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

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

Summary

Motivation

Accretion is ubiquitous in astrophysics

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

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...

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

Summary

The Viscosity Problem

Compare the viscous force density:

f visc ∼ ρν ∂ 2vφ∂ r 2

∼ ρν vφ

to the inertial term ρv · v in the momentum equation:

Re ≡inertia

viscosity∼

ρv2φ/r

ρν vφ/r 2=

For astrophysical accretion flows, microscopic viscosity

gives Re ∼ 1012 typically! ⇒ dynamically unimportant

⇒ microscopic kinematic viscosity cannot be responsible

for accretion

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

Summary

Also, keplerian flows have r 2Ω ∝ r 1/2 ⇒ obey Rayleigh

stability criterion for hydrodynamic turbulence:

dr (r 2

Ω) > 0

Experimentally verified

in high– Re laboratory

Couette flow

(Ji +, Nat. 444, 343, 2006).

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

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 ....)

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

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−

∂ 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 )

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

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)

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

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)

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

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

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

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)

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

Theory

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 i vφr

d ˙ M a

dr + 4πr 2

B+φ B+ z

4π dr

Internal energy:

F d(r ) ≈

2T r φr ∂ Ω∂ r

4T r φΩ

where internal turbulent MHD stresses T r φ determined by

rate of angular momentum flux transported radially

outwards

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?

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

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

MHD Accretion

Z. Kuncic

Introduction

MHD Accretion

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

MHD Accretion

Z. Kuncic

Introduction

MHD AccretionTheory

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

MHD Accretion

Z. Kuncic

Introduction

MHD AccretionTheory

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

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