A theoretical study of problems in classical nova evolution
Shankar, Anurag
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https://hdl.handle.net/2142/19388
Description
Title
A theoretical study of problems in classical nova evolution
Author(s)
Shankar, Anurag
Issue Date
1990
Doctoral Committee Chair(s)
Truran, James W.
Department of Study
Astronomy
Discipline
Astronomy
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Astronomy and Astrophysics
Language
eng
Abstract
Three distinct issues in classical nova evolution are addressed with the aid of one- and two-dimensional numerical hydrodynamics.
The effects of convection on nova outbursts are examined first within the confines of the mixing length theory. It is found that increasing the efficiency of convection enhances the violence of the thermonuclear runaway (TNR). This also relates to the question of the feasibility of obtaining nova outbursts on magnetic white dwarfs among the AM Her systems. In particular, the thesis explores the effects of a strong magnetic field on the TNR. The field interferes with the development of convection during the TNR, which results in lower ejection velocities. However, for field strengths typical of cataclysmic variables, the violence of strong outbursts is affected only moderately.
"The second question concerns the conditions necessary for the production of strong TNRs in the ""hibernation"" model of cataclysmic binary evolution, which accounts for non-steady mass transfer. The thesis investigates the feasibility of obtaining strong nova outbursts when the accretion rate during hibernation is decreased by a factor of 100. It is found that such a reduction, for periods of longer than a couple thousand years, is sufficient to ensure violent outbursts, even in the presence of large pre-outburst accretion rates."
The final section of the thesis deals with the effects of a common envelope phase (CEP) on the outburst. The motion of the secondary through an expanding common envelope is resisted by frictional drag. This dissipates both energy and angular momentum from the orbit, which get deposited in the envelope, inducing hydrodynamic motion. Significant departures are found to occur in the manner in which mass is lost when the effects of drag are taken into account. Specifically, a CEP is found to accelerate and enhance mass loss. Ejection is found to be concentrated in the orbital plane, with velocities of a few thousand km $s\sp{-1}$. Possible explanation for observed morphologies of the ejected shells of slow novae may be provided by a CEP. In addition, accelerated mass loss may naturally account for rapid emergence of X-rays and a nebular phase in a number of systems.
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