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Multidimensional Diffusive Shock Acceleration in Winds from Massive Stars

This study investigates the role of multidimensional effects in shock acceleration within the winds generated by massive stars, specifically focusing on Wolf-Rayet (WR) and O-type stars. Through advanced MHD simulations using the AstroBEAR code, we analyze internal shocks, cosmic ray acceleration, and the impact of colliding winds. The results indicate that cosmic ray pressure is not significant in the inner wind, but protons and electrons can achieve high energy levels. Future work will delve into more complex scenarios and synthetic observations.

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Multidimensional Diffusive Shock Acceleration in Winds from Massive Stars

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  1. Multidimensional Diffusive Shock Acceleration in Winds from Massive Stars Paul P. Edmon University of Minnesota Collaborators: Tom Jones (U of M), Andrew Cunningham (LLNL), Adam Frank (U of R) 5th Korean Astrophysics Workshop: Shock Waves Turbulence, and Particle Acceleration 11-19-09

  2. Outline • Background • Numerical Method • Simulation Setup • Results • Summary

  3. Background • Non-thermal Emitting WR and O-Stars • Radio: About a quarter of all WR and O-stars • X-ray: Thought to be a few from WR Stars • TeV Gamma-Ray: Seen in some young star clusters and single WR Stars Credit: HESS page on Westerlund 2

  4. Model • Diffusive Shock Acceleration: • Internal Wind Shocks • Shocks caused by Line Driving Instability • Colliding Wind Binary • Current Status • Dougherty & Williams (2000): Non-thermal Radio emitting WR Stars are most likely Binaries with Colliding Winds • Van Loo (2005, 2006): 1-D Hydro Simulations of O-stars indicate that non-thermal emitting O-stars are also Binaries with Colliding Winds Credit: Owocki AIPC 1175 (2009) 173

  5. Open Questions • Is cosmic ray (CR) acceleration efficient enough to significantly modify the shocks? • Is electron reacceleration important in the internal shock scenario? • What role do multidimensional effects play in this problem?

  6. Numerical Method • AstroBEAR (Astronomical Boundary Embedded Adaptive Refinement) MHD Code (Cunningham et al. 2009) • Coarse-Grained Momentum finite Volume (Jones & Kang 2005) • Method reduces number of momentum zones by an order of magnitude • Assumes CR spectrum can be represented with a powerlaw • Crank-Nicolson Scheme with a tridiagonal solver for Diffusion

  7. Simulation Setup • 2.5-D Cylindrical • Effective Resolution: 6144 x 3072 x 14 momentum bins • Isotropic Bohm-like Diffusion • O-type Star: 22 Msun • Vw: 2000 km/sec • Mdot: 10-6Msun/yr • Stellar Radius: 8.5 Rsun • Inner Radius: 50 Rstar • dx: 0.3 Rstar • Field Geometry • Radial Field~ 1/R2 • Azimuthal Field~ 1/R • Field Strength at Star: 10 G • Frame dt: 4.7 hrs • Final Frame: 9 days Credit: Hamann, et al. AIPC 1175 (2009) 136

  8. Log of Number Density

  9. Log of Cosmic Ray Pressure

  10. Log of the Ratio of Pc/Pg

  11. Cosmic Ray Protons

  12. Cosmic Ray Electrons

  13. Summary and Future Work • Currently running and analyzing single O-star wind with internal shocks • CR pressure appears to be not important in the inner wind • Protons can be accelerated to at least tens of GeV • Electrons can be accelerated to at least a GeV • Future Work • Implement CR solver into WOMBAT • Single Wind Runs • Synthetic Observations of Synchrotron, Bremsstrahlung, Inverse Compton and Neutral Pion Decays • Detailed analysis of CR interactions • Colliding Wind Binaries

  14. Acknowledgements • NASA Grant NNG05GF57G • NSF Grant AST0607674 • Minnesota Supercomputing Institute for Advanced Computational Research

  15. Crank-Nicolson Scheme • Uses a CN Scheme with a tridiagonal solver to solve the Diffusion Equation • Adaptive Subcylcing • Minimum amount of subcycling based on size of grid patch relative to the diffusion length for that momentum • Internal Boundary Conditions • Floating linear interpolation for internal boundaries

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