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Accretion disc dynamos

Explore the dynamics of disc dynamos, energy deposition inside and outside the disc, and the interplay between ambient fields and dynamos. Investigate the possibilities of current sheets and the spin-up vs. spin-down effects, as discussed in studies by B. von Rekowski et al. Uncover the unique aspects of the disc dynamo, its impact on global models, and the roles of local mean-field and global mean-field models. Discover the implications of vertical stratification, heating near disc boundaries, and the influence of ambient fields on jet direction. Delve into the jet-disc-dynamo model, feedback mechanisms, and the necessity of external fields in jet formation. Discuss disc cooling mechanisms, coronal heating, structured outflows, and momentum transport into the wind. Gain insight into stellar spin alignment, the formation of X-points, and the role of magnetic fields in stellar wind dynamics. Learn about the Pencil Code simulation tool and its contributions to understanding disc dynamos. Conclude by exploring future research directions, including self-maintained entropy gradients and radiative cooling effects in disc surfaces.

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Accretion disc dynamos

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  1. Accretion disc dynamos • Energy deposition • Inside/outside the disc • Ambient field vs dynamo • current sheets possible • Spin-up vs spin-down B. von Rekowski, A. Brandenburg, 2004, A&A 420, 17-32 B. von Rekowski, A. Brandenburg, W. Dobler, A. Shukurov, 2003 A&A 398, 825-844

  2. Disc dynamo: shearing sheet simulationsoscillatory mean fields Dynamo cycle period: 30 orbits (=turbulent diffusion time) B-vectors in midplane: reversals

  3. Unusual dynamo aspects Magnetic field as catalyst Negative alpha effect

  4. Dynamo 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)

  5. Vertical stratification and heating Brandenburg et al. (1996) z-dependence of

  6. Heating near disc boundary weak z-dependence of energy density where Turner (2004)

  7. Ambient field versus dynamo:different jet directions Hodapp & Ladd (1995)

  8. Correlation with ambient field •  All objects • have outflows • Jets more pronouced when aligned Menard & Duchene (2004) Taurus-Auriga

  9. Bridging the gaps: jet-disc-dynamo Model of disc dynamo, with feedback from disc, and allowing for outflows Jet theory (Pudritz) Do jets require external fields? Do we get dipolar fields? Are they necessary? Disc theory (Hawley) Dynamo theory (Stepinski)

  10. Our inspiration • Disc modeled as b.c. • Adiabatic EOS • Virial temperature • Need a cool disc • Kepler rot. only for Ouyed & Pudritz (1997)

  11. Modeling a cool disc • vertical pressure eq. • low T  high density • Entropy lower in disc • bi-polytropic model Coronal heating: by reconnection Radiative cooling Vertical cross-section through the disc

  12. Structured outflow Disc temperature relative to halo is free parameter: Here about 3000K Cooler disc: more vigorous evolution

  13. Disc temperature about 3000 K Acceleration mechanism Ratio of magneto-centrifugal force to pressure force

  14. Unsteady outflow is the rule Momentum transport from the disc into the wind

  15. Comparison with Ouyed & Pudritz (1997) Very similar: Alfven Mach >1 Toroidal/poloidal field ratio increases

  16. Lagrangian invariants

  17. Conical outflows (similar to BN/KL) Greenhill et al (1998)

  18. Stellar spin-up or spin-down? Matt & Pudritz (2004)

  19. Which way around does it go? Stellar field aligned with ambient field  formation of X-point • Stellar field anti-aligned • current sheet • Source of heating If external field is dragged in. or if stellar field shows reversals If field is due to dynamo

  20. Wind and accretion X-wind (Shu et al. 1994) Simulation (Fendt & Elstner 2000)

  21. Simulations by Miller & Stone (1997) Simulation time: several days

  22. Stellar spin-up or spin-down? Strong accretion flow Küker. Henning, & Rüdiger (2003) Fieldline loading?

  23. Alternating fieldline uploading and downloading Similar behavior found by Goodson & Winglee (1999) von Rekowskii & Brandenburg 2004 (A&A) Star connected with the disc Star disconnected from disc

  24. Similar behavior found by Goodson & Winglee cf. field line opening: Uzdensky (2002), Pudritz & Matt (2004)

  25. Stellar breaking: winds from stellar dynamo Speed: 300 km/s Highly time-dependent; Switch dipole/quadrupole Magneto-centrifugal acceleration

  26. Winds from stellar dynamo Current sheet configuration as a result of the simulations

  27. Pencil 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

  28. Conclusions • Heating of disc corona is quite natural • 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

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