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J.R. Campbell (Physics & Astronomy Department, Dartmouth College), Kazimierz J. Borkowski, J. Blondin (Physics & Astronomy Department, N.C. State University)
In the simplest model of the expansion of a pulsar wind-blown bubble, the density of the supernova (SN) ejecta is assumed to be spherically symmetric, and thus not contributing to any asymmetries in the bubble itself. However, wind-blown bubbles are observed to be asymmetric. A possible cause of this asymmetry is intrinsic asymmetry in the SN ejecta, because it is now well established that SN explosions are not spherically symmetric. Analytical work has been done to determine how polar asymmetry in SN ejecta would affect the shape of the bubble. Giuliani's thin-shell approximation has been used to model the swept shell enclosing the bubble. The magnetic fields surrounding the pulsar are assumed to be turbulent, and thus not contributing to any asymmetry. At late times it is assumed that the bubble asymptotically attains a self-similar solution over time as it expands into the ejecta. At early times, when the pulsar starts blowing a bubble after an initial delay, the bubble may also be described by a self-similar solution. In both cases the bubble shape is asymmetric and time invariant. Surprisingly, multiple solutions are allowed within the framework of the thin-shell approximation. Hydrodynamical simulations with the VH-1 hydrocode have been performed to corroborate these analytical findings. At late times the Raleigh-Taylor instability in the accelerating bubble reduces its asymmetry much below the analytical predictions. At early times a better agreement is found between the analytical predictions and the simulations. We explore a transition between these two self-similar stages through hydrodynamical simulations.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.