Multidimensional diffusive shock acceleration in the Winds from massive stars

Abstract

Nonthermal radio emission has been seen in the winds around a quarter of all O-stars. The emission is attributed to shock accelerated cosmic rays. The shocks thought to be causing the acceleration are either wind-embedded shocks due to radiative line driving instabilities in the stellar wind, or shocks due to the colliding winds in a binary system. Very few numerical Diffusive Shock Acceleration (DSA) simulations exist for these systems due to the complicated, multidimensional nature of the winds. We present the first 2-D magnetohydrodynamic DSA (MHD-DSA) simulations of massive stellar winds using the Multidimensional Adaptive Subcycling Tridiagonal solver (MAST), which has been incorporated into the WOMBAT (sWift Objects for Mhd BAsed on Tvd) code to solve diffusive shock acceleration for cosmic rays. Shock modification due to cosmic ray pressure is shown to be important in describing the shock dynamics of the colliding wind binary scenario. With 0.01% of the gas particles passing through the shocks being injected as cosmic rays, about 15 % of the wind ram pressure is converted into cosmic ray pressure. In the wind-embedded shock scenario, the isothermal conditions in the wind, due to radiative heating and cooling, precluded inclusion of cosmic ray feedback. Future 1-D simulations of cosmic ray modified radiative shocks are suggested, as the combined effects of radiative line cooling and cosmic ray feedback dramatically change the shock dynamics from adiabatic analogues. Both cases show efficient cosmic ray acceleration. In the case of the wind-embedded shocks, the isothermal nature of the wind creates shocks capable of accelerating electrons up to 100 MeV and protons up to 1 GeV with a spectral slope of 4. The colliding wind binary scenario produces very strong shocks which are capable of accelerating electrons up 1 GeV and protons up to 1 TeV with a spectral slope of 4. While full radiation models will be performed in the future, preliminary estimates indicate that the radio emission from the wind-embedded shock scenario may be extinguished due to free-free absorption. This would exclude the wind-embedded shock scenario from being able to explain the observed radio emission.