晚期恆星之環星物質的結構研究; Structure of envelopes around late type stars

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题名:	晚期恆星之環星物質的結構研究;Structure of envelopes around late type stars
作者:	邱柏然;Po-Jian Chiu
贡献者:	天文研究所
关键词:	晚期恆星;環星物質;circumstellar envelope;AGB star
日期:	2005-06-27
上传时间:	2009-09-22 10:12:37 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	行星狀星雲或原行星狀星雲中的雙極流以及環星盤等結構的形成機制至今仍不是十分清楚，甚至連這個機制是發生在恆星演化的哪一個階段也不明瞭，因此我們想藉由研究晚期恆星的環星包層（circumstellar envelope）的結構來嘗試找到一些線索。在此篇論文中我們呈現了兩個獨立的研究結果。在第一部份的研究，我們在去年九月利用位在夏威夷（Muna Kea）的次毫米波陣列（SMA）來觀測一個漸進巨星分枝恆星﹙AGB star﹚，π Gru，其環星物質的CO(2-1)分子轉動譜線。從高解析度影像中，我們發現CO譜線在低速度和高速度的部份呈現兩種不同的分佈與速度梯度。我們的解釋為低速度的CO輻射來自於一個正在膨脹的扁平盤狀結構，其盤面與視線方向約有55∘的夾角而膨脹速度約為15 km/s；而高速度的CO輻射則是來自於一個垂直於盤面的高速雙極流，其速度約為30-60 km/s。我們也模擬了盤狀結構的輻射轉移模型來和我們的觀測結果比較，我們發現盤狀模型與我們觀測的結果十分符合。因此我們的觀測發現了第二個有環星盤以及雙極流等結構的的AGB star，也指出這種在行星狀星雲中常見的雙極流及環星盤結構早已存在於AGB star的時期。另外我們利用位於美國新墨西哥的極大天線陣列(VLA)觀測了原行星狀星雲“蛋星雲”中的氨分子的(1,1)，(2,2)和(3,3)反轉譜線。我們發現觀測的結果與前人所提出來的膨脹盤狀模型並不符合；而藉由與文獻中的CO譜線以及氫分子譜線的觀測結果比較，我們認為在蛋星雲中的氨分子譜線應該是來自於被數個高速噴流擠壓並被加熱的分子氣體所發出來的輻射，這也間接加強了蛋星雲中存有多重噴流的模型的可信度，且指出在蛋星雲中此高速噴流支配了整個環星物質的動力學。另外我們也由氨分子譜線的強度比以及光譜形狀的對照來推估分子氣體的物理性質，經過計算發現分子氣體的溫度在兩極區較高，在赤道方向較低；而密度則是在兩極區較低，在赤道方向較高。 We present two observational results of the molecular envelopes around evolved stars. First, we have imaged at high angular resolution the CO (J=2-1) line of pi Gru, a nearby S star, using the Sub-Millimeter Array. We detected CO emission with the very broad velocity width of ~90 km/s. The high resolution images show that the CO traces a flattened envelope expanding at a velocity of ~15 km/s, and a bipolar high velocity outflow with a velocity of up to ~50 km/s perpendicular to the equatorial plane. The morphology of low-velocity CO emission can be reproduced by model consisting of a slow equatorial wind with expanding velocity of 15 km/s and inclined to the line of sight about 55 degree, together with a central cavity of 200 AU in radius. In contrast to the spherical morphology commonly seen in AGB envelopes, the disk-outflow structure observed in pi Gru adds to the increasing evidence that bipolar structures seen in many planetary nebulae and proto-planetary nebulae already exist in the late stage of the AGB phase. The presence of high-velocity outflow in pi Gru also suggest high-velocity plays an important role in the shaping of circumstellar envelopes around post AGB stars. Second, using the Very Large Array, we observed the NH3(1,1), (2,2), (3,3) transitions lines from the Egg Nebula, a famous nearby proto planetary nebula. The morphology and kinematics in three different transitions all shows four distinct lobes align with the polar axis and equatorial direction. Combining with resent H2 and CO line observation suggest that the NH3 emission traces the hot molecular gas behind the shock front which is shocked and heated by the interaction between the high velocity outflows and the surrounding AGB envelope. The velocity gradient along N-S direction in the east lobe can be interpreted as the different velocities in different outflows, which suggest high velocity outflow dominate the kinematics in the circumstellar envelope. Besides, from the NH3 line ratio we suppose that the gas temperature is higher in the bipolar direction which might imply the interaction is much stronger in the bipolar than in equatorial direction.
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