| dc.description.abstract | Planetary nebulae can be classified into round, elliptical, bipolar, and irregular. We are especially interested in bipolar planetary nebula. It is conceivable that the stellar wind from the star should be spherical. However, from observations the morphology of most planetary nebulae is bipolar not spherical. There are two main reasons for planetary nebula to have bipolar structure: one is binary system, and the other is magnetic field. We would like to understand the formation of bipolar planetary nebula, and we use numerical simulation for our study. We choose the magnetohydrodynamic simulation code FLASH as our main tool. In this thesis, we focus on binary system as the major cause of bipolar structure in planetary nebula (in particular in proto-planetary nebula). We assume that at the end of stellar evolution, the interaction of the stellar wind from the star with the companion star would form a torus around the star. The torus impedes outflow in the equatorial direction. Thus subsequent stellar wind prefers polar direction. As a result, bipolar planetary nebula (proto-planetary nebula) is formed.
Our model involves a gas torus surrounding the central star. The star pulsate periodically and stellar wind bursts at each pulsation. The whole system is embedded in a low density uniform interstellar medium. We study the interaction of the stellar wind with the torus using numerical simulations, and examine the proposition that torus in the binary system is the cause of bipolar (proto-)planetary nebula.
From our simulation result, the gas torus is able to impede the stellar wind, and the shape of the (proto-)planetary nebula is bipolar. We also investigate the effect of the density of the gas torus on the result. As expected, when the density of the tours is lower, it becomes more difficult to hinder the stellar wind, and it may be totally disrupted if its density is low enough. With these simulations, we successfully confirm that gas torus can be one of the main reasons for the formation of bipolar morphology of (proto-)planetary nebulae.
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