Solving the Ubiquitous Problem of Stellar Radii
Date: 23 March 2020
University of Exeter
Doctor of Philosophy in Physics
This thesis will address the problem of measuring stellar radii, which is ubiquitous across many fields of modern astrophysics. A technique is introduced which integrates the area beneath the stellar spectral energy distribution (SED) of a star to measure its luminosity, and the shape of the SED to measure its temperature - from which ...
This thesis will address the problem of measuring stellar radii, which is ubiquitous across many fields of modern astrophysics. A technique is introduced which integrates the area beneath the stellar spectral energy distribution (SED) of a star to measure its luminosity, and the shape of the SED to measure its temperature - from which follows its radius. This method addresses many of the problems facing of existing methods, which are reviewed, as it provides accurate measurements of stellar radius using only multiband photometry and precision parallaxes. It is well known that the radii and temperatures of M-dwarf prescribed by models are in disagreement with observations, both on the pre-main-sequence (pre-MS) and the main-sequence (MS). This methodology is applied to pre-MS M-dwarfs in the Pleiades and Praesepe clusters to perform a direct comparison to the radii predicted by stellar interiors. Assessment of the physicality and accuracy of the stellar atmosphere models is also performed by comparing synthetic spectra generated from them to flux--calibrated spectroscopic observations. The parameters for the synthetic spectra are provided by the SED fitting, allowing verification of the methodology itself to be performed. The advent of Gaia DR2 means that reliable distances are now available for field M-dwarfs, permitting the extension of this investigation to MS stars. Through this investigation, the nature of radius inflation in MS M-dwarfs is studied as a function of mass. This crucially allows insight into the physics behind the observed radius inflation, allowing current theories underpinning radius inflation to be critically assessed. The conclusion of this investigation is that magnetic models are currently unable to explain radius inflation in M-dwarfs. Given the successful application of the SED fitting methodology in measuring the stellar radii of miscellaneous field stars, this work is built upon to address the problem of determining the stellar parameters of exoplanet host radii. In doing so, it is demonstrated that the SED fitting technique extends well to the mass range of stars currently being scrutinised to discover and characterise exoplanets. Given its wide applicability for exoplanet host characterisation, the potential systematic errors that may prove problematic are reviewed and methods for their mitigation are suggested.
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