Minimum Torque and Minimum Dissipation Black Hole-Driven Winds

Details Minimum Torque and Minimum Dissipation Black Hole-Driven Winds Minimum Torque and Minimum Dissipation Black Hole-Driven Winds

Oct 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...506..790p&link_type=abstract

The Astrophysical Journal, Volume 506, Issue 2, pp. 790-804.

Physics

10

Black Hole Physics, Magnetohydrodynamics: Mhd, Stars: Mass Loss

Scientific paper

This article is an investigation of paired winds of magnetized plasma on the magnetic flux tubes that thread a rotating black hole. The paired wind system includes an ingoing accretion wind and an outgoing magnetically and centrifugally slung wind. We consider wind systems in which the field-line angular velocity, Omega_F, is established by the causal physics of plasma injection into the azimuthally symmetric magnetic flux tubes, Omega_F << Omega_H, where Omega_H is the angular velocity of the black hole. There is another school of thought that holds that Omega_F ~ Omega_H/2 is the proper choice of field-line angular velocity. However, it has been argued previously by this author that there is no known microphysics associated with the gravitational field of a rotating black hole that can produce the predominantly field-aligned poloidal current flows associated with Omega_F ~ Omega_H/2. Furthermore, the detailed structure of MHD waves in the background of the geometry of the hole argues against the causal structure implicit in assuming Omega_F ~ Omega_H/2. Consequently, for the purposes of this paper we assume that the relatively low angular velocity of the injected plasma sets Omega_F << Omega_H throughout the following. Two families of solutions are analyzed. A ``minimum dissipation solution,'' which obeys the perfect MHD approximation everywhere, is shown to be the analog of the Blandford-Znajek solution for an imposed value of Omega_F << Omega_H. A second paired-wind system is studied in which the outgoing wind is the perfect MHD ``minimum torque solution.'' The paired ingoing wind in the perfect MHD limit cannot reach the event horizon. It is shown that the ingoing perfect MHD wind joins onto a deflagration wind. It is argued that the second paired-wind system is physically more reasonable on the basis of causality, minimum energy, and minimum dissipation principles. The minimum dissipation aspect of the discussion requires a general relativistic understanding of entropy generation in the event horizon.

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