The electron-degenerate core of a white dwarf is an extreme environment that provides a testbed of physics in regimes not achievable with ground-based experiments. In this dissertation, observations of white dwarfs are used to test white dwarf cooling theory and look for evidence of novel physical phenomena. The cooling and kinematics of ultramassive white dwarfs in the solar neighbourhood are analysed using Gaia EDR3 observations to reinvestigate an anomalous cooling delay previously reported based on earlier Gaia DR2 observations, which challenged the conventional understanding of white dwarf cooling. The cooling of white dwarfs in the globular cluster 47 Tucanae is also analysed in detail by comparing cooling models to archival HST data to both test the implementation of element diffusion in stellar evolution simulations and determine the values of parameters important for modelling white dwarf cooling like the white dwarf mass and envelope thickness. A thorough understanding of these properties enables the cooling of white dwarfs to be used to indirectly search for evidence of novel particles such as axions. The emission of axions produced in the core of a white dwarf via axion bremsstrahlung would provide an additional energy loss mechanism and thus affect the cooling rate. A new bound on the axion-electron coupling of $g_{aee} \leq 0.81 \times 10^{-13}$ is derived from the cooling of white dwarfs in 47 Tucanae. This improves upon previous constraints by nearly a factor of two and excludes the range of values favoured by the hints of axions suggested by galactic white dwarf luminosity functions. Axions could also be produced in the core of a very hot, magnetic white dwarf like ZTF J1901+1458 via the Fe-57 transition and then convert to photons in the magnetosphere, and it is shown that X-ray observations of ZTF J1901+1458 by NuSTAR to search for the corresponding signal couldconstrain the product of an effective axion-nucleon coupling and axion-photon coupling at a level more stringent than both current and future planned ground-based observations. Event Location:
Henn 309