原恆星噴流與其團塊:形成理論及案例研究

Abstract

雙極噴流在恆星演化前期佔有非常重要的地位，因為他們會帶走吸積盤的角 動量，讓物質可以吸積進原恆星。他們從非常靠近原恆星的地方發射出來，但我們 對於他們的發射機制還不是很了解。目前有兩個模型來研究原恆星吸積的情況:X 風模型和盤面風模型。我們可以從噴流的形態學(morphology)和動力學 (kinematics)來比較兩個模型的優缺點，尤其是當這些噴流有形成團塊結構 (knotty structure)時，就可以從這些團塊推敲出噴流是如何形成的。 本篇論文的研究對象主要是原恆星系統中的噴流與其中的團塊。論文的第一 部份，主要的觀測對象是 HH 211 的系統，並利用阿塔卡瑪大型毫米及次毫米波陣 列(ALMA)作觀測。HH 211是一個雙極噴流，並有明顯的團塊結構，是一個研究噴 流特性及團塊形成裡，非常好的選擇。而利用 ALMA 高解析度的觀測，我探討噴流 的擺動結構(wiggle structure)，並在雙星系統(binary system)中，利用環繞 軌道運行噴流源模型 (orbiting source jet model) 來進行模擬，結果和之前所 得到的結論相同。我也同時分析此系統團塊結構(knotty structure)及首震波 (bow shock)，且在觀測結果顯示:這種團塊的結構來自於原恆星系統在噴出物質 時，噴出的速度會呈現週期性的變化，這種週期性變化會讓後方速度較快的物質， 追撞到前方速度較慢的物質，而形成內衝擊波(internal shock)。再利用不同年代 的觀測結果，來研究團塊的自行運動(proper motion)，依此得到噴流的速度及原 恆星系統的質量損失率(mass-loss rate)。除了觀測到此種現象外，我另外還架 構出一個簡單的動力模型來模擬內衝擊波的物理結構:這個動力模型包含了前衝 擊波(forward shock)和後衝擊波(backward shock)，也加入了內衝擊波，並應用在觀測中得到的一個完整解析的團塊(Knot BK3)。也因為如此高解析度的觀測， 我可以更深入探究一氧化矽(SiO)、一氧化碳(CO)、一氧化硫(SO)在團塊裡的分佈 情況，發現到不同物質會各自形成洋蔥般的層狀結構，這是因為不同層有不同的密 度，而不同密度會有不同物質被激發、發出光線，進而被觀測到，也可以推敲出團 塊在不同的位置有不同的衝擊波力道。 另外，我也用 ALMASOP 這個觀測計畫的數據，這個觀測計畫包含了 37 個一氧 化碳的外流，其中有 20 個擁有高速度的一氧化矽的噴流。我們挑了 6 個特定的噴 流來研究，這些噴流都有一長串的團塊，且每個團塊間的距離大約相同。在這些狀 態下的噴流，我可以利用衝擊波形成模型(shock-forming model)，來模擬後方速 度較快的物質，追撞到前方速度較慢的物質，而形成內衝擊波(internal shock)， 於此來計算各個噴流的速度(jet velocity)和系統的傾角(inclination angle)， 而這個方法所得到的傾角跟同一天體的外流(outflow)的觀測及廣角風驅動模型 所推算的結果一致。另外，我也把衝擊波形成模型套用在其他天體(HH 211、HH 212 及 L1448C(N))上，模型得到的參數數據(噴流速度和傾角)也符合觀測結果。 這些案例顯示了:噴流速度的確呈現週期性的變化，且這類的變化可以形成一長串 的團塊。 未來，我會持續觀測並研究 HH 211，利用 ALMA 更高解析度的觀測，來尋找可 能的噴流的旋轉，並計算噴流噴出的位置。我也計劃利用ALMA得到的HH 111 及 HH 212 觀測資料，來研究他們的外流是風驅動還是噴流驅動。

Protostellar jets play an important role in star formation, carrying away an- gular momenta from the accretion disks, allowing the disk material to feed the central protostars. They are believed to be launched from the innermost part of the accretion disks around the protostars. Currently, two compet- ing MHD models, the X-wind and disk-wind models, are proposed to launch them. Here we study the physical properties (e.g., morphology and kinemat- ics) of the jets in order to constrain their launching mechanism and determine which model is better. In particular, since most of the jets have knotty struc- tures (knots), studying the formation of the knots may help us to constrain where the jets are launched, e.g., the launching radius. In my thesis, I studied protostellar jets in star formation. In my first study, I focused on the well-studied jet HH 211, which consists of a chain of well-defined knots and thus is a good candidate for studying the physical properties of the jet and the formation of the knots. With the high-resolution observations of Atacama Large Millimeter/submillimeter Array (ALMA), I investigated the wiggle of the jet and modeled it with an orbital jet source model in a binary system. I analyzed the knotty structures and the bow- shock structures in the jet, and found them to be internal shocks produced by quasi-periodical variations in ejections as the fast jet material catches up with the slow jet material. I also measured the proper motion of the jet with multi-epoch observations, derived the jet velocity and revised the mass-loss rate. These jet properties help me to constrain the launching mechanism, e.g.,the launching radius. Thanks to the high-resolution observations of ALMA, I uncovered the onion-like morphological relationship among the SiO, CO, and SO emission structures in a spatially resolved knot. These molecules form layers and trace different densities in the knot, likely because they trace different shock strengths in the knot. In my second study, I used the data from ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP). This survey detected a total of 37 sources with CO outflows, and 20 of them also showing high-velocity jets in SiO. In my study, I selected 6 of them, which have a chain of knots with equal spacing and well-defined jet and outflow structures. Then, I adopted a shock-forming model, in which the knots are considered as internal shocks formed as the fast jet material catches up with the slow jet material, to cal- culate the velocity and inclination angle of these jets. The inclination angles of these jets are broadly consistent with those of their associated outflows derived from the wide-angle wind-driven shell model. Furthermore, I ap- plied the shock-forming model to other jet sources - HH 211, HH 212, and L1448C(N)- and the results, such as inclination angle and jet velocity, are also consistent with those derived from proper motion and radial velocity measurements. These consistencies support that the jets indeed have quasi- periodical variations in their velocity, and these variations can produce a chain of knots in them. Such velocity variations are likely formed by period- ical variations in the ejection velocity and possible origins are discussed. In the future, with ALMA, I will continuously study HH 211 with even higher resolution to search for a rotation in this jet in order to constrain the launching radius. Also, I will study the protostellar outflows in HH 111 and HH 212 to find out whether they are wind- or jet-driven, and compare their driving mechanisms.