In the present work, I combine observations of pulsating white dwarf stars with theoretical models of these stars to constrain the mass of axions and the emission rate of plasmon neutrinos (neutrinos that result from the decay of a photon coupled to a plasma). Axions, while hypothetical, are of great interest in Astrophysics because they are good candidates for the mysterious dark matter that pervades our universe. Measuring plasmon neutrino emission rates gives us a unique way to test the theory of weak interactions in the Standard Model of particles physics.; Axions arise from an elegant solution to a problem with the Standard Model of particle physics. Along with supersymmetric particles, axions are currently favored candidates for dark matter. But they have not been discovered (neither have supersymmetric particles) and the theory of axions fails to place any constraint on their mass. The possible contribution of axions to dark matter depends of course on their mass. The mass of axions determines how strongly they interact with the matter we know, with more massive axions interacting more strongly. In turn, the stronger the interaction of axions with matter or light, the larger their emission rate. With pulsating white dwarfs, we can constrain the axion emission rates and therefore their mass.; While we know a lot about neutrinos produced in nuclear reactions inside the Sun, plasmon decay has never been detected. This is because plasmon neutrino emission rates are expected to be significant only in very dense plasmas, such as in the degenerate interiors of white dwarfs. We cannot reproduce those conditions in the lab, and they are also not present in our nearest neutrino emitter, the Sun.; Both axions and plasmon neutrinos should stream freely out of white dwarfs, contributing efficiently to their cooling. We can measure the cooling rate of pulsating white dwarfs by measuring the rate at which the pulsation period of a given mode slows down with time (P˙). The faster the cooling, the larger P˙ is. By comparing the P˙ theoretically expected from the cooling with the P˙ we actually measure, we can deduce the emission the emission rates of plasmon neutrinos and axions and the mass of axions.; I begin by providing useful background information. I talk about non-radial stellar oscillations, give an overview of the observational methods behind the determination of ˙P˙'s, and list the P˙'s we have so far. Then I describe the approach I took to asteroseismology. Computers have become powerful enough that I was able for the first time to perform a brute force, systematic fine grid search of the relevant stellar parameter space. Next I present the theory behind axions, connect them to the dark matter problem through axion cosmology, and describe experiments and astrophysical observations (other than pulsating white dwarfs) that have already helped place upper limits on the axion mass. None of those attempts measures up to the method presented in the present work, where I use my own best fit models of G117-B15A (fit in parallel with R548) and the P˙ measured by Kepler et al. (2005c) to place a strong upper limit on the axion mass of 26.5 meV. I conclude with a detailed discussion of plasmon neutrinos and derive constraints we can very soon hope to place on their production rates, using the hot DBV EC20058. Along the way, I perform detailed asteroseismological analyses of G117-B15A, her sister R548, and of EC20058 and gain further physical insight into what determines the pulsation periods in those stars.
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