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Stationary Spherical Optically Thick Accretion into Black Holes

Citation

Flammang, Richard Alan (1982) Stationary Spherical Optically Thick Accretion into Black Holes. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:08252017-133002176

Abstract

As its title indicates, this thesis treats the problem of stationary, spherical, optically thick accretion into black holes. By the phrase "optically thick" it is meant that (1) radiative energy transport can be adequately described by the diffusion approximation and (2) the photons are everywhere in local energy equilibrium (LTE) with the accreting gas particles.

In Chapter 1, a general set of equations governing time-independent spherical accretion into black holes is formulated. The equations are fully general relativistic and are applicable to optically thick regions, optically thin regions, and the transition regions which join them. The radiation is treated using frequency-integrated moments. The full, infinite series of moment equations is given, together with the limiting forms the equations take in the optically thick regime.

In Chapter 2, we present the mathematical theory of stationary spherical optically thick accretion. We analyze the integral curves of the differential equations describing the problem. We find a one-parameter family of critical points, where the inflow velocity equals the isothermal sound speed. Physical solutions must pass through one of these critical points. We obtain a complete set of boundary conditions which the solution must satisfy at the horizon of the black hole, and show that these, plus the requirement that the solution pass through a critical point, determine a unique solution to the problem. This analysis leads to a generalization of the well-known Bondi critical point constraint, which arises in the adiabatic accretion problem and which is effective at the point where the inflow velocity equals the adiabatic sound speed. We show that this point can be regarded as a "diffused critical point" in our problem. The analysis also yields a simple expression for the diffusive luminosity at radial infinity. Finally, we find a satisfying explanation for the rather peculiar critical point structure of this problem in an analysis of the characteristics and subcharacteristics present in the problem and in a "hierarchical" analysis of the waves which propagate along them.

In Chapter 3, we apply the theory of optically thick accretion developed in Chapter 2 to a wide range of physically different accretion regimes. Numerical solutions are presented and their physical properties are discussed. For solutions in which radiation pressure PR dominates gas pressure PG, but in which gas energy density (including its rest-mass) ρG dominates radiation energy density ρR, we pay particular attention to the adabaticity of the flow. Our quantitative results in this regime agree very well with Begelman's (1978) theory. We find the dimensionless number which governs the importance of heat diffusion in our problem and show that it reduces to the idea of "trapping of photons" and to the Péclet number in the appropriate limits. We find that solutions with PR > PG and ρR > ρG are always essentially adiabatic, owing in part to a relativistic suppression of heat flux which becomes important in this regime. The diffusive luminosity at infinity for these solutions is the Eddington limit of the black hole; with the adiabatic accretion rate, "efficiencies" of up to order unity are possible. We give preliminary consideration to the question of the stability of our solutions against convection and conclude that the Schwarzschild criterion is applicable, even for our non-static accretion flows. We show that solutions with PR > PG are everywhere stable against convection. On the other hand, solutions which start out at radial infinity with PG > PR are unstable to convection (if the adiabatic index of the gas γG exceeds 17/12) from radial infinity down to the point where PR ~ PG and the radiation-gas mixture has attained an adiabatic index of 17/12. The diffusive luminosity at infinity for these solutions is reduced from the Eddington limit of the black hole by the factor (γG - 1)4PR∞GPG∞; it is further reduced by the ratio of the electron scattering opacity to the actual opacity at infinity, if this differs from unity. In most cases, energy diffusion has a negligible effect on the accretion rate of these solutions.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Physics
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Thorne, Kip S.
Group:TAPIR
Thesis Committee:
  • Thorne, Kip S. (chair)
  • Blandford, Roger D.
  • Whitcomb, Stanley E.
  • Simon, Barry M.
Defense Date:4 January 1982
Funders:
Funding AgencyGrant Number
NSFUNSPECIFIED
Schlumberger FoundationUNSPECIFIED
Danforth FoundationUNSPECIFIED
Record Number:CaltechTHESIS:08252017-133002176
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:08252017-133002176
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:10391
Collection:CaltechTHESIS
Deposited By: Benjamin Perez
Deposited On:28 Aug 2017 21:34
Last Modified:07 Mar 2018 00:32

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