Core collapse supernovae: effect of a « moderate » magnetic field

1. Core collapse supernovae: effect of a « moderate » magnetic field Jérôme Guilet En collaboration avec Thierry Foglizzo, Sébastien Fromang & Jun’ichi Sato

2. From core collapse to explosion massive star Stalled shock collapse of the iron core How to revive the shock and obtain an explosion ?

3. Neutrino driven explosion Magnetic explosion Burrows et al. 2007 • Obtains for VERY rapid rotation • Magnetic field amplification: B ~1015 G • Strong explosion through magnetic jets Marek & Janka 2009 • Neutrino heating below the heating • Aided by the Standing Accretion Shock Instability (SASI) • -> Maybe a marginal explosion in 2D? Solid explosion in 3D?

4. Large scale asymetry caused by SASI What is SASI? Standing Accretion Shock Instability Causes dipolar shock oscillation Driven by an unstable advective-acoustic cycle Important consequences : Neutron star kick : up to 1000km/s (Scheck et al 2006) Neutron star spin driven by spiral SASI mode (Blondin & Mezzacappa 2007) Gravitational waves signature Element mixing during explosion Asymetry of the explosion Blondin & Mezzacappa 2007

5. SASI with a magnetic field : (Guilet & Foglizzo 2010) Dependence of the pulsar kick on its magnetic field ? Kick of magnetars ? Dynamics of an Alfvén surface : (Guilet, Foglizzo & Fromang submitted) Help the explosion ? Explosion geometry ? What is the effect of a magnetic field ?

6. A toy model v shock B or deceleration by an external potential neutron star shock « neutron star » SASI : an advective acoustic cycle Foglizzo et al 2007 Two steps in the cycle : - An acoustic wave deforms the shock creating an entropy-vorticity wave - The entropy-vorticity wave is decelerated creating an acoustic feedback With a magnetic field : vorticity can propagate through Alfvén and slow waves !

7. Cycle efficiencies : 6 cycles (instead of 2) Coupling at the shock shock 1 cycle fast coupling in the gradients fast magnetosonic 1 cycle entropy 2 cycles slow 2 cycles Alfvén « neutron star »

8. Propagation of the vorticity Total efficiency oscillates : Qtot =  Q Growth rate : i~ log(Qtot)/ growth rate magnetic field strength Interference between the cycles different cycles are out of phase ~vA~B choc Entropy Wave - Wave + vorticity

9. Vertical B : No effect Horizontal B : Strong amplification of the vorticity cycles when field lines are bent (k // B) Increase of the growth rate Significant effect if Coupling efficiency coupling efficiency growth rate vA~v Conclusion : Ambiguous result... magnetic field strength

10. Dynamics of Alfvén surface

11. The alfvén surface is defined by : v = vA Alfvén speed : vA2 = B2/( AnAlfvén wave propagates against the flow at the speed : v-vA Accumulation of Alfvén waves at the Alfvén surface ! We performed 1D simulations with the code RAMSES Alfvén surface simulations

12. The Alfvén wave amplifies while its wavelength decreases : When the wavelength is as small as the dissipative scale, the Alfvén wave is dissipated Creation of a pressure feedback that increases the upstream pressure Alfvén wave amplification and pressure feedback

13. High frequency : Low frequency : Pressure feedback : Non linear saturation : Analytical estimate

14. Important effect if the Alfvén surface is above the proto-neutron star surface Magnetic field required : SASI creates Alfvén waves with an amplitude : Amplitude of the pressure feedback : Pressure increase pushes the shock and could help the explosion How does it affect core collapse ? Important effect !!!

15. The magnetic field has an important effect when vA ~ v, even if the magnetic pressure is negligible Alfvén wave amplification at the Alfvén surface creates an important pressure feedback Magnetic effect on SASI depends on the field geometry : ambiguous result Effect of more realistic field geometry ? Multidimensional dynamics at the Alfvén surface? Conclusions Dipolar magnetic field Merci de votreattention!