The vertical structure and stability of accretion disks surrounding black holes and neutron stars

Details The vertical structure and stability of accretion disks surrounding black holes and neutron stars The vertical structure and stability of accretion disks surrounding black holes and neutron stars

Feb 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...421..668m&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 421, no. 2, p. 668-676

Astronomy and Astrophysics

Astronomy

18

Accretion Disks, Black Holes (Astronomy), Neutron Stars, Stellar Gravitation, Stellar Mass Accretion, Stellar Models, Stellar Structure, Systems Stability, Gravitational Collapse, Vertical Distribution

Scientific paper

The structure and stability of the inner regions of accretion disks surrounding neutron stars and black holes have been investigated. Within the framework of the alpha viscosity prescription for optically thick disks, we assume the viscous stress scales with gas pressure only, and the alpha parameter, which is less than or equal to unity, is formulated as alpha0(h/r)n, where h is the local scale height and n and alpha0 are constants. We neglect advective energy transport associated with radial motions and construct the vertical structure of the disks by assuming a Keplerian rotation law and local hydrostatic and thermal equilibrium. The vertical structures have been calculated with and without convective energy transport, and it has been demonstrated that convection is important especially for mass accretion rates, M-dot, greater than about 0.1 times the Eddington value, M-dotEdd. Although the efficiency of convection is not high, convection significantly modifies the vertical structure of the disk (as compared with a purely radiative model) and leads to lower temperatures at a given M-dot. The results show that the disk can be locally unstable and that for n greater than or = 0.75, an S-shaped relation can exist between M-dot and the column density, sigma, at a given radius. While the lower stable branch (derivative of M-dot/derivative of sigma greater than 0) and middle unstable branch (derivative of M-dot/derivative of sigma less than 0) represent structures for which the gas and radiation pressure dominate respectively, the stable upper branch (derivative of M-dot/derivative of sigma greater than 0) is a consequence of the saturation of alpha. This saturation of alpha can occur for large alpha0 and at M-dot less than or = M-dotEdd. The instability is found to occur at higher mass accretion rates for neutron stars than for black holes. In particular, the disk is locally unstable for M-dot greater than or = 0.5 M-dotEdd for neutron stars and for M-dot greater than or = M-dotEdd for black holes for a viscosity prescription characterized by n = 1 and alpha0 = 10.

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