Convection in a spherically symmetric accretion flow

Details Convection in a spherically symmetric accretion flow Convection in a spherically symmetric accretion flow

Nov 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995mnras.277...11m&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 277, Issue 1, pp. 11-24.

Astronomy and Astrophysics

Astronomy

5

Accretion, Accretion Discs, Convection

Scientific paper

We develop a simple model of convection in a fluid spherically accreting on to a black hole. The model is based on a generalization of the standard mixing length theory of convection and is applicable if the convective velocities are either larger or smaller than the accretion velocity - as long as both are significantly lower than the speed of sound. The change in the accretion velocity with radius leads to a radial stretching of the inflowing fluid which impedes convection. Thus, in order for convection to occur, the specific entropy has to grow inwards at a rate higher than some threshold value [dS/dr<(dS/dr)_threshold<0], which modifies the familiar Schwarzschild criterion for convection. However, this minimum rate is of such a small magnitude that even, for instance, optically thick, gas-pressure-dominated accretion flows, as discussed by Flammang, would necessarily involve convection in their subsonic portions, although the convective luminosity would be small compared with the total luminosity. We illustrate our model through its application to accretion on to a small primordial black hole at the centre of a Sun-like star.

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