金牛座T型星噴流之可見光與紅外光波段研究

Abstract

從恆星系統產生的噴流與風是年輕恆星成長必經的過程。在各種不 同質量與不同年齡的恆星之間,金牛座 T 型星(質量約介於 0.08 至 2 倍太陽質量及年齡介於數十萬至一百萬年的年輕恆星)產生的噴流與 風,因其在可見光與紅外光波段的明亮線光譜,而得以最早獲得學界 注意,並隨之展開大規模的研究。此階段的年輕恆星因為周圍環星包 層在恆星形成過程中漸趨稀薄而首次顯露,而其鄰近的噴流形成區域 才可能得以直接被觀測。 觀測技術的日新月異革新了學界對金牛座 T 型星噴流與風的了解。 來自哈伯太空望遠鏡與配備調適性光學的八米等級地面望遠鏡的觀測 資料,具備比 1 角秒更精細的解析能力,讓學界得以更了解噴流與風 在數十天文單位尺度下的結構。高速度解析度的光譜儀,具備每秒數 十公里的解析能力,得以描述線光譜之譜線形,借此分析噴流的動力 學細節。憑藉這些觀測資料的細膩度與精準度,許多以往有關金牛座 T 型星噴流難解的謎題,開始有了新的討論與解決的方向。 本篇論文,結合可見光與紅外光波段光譜與光度測量,討論以下四 項在金牛座 T 型星噴流研究中之重要議題:(一)雙極噴流中,兩極之 速度與物理性質的不對稱性;(二)噴流與風之動力學結構研究;(三) 噴流之游離、激發、與冷卻機制;(四)恆星、拱星盤、與噴流產生交互 作用區域之磁場結構。藉由哈伯太空望遠鏡(Hubble Space Telescope) 與地面大型望遠鏡,包括昴星望遠鏡(Subaru Telescope)與甚大望遠 鏡(Very Large Telescope)的觀測,輔以 X-風模型計算而得的合成光 譜模型(如 Shang et al. 2002, 2010),本論文欲對這些議題提供與文獻 資料中不同角度觀點的解釋。

Jets and winds emerging from young stars are ubiquitous during their formation. Among stars with different masses and ages, jets and winds from T Tauri Stars (stars of ∼ 0.08 to ∼ 2 solar masses with ages from ∼ 10^5 to ∼ 10^6 years) are the first known and most extensively studied because they are bright in line emission in the optical and near- infrared wavelengths. The T Tauri phase marks the first direct appearance of the star from which the surrounding envelope is dissipating, which enables direct observations toward the regions where the jets and winds are driven. Recent development of observational instruments has revolution- ized our understanding to T Tauri jets and outflows. With sub-arcsecond resolution observations obtained by the Hubble Space Telescope (HST) and large (> 8 m) ground-based telescopes aided with adaptive optics (AO), the structures of the jets and outflows are revealed down to a few tens of astronomical units (AU). High-dispersion spectrographs reach the velocity resolution of 10 to 50 km/s that resolves the line profiles and enables detailed kinematics studies of jets and winds. Detail information obtained from these observations confirms and at the same time shed light on solutions to some well-known puzzles in the research of T Tauri jets. In the thesis, the following aspects of T Tauri jets and winds are ad- dressed through spectroimaging studies: (1) asymmetries in velocities and physical conditions between the two sides of a bipolar outflow; (2) kinematic structure of the jets and winds; (3) ionization, excitation, and cooling of the jet; and (4) magnetic field structure of the region where field lines from the star, disk, and jet interact. Using archival spectroimaging data obtained from the HST and large ground-based telescopes, including the Subaru Telescope and the Very Large Telescope (VLT), aided with synthetic spectra calculated through the magnetocentrifugal X-wind models (e.g., Shang et al. 2002, 2010), these aspects are consistently viewed at a different angle.