Journal
ASTROPHYSICAL JOURNAL
Volume 577, Issue 1, Pages 295-301Publisher
UNIV CHICAGO PRESS
DOI: 10.1086/342159
Keywords
accretion, accretion disks; black hole physics; convection; instabilities; MHD; turbulence
Categories
Ask authors/readers for more resources
Radiatively inefficient accretion flows onto black holes are unstable due to both an outwardly decreasing entropy (convection) and an outwardly decreasing rotation rate (the magnetorotational instability [MRI]). Using a linear MHD stability analysis, we show that long-wavelength modes with lambda/H >> beta(-1/2) are primarily destabilized by the entropy gradient and that such convective modes transport angular momentum inward (lambda is the wavelength of the mode, H is the scale height of the disk, and beta is the ratio of the gas pressure to the magnetic pressure). Moreover, the stability criteria for the convective modes are the standard Hoiland criteria of hydrodynamics. By contrast, shorter wavelength modes with lambda/H similar to beta(-1/2) are primarily destabilized by magnetic tension and differential rotation. These MRI modes transport angular momentum outward. The convection-dominated accretion flow (CDAF) model, which has been proposed for radiatively inefficient accretion onto a black hole, posits that inward angular momentum transport and outward energy transport by long-wavelength convective fluctuations are crucial for determining the structure of the accretion flow. Our analysis suggests that the CDAF model is applicable to an MHD accretion flow provided that the magnetic field saturates at a value sufficiently below equipartition (beta >> 1), so that long-wavelength convective fluctuations with lambda/H >> beta(-1/2) can fit inside the accretion disk. Numerical MHD simulations are required to determine whether such a subequipartition field is in fact obtained.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available