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Paper: Formation of Bipolar Planetary Nebulae Due to Accelerated Stellar Winds
Volume: 359, Numerical Modeling of Space Plasma Flows: Astronum-2006
Page: 232
Authors: Washimi, H.; Zank, G.P.; Tanaka, T.; Balick, B.
Abstract: Some of the most spectacular astrophysical objects are planetary nebulae and we describe here one example of global heliospheric modeling applied to the bipolar planetary nebula M2-9. M2-9 is composed of three main components, namely two bright, narrow and highly axis-symmetric bipolar lobes, two faint outer flows along the bipolar lobes, and a bright compact central core. The surrounding matter seems to be much more dilute than the bipolar flow. We assume that the magnetic pressure of the toroidal field in the stellar wind plays a dominant role in forming the bipolar flow along the rotation axis, and we examine this mechanism by using global 3-D MHD simulations. We find that we can explain the M2-9 structure when the magnetic pressure in the stellar wind is comparable to the stellar-wind ram pressure. The stellar wind, which expands in the radial direction, bends poleward with distance from the star due to a magnetic pinch effect, hence the wind density at high latitudes is enhanced and bipolar lobes are formed. The material surrounding the bipolar lobes originates from middle and lower latitudes, which results in the formation of two faint outer flows. The overall structure is a self-consistent consequence of the stellar wind itself, corresponding to two kinds of self-consistent circumstellar gas distributions, one a collimated flow and the other enveloping the collimated flow, thus our model shows a 'solitary stellar-wind' forming in a dilute circumstellar gas density.
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