Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and...
Transcript of Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and...
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
Enhanced MHD Transport in
Astrophysical Accretion Flows:
Turbulence, Winds and Jets
Zdenka Kuncic1, Peter Dobbie1
Geoffrey Bicknell2, Raquel Salmeron2
1School of Physics, University of Sydney
2Research School of Astronomy & Astrophysics,Australian National University
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
Motivation
Accretion is ubiquitous in astrophysics
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
Accretion Flows
How does accreting matter get rid of orbital angularmomentum and binding energy?
Specific angular momentum:
r 2Ω ∝ r 1/2 ⇒ shearing motions
But – kinematic fluid viscosity f visc = · (2ρν s) is too
slow...
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
The Viscosity Problem
Compare the viscous force density:
f visc ∼ ρν ∂ 2vφ∂ r 2
∼ ρν vφ
r 2
to the inertial term ρv · v in the momentum equation:
Re ≡inertia
viscosity∼
ρv2φ/r
ρν vφ/r 2=
rvφ
ν
For astrophysical accretion flows, microscopic viscosity
gives Re ∼ 1012 typically! ⇒ dynamically unimportant
⇒ microscopic kinematic viscosity cannot be responsible
for accretion
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
Also, keplerian flows have r 2Ω ∝ r 1/2 ⇒ obey Rayleigh
stability criterion for hydrodynamic turbulence:
d
dr (r 2
Ω) > 0
Experimentally verified
in high– Re laboratory
Couette flow
(Ji +, Nat. 444, 343, 2006).
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
The Magneto-Rotational Instability (MRI)
Weakly magnetised, differentially rotating flows are
unstable to the growth of MHD waves which in thenonlinear regime develop into MHD turbulence
(Velikhov 1959, Chandresekhar 1960, Fricke 1969, Acheson
1978, Balbus & Hawley 1991, 1992, 1998 ....)
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
Turbulent field lines correlate spatially distinct fluid
elements ⇒ transmit stresses without direct contact
between the fluid elements:
∂ρviv j∂ x j
= −ρ∂φG
∂ xi−
∂ p
∂ xi+
∂
∂ x j
t mhdij + 2νρsij
(weak) MHD turbulence provides effective viscosity
needed for transport in accretion flows
Simulations of a quasi-keplerian fluid showing density contours.Left: hydro case only; Right: MHD case, showing development ofturbulent motions.(From www.astro.virginia.edu/VITA/accdisk.php )
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
MHD Accretion Theory
MHD turbulence originally proposed as most viable
candidate for effective viscosity in standard accretion disk
theory: (Shakura & Sunyaev, 1973, A&A, 24, 337)
Today, numerical simulations
confirm that weakly magne-
tised keplerian disks are un-
stable to MHD turbulence.
(www.astro.princeton.edu/ ∼ jstone)
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
Outstanding Issues
Turbulent MHD transport alone cannot explain
accretion rates in the most luminous sources:
L 1048 ergs−1 ≈ 0.1 ˙ M ac2 =⇒ ˙ M a 200 M yr−1
Turbulent MHD transport cannot explain universality
of outflow phenomena, suggestive of large-scale
MHD effects
(Karovska + 2002, ApJ 577, 114; Kovalev + 2007, ApJ L27)
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
More Issues
MHD simulations are unable to recover high ˙ M awithout imposing a priori a large-scale poloidal
B-field on accretion disk For black hole accretion, it is not known whether a
net B z can evolve out of stochastic field in the disk
Standard accretion disk theory cannot explain
high-energy, nonthermal emission
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
A Generalized MHD Accretion Model
Transport in accretion flows is facilitated by bothlarge-scale MHD torques and small-scale MHD
turbulence
Vertical transport by MHD torques is primarily
responsible for efficient removal of angular
momentum (c.f. Blandford & Payne 1982; Königl & Pudritz 2000)
High-energy emission generated in a magnetized
corona and/or jet
Presence of jets indicates a high ˙ M a, even if the
radiative luminosity is low Disk emission spectrum is modified by jets and winds
(Kuncic & Bicknell, 2004, 2007a,b)
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
Theory
MHD AccretionSimulations
Summary
Mass continuity:
˙ M = ˙ M a(r )+ ˙ M w(r ) = const .
Angular momentum transport:
˙ M avφr − ˙ M a(r i)vφ(r i)r i = 2πr 2T r φ
+ r
r i vφr
d ˙ M a
dr + 4πr 2
B+φ B+ z
4π dr
Internal energy:
F d(r ) ≈
1
2T r φr ∂ Ω∂ r
=
3
4T r φΩ
where internal turbulent MHD stresses T r φ determined by
rate of angular momentum flux transported radially
outwards
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
TheoryMHD AccretionSimulations
Summary
Black Hole Accretion
Can stochastic reconnection events promote an inverse
cascade process to produce large-scale fields?
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
TheoryMHD AccretionSimulations
Summary
MHD Accretion Simulations
MHD disk simulations to date limited by: non-conservative schemes shearing box approximation neglect of finite resistivity neglect of radiation
(e.g. Hawley + 2001; Hawley & Balbus 2002)
MHD jet-disk simulations first
performed over 20 years ago!
(Uchida & Shibata 1985, PASJ 37, 515)
net B z required to launch jetsfrom disk
(e.g. Kigure & Shibata 2005, ApJ 634, 879)
MHD A i
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD Accretion
TheoryMHD AccretionSimulations
Summary
We are developing global, 3D, non-ideal, radiativeMHD simulations to model black hole accretion usingFLASH 1: adaptive mesh refinement improved Direct Eulerian piecewise parabolic method conservative numerical scheme constrained transport fully modular resistive MHD option
1flash.uchicago.edu
MHD A ti
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD AccretionTheory
MHD AccretionSimulations
Summary
Our goals:
1. To understand the MHD microphysics inaccretion flows and how they govern
macrophysical MHD phenomena.
2. To test the hypothesis that an inverse
cascade triggered by stochasticreconnection generates large-scale fields
out of fluctuating fields.
3. To calculate the emission spectrum of an
MHD accretion flow around a black hole and
compare with observational data.
MHD Accretion S
8/3/2019 Zdenka Kuncic et al- Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
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MHD Accretion
Z. Kuncic
Introduction
MHD AccretionTheory
MHD AccretionSimulations
Summary
Summary
accretion is ubiquitous in astrophysics and accretion
flows are unstable to MHD turbulence
MRI-driven turbulence enhances transport overviscosity, but is still too inefficient
MHD turbulence cannot explain universality of
outflows and nonthermal emission
a generalized MHD accretion has been proposed:accretion is primarily attributed to vertical transport of
angular momentum by large-scale MHD torques;
radial transport by turbulent stresses plays a
secondary role
resistive MHD simulations are needed to understand
evolution of magnetic field topology
radiative MHD simulations are needed to directly
compare predicted and observed accretion disk
emission spectra