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.
中文摘要 iii
英文摘要 iv
致謝 v
圖片列表 vii
表格列表 vii
一、 序論 1
1-1 行星狀星雲的演化 1
1-2 行星狀星雲的分類 2
1-3 長條狀行星狀星雲與雙星系統 3
二、 模擬程式與行星狀星雲模型 5
2-1 模擬程式：FLASH 5
2-2 行星狀星雲模型 6
2-2-1 甜甜圈模型（Torus） 8
2-2-2 AGB星及質量流失率 10
2-3 邊界條件 10
2-4 單位 11
三、 模擬測試 12
3-1. 恆星 12
3-2. 甜甜圈模型（Torus） 13
3-3. 解析度 14
3-4. 質量流失率（Mass-loss rate） 17
3-5. 模擬區域切半 18
3-6. 星際介質（Interstellar Medium，ISM）密度 20
四、 結果與討論 22
4-1. 環的影響 22
4-2. 比較甜甜圈模型的密度 25
4-3. 和模擬結果與觀測比較 31
五、 結論 34
六、 參考文獻 36
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