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Accretion disc dynamosAccretion disc dynamos
B. von Rekowski, A. Brandenburg, 2004, B. von Rekowski, A. Brandenburg, 2004, A&A A&A 420420, , 17-3217-32B. von Rekowski, A. Brandenburg, W. Dobler, A. Shukurov, 2003 B. von Rekowski, A. Brandenburg, W. Dobler, A. Shukurov, 2003 A&A A&A 398398, , 825-825-
844844
(i)(i) Energy depositionEnergy depositionInside/outside the discInside/outside the disc
(ii)(ii) Ambient field vs dynamoAmbient field vs dynamo current sheets possiblecurrent sheets possible(i)(i) Spin-up vs spin-downSpin-up vs spin-down
Disc dynamo: shearing sheet simulationsDisc dynamo: shearing sheet simulationsoscillatory mean fieldsoscillatory mean fields
Dynamo cycle period: 30 orbits
(=turbulent diffusion time) B-vectors in midplane: reversals
Dynamo input to global modelsDynamo input to global models
• Local disc simulations: cyclic fields– Quadrupolar for vacuum condition– Steady dipolar field for perfect conductor b.c.
• Dynamo alpha negative– Local mean-field models: similar results
• Global mean field models: always dipolar– Bardou et al. (2001), Campbell (2001)
Vertical stratification and heatingVertical stratification and heating
Brandenburg et al. (1996)
const turb ss cHc
HczHc ss )(turb z-dependence of
Heating near disc boundaryHeating near disc boundary
Turner (2004)
radp
radp
gasp
2
2...J
u
t
Tcv
022 / Bu
weak z-dependence of energy density
0/ BJ where
Ambient field versus dynamo:Ambient field versus dynamo:different jet directionsdifferent jet directions
Hodapp &
Ladd (1995)
Correlation with ambient fieldCorrelation with ambient fieldM
enar
d &
Duc
hene
(20
04) All objects
have outflowsJets morepronoucedwhen aligned
Taurus-Auriga
Bridging the gaps: jet-disc-dynamoBridging the gaps: jet-disc-dynamo
Dynamo theory
Disc theory
Jet theory(Pudritz)
(Stepinski)
(Hawley)
Model of disc dynamo,with feedback from disc,and allowing for outflows
Do jets require external fields?Do we get dipolar fields?Are they necessary?
Our inspirationOur inspiration
• Disc modeled as b.c.
• Adiabatic EOS– Virial temperature
• Need a cool disc
• Kepler rot. only for
Ouyed & Pudritz (1997)r
GMTcp
0T
Modeling a cool discModeling a cool disc
• vertical pressure eq.
• low T high density
• Entropy lower in disc– bi-polytropic model
Vertical cross-sectionthrough the disc
z
sTh
z
0
s
Thu2
Coronal heating:by reconnection
Radiative cooling
Structured outflowStructured outflow
Disc temperature relative to halo is free parameter:Here about 3000K
Cooler disc: more vigorous evolution
Acceleration mechanismAcceleration mechanismDisc temperatureabout 3000 K
Ratio of magneto-centrifugal forceto pressure force
Unsteady Unsteady outflow is outflow is the rulethe rule
Momentum transport from the disc into the wind
Comparison with Ouyed & Pudritz (1997)Comparison with Ouyed & Pudritz (1997)
Very similar: Alfven Mach >1Toroidal/poloidal field ratio increases
Which way around does it go?Which way around does it go?
Stellar field aligned with ambient field
formation of X-point
Stellar field anti-aligned
current sheetSource of heating If external field is dragged in.
or if stellar field shows reversals If field is due to dynamo
Simulations by Miller & Stone (1997)Simulations by Miller & Stone (1997)
Simulation time: several days
Stellar spin-up or spin-down?Stellar spin-up or spin-down?K
üker
. Hen
ning
, & R
üdig
er (
2003
)
Fieldlineloading?
Strongaccretion
flow
Alternating fieldline uploading and downloadingAlternating fieldline uploading and downloading
Star connected with the disc Star disconnected from disc
Simil ar beh avior fo und b y G
oo dso n & W
ingl ee (19 99)von
Rek
owsk
ii &
Bra
nden
burg
200
4 (A
&A
)
Similar behavior found by Goodson & WingleeSimilar behavior found by Goodson & Winglee
cf. field line opening: Uzdensky (2002), Pudritz & Matt (2004)
Stellar breaking: winds from stellar dynamoStellar breaking: winds from stellar dynamo
Magneto-centrifugal accelerationSpeed: 300 km/sHighly time-dependent;Switch dipole/quadrupole
Winds from stellar dynamoWinds from stellar dynamo
Current sheet configurationas a result of the simulations
Pencil CodePencil Code
• Started in Sept. 2001 with Wolfgang Dobler
• High order (6th order in space, 3rd order in time)
• Cache & memory efficient
• MPI, can run PacxMPI (across countries!)
• Maintained/developed by many people (CVS!)
• Automatic validation (over night or any time)
• Max resolution so far 10243
ConclusionsConclusions
(i) Heating of disc corona is quite natural(ii) Stellar field anti-aligned against exterior
– Current sheet, not X-point configuration
(ii) Conical outflow– Collimation: external field required– Larger distances? Mass loading?
(iii) Disc field opens up: magnetic spin-up– Spin-down by stellar wind (depends on strength)
Future work: entropy gradient self-maintained– Requires radiative cooling of disc surface