前行星狀星雲 IRAS 17150-3224 的環狀物及噴流研究

Abstract

IRAS 17150-3224 為一顆周遭富含塵埃的雙極前行星狀星雲（Bipolar proto-planetary nebulae）。前行星狀星雲是在類太陽質量恆星演化末 期出現的一個極短現象，介於漸進巨星（Asymptotic giant branch）和行 星狀星雲（Planetary nebulae）之間。在這篇論文中，我們使用次毫米波 陣列（Sub-millimeter Array, SMA），並利用CO J=2-1 輻射線觀測這顆 天體。此次的觀測角解析度約為一角秒（arcsec），並利用光譜觀測得 知系統速度（VLSR）約為15.5 km s−1。從觀測中發現有一環面（torus） 的結構存在於低速帶，並且與雙極的外向流（outflow）的方向呈垂直。 由位置-速度關係圖（Position-Velocity diagram）可得知此環面結構約 以13 km s−1 的速度向外膨脹。為了了解此星體的物理性質，包含：結 構、密度分佈、溫度分佈即速度分佈，利用一個輻射轉移（Radiative transfer）的模型去做擬合。根據光學與SMA 的觀測資訊，在模型中加 入兩種結構：一個環形的恆星包層和一個雙極的外向流。我們的最好 模型顯示出，IRAS 17150-3224 的質量損失率（mass-loss rate）大約是 5.5 × 10−4M⊙ yr−1，約小於Meixner et al. 在2002 年提出的數字（1.2 × 10−3M⊙ yr−1）兩倍。另外，可以從此天體的大小及速度分佈可知此環 面結構已存在將近1600 年，略大於外向流的1000 年。因此認為是恆 星將物質向外噴時，形狀受到環面結構的限制，變成雙極的外向流形 狀。目前還不知道環形結構是如何生成，一般認為可能受到偶極磁場 （dipole magnatic field）又或是雙星系統（binary companion）影響而造 成。而IRAS 17150-3224 在1993 年就被測出有偶極磁場的存在，或許 就是此類形狀的前行星狀星雲的成因。

IRAS 17150-3224 is a bipolar proto-planetary nebula (PPN) with a series of dust shells in optical image. It is one of the best candidates for studying this short-lived phase between the asymptotic giant branch phase and the planetary nebula phase duringthe low- to intermediate-mass stellar evolution. We have mapped it in CO J = 2-1 with the Submillimeter Array (SMA) at 1 ′′ resolution. The observed CO J = 2-1 spectrum shows that the systemic velocity VLSR 15.5 km s−1. At low velocity, a dusty torus is seen in continuum and CO perpendicular to the outflow axis. Judging from the Position-Velocity (PV) diagram, we find that the torus is expanding at 13 km s−1 away from the central star. A radiative transfer model is used to obtain the physical properties of the torus, including the structure, density, temperature, and velocity distributions. Based on the optical and SMA observations, we assume two components in our model: a toroidal envelope and a bipolar outflow. The mass-loss rate of the torus is found to be 5.5 × 10−4 M⊙ yr−1, which is 2 times lower than that found in Meixner et al. (2002). From the size and the speed of the torus, we find that the dynamical age of the torus is 1600 yrs, longer than that of the outflow. We believe that the outflow is produced by an interaction of an underlying mass ejection with the torus. This mass ejection could be collimated, producing the bipolar outflow. The outflow could be confined further in the equatorial plane by the torus. The formation mechanism of the torus is still unclear, and it could be related to the dipole magnetic field detected in this source (Hu et al. 1993) or an orbital motion of an unseen binary companion