Convection causes enhanced magnetic turbulence in accretion disks in outburst

Journal Article · Tue May 20 00:00:00 EDT 2014 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/787/1/1· OSTI ID:22356888

Hirose, Shigenobu ^[1]; Blaes, Omer; Coleman, Matthew S. B. ^[2]; Krolik, Julian H. ^[3]; Sano, Takayoshi ^[4]

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1. Department of Mathematical Science and Advanced Technology, JAMSTEC, Yokohama, Kanagawa 236-0001 (Japan)
2. Department of Physics, University of California, Santa Barbara, CA 93106 (United States)
3. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 (United States)
4. Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871 (Japan)

We present the results of local, vertically stratified, radiation magnetohydrodynamic (MHD) shearing box simulations of magneto-rotational instability (MRI) turbulence appropriate for the hydrogen ionizing regime of dwarf nova and soft X-ray transient outbursts. We incorporate the frequency-integrated opacities and equation of state for this regime, but neglect non-ideal MHD effects and surface irradiation, and do not impose net vertical magnetic flux. We find two stable thermal equilibrium tracks in the effective temperature versus surface mass density plane, in qualitative agreement with the S-curve picture of the standard disk instability model. We find that the large opacity at temperatures near 10{sup 4} K, a corollary of the hydrogen ionization transition, triggers strong, intermittent thermal convection on the upper stable branch. This convection strengthens the magnetic turbulent dynamo and greatly enhances the time-averaged value of the stress to thermal pressure ratio α, possibly by generating vertical magnetic field that may seed the axisymmetric MRI, and by increasing cooling so that the pressure does not rise in proportion to the turbulent dissipation. These enhanced stress to pressure ratios may alleviate the order of magnitude discrepancy between the α-values observationally inferred in the outburst state and those that have been measured from previous local numerical simulations of magnetorotational turbulence that lack net vertical magnetic flux.

• OSTI ID: 22356888
• Journal Information: Astrophysical Journal, Vol. 787, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ACCRETION DISKS
AXIAL SYMMETRY
COMPUTERIZED SIMULATION
DENSITY
EQUATIONS OF STATE
INSTABILITY
MAGNETIC FIELDS
MAGNETIC FLUX
MAGNETOHYDRODYNAMICS
NMR IMAGING
NOVAE
OPACITY
RADIANT HEAT TRANSFER
SOFT X RADIATION
STRESSES
SURFACES
THERMAL EQUILIBRIUM
TRANSIENTS
TURBULENCE