Jaymie Mark Matthews

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Transiting super-Earth exoplanets : search and characterisation (2013)

The only way to measure the diameter of a planet outside the Solar System is if it transits its host star. Transit data combined with radial velocity (RV) data give an exoplanet's mass. Mass and volume yield density, so transits give unique insights into the interior structure of an exoplanet. Super-Earths are a class of exoplanet with masses a few times that of Earth, for which no examples exist in our Solar System. The goals of this thesis: to detect and characterise transits of exoplanets (particularly super-Earths) discovered via RV surveys orbiting bright stars. The observational tool: MOST (Microvariability & Oscillations of STars) - a microsatellite housing an optical telescope which can monitor stars with high precision. We searched 12 super-Earth candidate systems, applying special data reduction and analysis techniques. The search gives upper limits on the transit depths of 11 of the planets. For GJ 581e (innermost planet of an M dwarf system), the 1σ upper limit excludes most transiting configurations for a planet with water ice or H/He compositions. MOST data of HD 97658 rejected a claim of a transit detection in this system. The timely MOST rejection cancelled observatory programmes dedicated to follow up on the transit claim, preventing the waste of valuable time on major facilities. In parallel with the MOST survey, we analysed a decade of ground-based photometry to exclude most transiting configurations for the massive exoplanet HD 192263b. We showed that the star's rotation period does not coincide with the planet's orbital period, as was previously reported. We also find evidence for an 8-year activity cycle in the host star. The twelfth star in the MOST survey is 55 Cancri, whose innermost planet (55 Cnc e) has an orbital period of less than a day and transits the star. We present 42 days of new MOST photometry of this system, and derive one of the most precise radius values (1.99 ± 0.08 Earth radii) known for any super-Earth. We explore the possibility of star-exoplanet interaction, and set a limit on the albedo (reflectivity) of the planet, which has implications for its atmospheric composition.

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Asteroseismic tuning of the magnetic star HR 1217 : understanding magnetism and stellar structure through MOST spacebased photometry (2010)

The chemically peculiar A (Ap) stars show extreme examples of astrophysical processes that have only recently been studied in detail in one other star—the Sun. These stars exhibit spectral anomalies caused by diffusion of some ionic species in a stellar atmosphere threaded by a strong (~ kG), organized magnetic field. A subset of the Ap stars rapidly oscillate (roAp) with periods ranging from 5 to 25 minutes. One of these roAp stars, HR 1217, is well studied with data from two global (ground-based) photometric campaigns that led to asteroseismic evidence of magnetically perturbed oscillation modes. This was the motivation to make HR 1217 a MOST space mission target. Our analysis of the almost 30 days of near-continuous MOST photometry on HR 1217 reveals a number of new periodicities that show spacings of ~ 15, 2.5, and 1.5 μHz. These new frequencies can be interpreted as magnetically perturbed oscillations and potentially second order spacings that could constrain the age and the magnetic interior of the star for the first time. These data are collected with a 95% duty cycle and reach a precision of 6 μmag, making this by far the best photometric data set on HR 1217. In addition, we present a grid of almost 52,000 stellar pulsation models including a large range of magnetic dipole field strengths (1-10 kG). This is the largest grid of stellar pulsation models of any Ap star to date and is critical to the interpretation of the MOST photometry. Our models can match the MOST observations to a fractional accuracy of about 0.05% with a mean deviation between theory and observation of a few μHz. A unique model match to the MOST observations could not be found. The results highlight the sensitivity to physics that has not been usually incorporated in Ap interior models, and the complex nature of the interaction of globally organized magnetic fields with stellar pulsation eigenmodes.

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Master's Student Supervision (2010 - 2018)
Accreting white dwarfs : a theoretical analysis of nuclear burning (2013)

Accreting white dwarfs can exhibit a variety of thermonuclear phenomena, such as shell flashes, classical and recurrent novae, as well as Type Ia supernovae. To better understand these processes, we consider the accretion of hydrogen-rich material onto the surface of a white dwarf. Our analysis is based on a semi-analytical approach that allows the investigation of properties of nuclear burning on accreting white dwarfs. In particular, we determine steady-state solutions and evaluate the stability of these solutions. As a first step, we follow Paczyński's one-zone model and confirm his results by following his analysis independently. We extend the framework to a sophisticated multi-zone model encompassing a variety of detailed physics. We determine accretion rates that may lead to stable or to unstable burning. Regimes of stable burning may result in mass increase and potentially identify progenitors of Type Ia supernovae. Unstable burning may lead to nova-like outbursts. The identification of both burning regimes is important, as these thermonuclear events influence the chemical and dynamical evolution of the Universe.

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Accreting white dwarfs : a theoretical analysis of nuclear burning (2013)

Accreting white dwarfs can exhibit a variety of thermonuclear phenomena, such as shell flashes, classical and recurrent novae, as well as Type Ia supernovae. To better understand these processes, we consider the accretion of hydrogen-rich material onto the surface of a white dwarf. Our analysis is based on a semi-analytical approach that allows the investigation of properties of nuclear burning on accreting white dwarfs. In particular, we determine steady-state solutions and evaluate the stability of these solutions. As a first step, we follow Paczyński's one-zone model and confirm his results by following his analysis independently. We extend the framework to a sophisticated multi-zone model encompassing a variety of detailed physics. We determine accretion rates that may lead to stable or to unstable burning. Regimes of stable burning may result in mass increase and potentially identify progenitors of Type Ia supernovae. Unstable burning may lead to nova-like outbursts. The identification of both burning regimes is important, as these thermonuclear events influence the chemical and dynamical evolution of the Universe.

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