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Title:X and Ultraviolet Radiation From Accreting Non-Magnetic Degenerate Dwarfs: A Two-Fluid Treatment of the Emission Region and the Effects of Steady Nuclear Burning
Author(s):Weast, Gordon John
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Physics, Astronomy and Astrophysics
Abstract:The first main chapter is a report of the results of calculations which^allow the temperatures of the electrons and the ions to differ and which^include electron thermal conduction. The calculation is done in spherical^geometry, with cooling by bremsstrahlung and cooling by the inverse^Compton effect added in the post shock emission region. Our two-fluid^calculations are needed only in the small region bounded by M(, )>(, )1.2^M(,(CIRCLE)) and M(, )>(, )5 x 10('-3) M(,E), where M(,E) is the Eddington accretion rate^at which gravitational and radiation pressure forces balance (M(,E) = 5.57^x 10('20)(R/5 x 10('8)) g s('-1)). When there is steady nuclear burning at the accretion rate, Compton cooling is enhanced and two-fluid effects are important for M(, )>(, )1.0 M(,(CIRCLE)) and accretion rates M(' )>(, )10('-4) M(,E).
It has been suggested that electron thermal conduction might significantly alter the structure and the observed spectrum produced by accreting degenerate dwarfs. Recently, King and Losota (1980, M.N.R.A.S., 191, 721) assumed that conduction can quench the hard X-ray production. The detailed calculations below show that even though two-fluid and conduction effects significantly alter the temperature and density structure of the emission region, the observed spectrum is altered only slightly. As compared with one-fluid spectra, the X-ray spectrum is slightly softer and the X-ray luminosity is slightly larger when a two-fluid treatment is used. However the changes are generally(, ) 1.0 M(,(CIRCLE))) stars when the optical depth is near 10, there is no high energy excess over an exponential spectrum. The excess is not present because the initial spectrum generated in the emission region is softer than in the one-fluid calculation and thus there is less flux at high energies, from which the tail was previously produced.
In the two-fluid calculations, the electron temperature is low and nearly constant when Compton cooling is important. As a result, electron thermal conduction is much less important than would be expected from estimates based on at the temperature gradient in the one-fluid calculations.
The second main chapter considers the result of steady nuclear burning of the accreted matter on the structure of the emission region.
The energy liberated by nuclear burning of matter accreting onto degenerate dwarfs varies from 240 times the gravitational potential energy for a 0.2 M(,(CIRCLE)) star to 3.7 times the for a 1.4 M(,(CIRCLE)) star. Because nuclear burning occurs at a large optical depth in the envelope of the white dwarf, the energy released comes out as blackbody radiation from the stellar surface. Through the mechanism of Compton cooling, this very large blackbody flux has a controlling influence on the X rays produced in the hot post-shock emission region just above the surface of the star. Here we report the results of calculations in which steady nuclear burning occurs at the accretion rate and compare them with our results without nuclear burning. We also explore several models in which the accretion rate is fixed but the burning rate is varied and, conversely, in which the burning rate is held constant but the accretion rate is varied.
When steady nuclear burning at the accretion rate occurs, the hard X-ray flux is greatly reduced, the observed X-ray temperature is reduced, and the soft X-ray flux is enhanced relative to the non-burning case.
When the constraint that burning occur at the accretion rate is relaxed, we find that there is a vast richness of observable behavior possible. A maximum T(,obs) and L(,hard) still exists for each mass star, but the observed values can vary with very few constraints.
Issue Date:1981
Description:131 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1981.
Other Identifier(s):(UMI)AAI8114496
Date Available in IDEALS:2015-05-13
Date Deposited:1981

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