Mathematics – Logic
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........27k&link_type=abstract
Thesis (PH.D.)--THE UNIVERSITY OF TEXAS AT AUSTIN, 1995.Source: Dissertation Abstracts International, Volume: 57-02, Section: B,
Mathematics
Logic
26
Cooling, White Dwarfs
Scientific paper
The history of our galaxy and a record of stellar evolution is within the white dwarf stars. The coolest observed white dwarfs are also the oldest; the Universe has not been around long enough for them to cool any further. If we can measure their rate of cooling, we can measure their age, obtaining a lower limit for the age of the galaxy. The white dwarfs have a very narrow mass distribution, yet their progenitors have a wide range of initial masses, most of which they must lose before arriving at nearly the same final mass. Inside the white dwarfs are the products of stellar evolution and nucleosynthesis. To measure their composition and structure is to gain insights into the processes of nucleosynthesis and mass loss. Fortunately, the white dwarfs give us ample opportunity to make such measurements. At three places along their cooling track, they become unstable to g-mode pulsations. Through asteroseismology, we can reveal the innards of the white dwarfs through these oscillations. Secular cooling changes the pulsation properties. Measure these changes and we have measured their cooling rates. The Whole Earth Telescope has enjoyed remarkable successes within two of the three white dwarf instability strips. The remaining strip, the cool DAVs, proved mysterious. Clemens recently solved the hotter DAVs by studying their group properties. The cooler DAVs, whose modes are more numerous, although unstable, could not be included in this analysis. By gathering multiple years of data on one DAV, G29-38, I reveal an underlying stable structure of modes suitable for asteroseismological measurements. I use data on additional cool DAVs to show they are all remarkably alike: they share the same modes of oscillation. Although existing theoretical models are few and incomplete, I use what is available to derive preliminary parameters of these objects. I find them consistent with having thick (near 10^{-4}M_star) H-layers, although the asteroseismological and spectroscopic masses disagree. The result of this effort opens the last major frontier of white dwarf asteroseismology. All the DAVs, the coolest and the oldest of the known white dwarf variables, are now available for asteroseismological analysis.
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