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Title: | 高質量恆星形成區域的磁場演化 Evolution of Magnetic Fields in High Mass Star Formation |
Authors: | Ya-Wen Tang 湯雅雯 |
Advisor: | 賀曾樸(Paul T . P. Ho) |
Keyword: | 星際間介質: 天體 W51 e2/e8,G5.89-0.39,W51 North,MMS 6,Orion BN/KL,星際間介質: 磁場,偏極化,恆星: 形成, ISM: individual (W51 e2/e8,G5.89-0.39,W51 North,MMS 6,Orion BN/KL),ISM: magnetic fields,polarization,stars: formation, |
Publication Year : | 2009 |
Degree: | 博士 |
Abstract: | 在此博士論文中,我分析磁場在高質量恆星形成區域所扮演的角色。
一般認為,磁場在恆星形成過程中扮演很重要的角色。它提供支持的力量,使得分子雲可以緩慢地塌縮,因此可以解決僅有極少比例的分子雲形成恆星的問題。另外,它可以重新分布磁通量以及轉動慣量,使得物質可以掉入中心的原恆星。然而,一直到近十年才開始有高解析度(約數角秒)的磁力線型態、而且極為有限的歸測資料。其中,次毫米波陣列是目前唯一可以偵測毫米與次毫米波段的偏極化光的陣列。這波段對恆星形成區域的塵埃輻射特別敏感。藉由觀測塵埃的熱輻射中的偏極化光,我們可以推得磁場投影的天球上的磁力線型態。利用次毫米波陣列,我研究四個在不同演化階段的高恆星形成區域:超緻密氫離子區 G5.89-0.39,塌縮原恆星核 W51 e2/e8 以及 W51 North,以及離我們最近的高質量恆星形成區域 Orion BN/KL。此外,獵戶座分子雲3 (OMC-3)中最亮的高密度核 MMS 6也是本論文的研究天體。 根據所分析的分子雲的運動狀態與所觀測到的複雜磁力線型態,超緻密氫離子區天體G5.89-0.39的磁場明顯的受到擾動。比較此天體中所存在的輻射壓力、氣體的力學能密度與磁壓力,我們估計輻射壓在靠近中心的高質量恆星位置的地方,有足夠的能量密度可以擾動磁場與推動附近的分子雲。在演化更早期的天體W51 e2/e8區域,我們看到被吸積盤面拉扯的磁力線型態。而在此塌縮核以外的分子雲氣中,磁場呈現非常規則的狀態。此結果傾向支持磁場主導的分裂過程。比較這兩個處於不同演化狀態的天體G5.89-0.39及W51 e2/e8,觀測所看到的磁力線分布型態,取決於恆星形成區域的演化狀態。若是中心的恆星已經釋放出足夠的能量,並且週遭的分子雲已經明顯的受到擾動,則所觀測到的磁力線可能極為複雜。而在演化比較早期的階段,磁力顯然扮演極為重要的角色—它主導分子雲氣演化到塌縮核的過程。而在演化最早期的天體MMS 6,磁通量明顯的比其他天體還低。此外,沒有更小尺度的磁力線型態存在,顯示獵戶座分子雲3的磁力線型態是非常均勻、規則的。因為磁場的投影影像容易受到投影作用的影響,我們需要更多的塌縮核的磁力線觀測數據。塌縮核W51 North磁力線已經受到分化過程的影響,其中大部分的高密度核中的磁力線方向約略一致,然而在密度最高的區域,磁力線明顯地受到規則地拉扯。我們需要一些數值模擬來解釋在這個區域所看到的磁力線型態。最後,觀測較近的高質量恆星形成區域的磁力線可以提供磁場在較小尺度的資訊。根據我們所觀測到Orion BN/KL天體內的磁力線型態,我們發現在以往所看到的較大尺度(約0.3 pc)均勻的磁場中,磁力線在比較小尺度內呈現明顯的規則、且較複雜的磁力線型態。在Orion BN/KL中,根據先前的的氨氣觀測數據顯示,此區域的分子雲處於非重力束縛態。此外,磁場方向有規則地變化方向。因此,我們推測所觀測到的磁場型態極可能是殘留的偽吸積盤面。 總結,在本論文中,我發現磁場所扮演的角色隨著恆星形成區域的演化階段而改變。高解析度的磁場型態資料、分析分子雲內的氣體運動狀態、以及比較小尺度與大尺度的磁場型態對研究恆星形成區域中的磁場極為重要。 In this thesis, I study the role of Magnetic (B) fields in the massive star forming process. It has been suggested that B field plays a key role in the star formation process - it sustains the molecular cloud from collapsing rapidly and helps to redistributes the flux density and angular momentum via ambipolar diffusion. However, there are limited measurements of B field strengths and also field morphologies, due to the weak signal and the limitation of instruments. The measurements of B-field morphologies associated with star-forming cores with high angular resolution, ~ a few arcseconds, are only available since recent decades. With the Submillimeter Array, the B field morphologies projected in the plane of sky (B_bot) are traced by mapping the thermal continuum emission of the dust grains at wavelengths of 870 micron. I study four massive star star forming regions in various evolutionary stages: the collapsing core W51 e2/e8 and W51 North:dust, the Ultracompact H II (UC H II) region G5.89-0.39, and the closest massive star forming site Orion BN/KL. The source in the earliest evolutionary stage, the dense core MMS 6 in OMC-3, is also observed. As inferred from the gas kinematics and the complicated B field morphology, the B field in G5.89 is most likely been overwhelmed by the stellar feedbacks, such as expansion of the UC H II region and the molecular outflows. While in the collapsing core W51 e2/e8, the hourglass-like B field associated with e2 seems to be located in a subcritical envelope at a scale of 0.5 pc, suggesting that the B field plays a dominant role in the formation process of the star-forming cores. The field geometry in W51 North:dust is complex but organized, correlated with the fragmentation and the rotation of the flattened structure. B field in Orion BN/KL shows a part of the larger scale (0.5 pc) hourglass morphology. In MMS 6, no smaller B field structure is detected, suggesting that the field is relatively uniform across the OMC-3 filament. In this thesis, I conclude that the role of the B field varies with the evolutionary stages of the central stars. The high angular resolution B field map is crucial when study the role of the B field in the star forming region. To understand the role of the B field, kinematics of the molecular cloud and linking the field geometry with larger scale field are necessities. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/9354 |
Fulltext Rights: | 同意授權(全球公開) |
Appears in Collections: | 物理學系 |
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