年輕分子噴流：理論模型與觀測

Abstract

源自第零類原恆星系統的分子外流攜帶著關於早期恆星形成過程的訊息。這些年輕外流的特徵是之一是具有「極高速噴流」和「低速殼層」的雙重組成，而此構成可以自然地用整合風模型（Shang et al. 2006）解釋。在此模型中，高速噴流的部分是源自磁化風自身在軸上的高密度分佈，而低速殼層的部分則是周圍被風推開的物質積聚而成。在這個基本框架之下，我們利用數值模擬實驗來增進理解並且計算分子發射譜線來更真實的比較模擬結果與觀測。 為了探討高溫的風在低溫環境中的傳播，我們在模擬中追蹤源自風的物質的分佈並依此進行「雙溫度」的模擬。結果顯示當磁場夠強時，即使在高溫之下高速噴流的部分仍可以維持其準直型態，否則將會因為熱壓力的作用而變得彌散。我們也發現周圍包層物質中的極向磁場能幫助抑制風與環境交互作用邊界上不穩定性的發展，進而造成較平直的低速殼層。 我們計算分子發射譜線的合成影象、位置-速度表、以及光譜，而 CO J=2-1 譜線的影像清楚的表現出極高速噴流和低速殼層的雙組成。模型的質量-速度分佈表之冪次法則指數大約落在 1 到 3 之間，我們發現其大小和風的磁場強度有所關聯。 我們分析在第零類原恆星系統 IRAS 04166+2706 噴流中觀測到的特殊鋸齒狀速度分佈，並且提出以球狀風的速度場和軸上高密度物質分佈的方式來解釋其成因。在此假設之下鋸齒特徵的斜率會自然地隨距離改變，正如觀測結果所示。當假設分子外流與視線的夾角為 52 度時，模型預側的速度分佈與觀測基本符合。

Molecular outflows associated with the youngest Class 0 protostars can bear clues of the early protostellar systems in the star formation process. These young outflows are characterized by dual components of extremely high velocity jets and low-velocity cavities, which are naturally understood in context of the unified wind model of Shang et al. 2006. The jet and cavity features are associated with the intrinsic density concentration of the magnetized primary wind and the swept-up ambient gas, respectively. Based on this framework, we develop understanding through numerical experiments and construct synthetic line emissions to bridge the gap between theory and observation. By using a wind tracer field, we employ a two-temperature scheme to study the problem of a warm wind running into a cold ambient. Our exploration shows that the jet can be well maintained even at a high temperature for a sufficiently magnetized wind, but can be otherwise diffused. We also find that the presence of poloidal magnetic field in the ambient mass can help suppress instabilities at the wind-ambient interface to produce a less corrugated shell boundary. Synthetic images, position-velocity diagrams, and spectra for molecular transition lines are presented, and the dual jet and shell components are clearly seen in CO J=2-1. The model power-law spectral index γ of the mass-velocity relation (m ~ v^-γ) falls in the range of ~1 to 3, and a dependency on the wind magnetization is revealed. We analyze the observed sawtooth-like velocity pattern of the Class 0 IRAS 04166+2706 outflow and propose that a spherical wind-like velocity field with mass concentration near axis could underly the pattern. The systematic change of the teeth slope with distance is a natural consequence in this case, and the overall pattern is consistently explained with an inclination angle of ~52 degree.