原恆星 L1157-mm 和 L1448C 的次毫米波觀測

Abstract

我們利用次毫米波陣列 (SMA) 觀測兩個低質量第零類原恆星: L1157-mm 和 L1448C 的分子譜線及塵埃連續輻射訊號。觀測的角解析度約為3' ~ 5'，相當於 650 ~ 1500 AU。 12CO (2-1)、 13CO (2-1)、C18O (2-1)、CS (7-6) 和 H2CO (5 -4 )是在兩個原恆星都偵測到的分子譜線。低速的 12CO (2-1) 譜線顯示靠近原恆星處的噴流瓣呈V 型，其張角約為30° ~ 40°。低速噴流的型態和運動學特徵可以用大張角原恆星風的模型解釋 (Wide Opening-Angle Wind)。除了V 型噴流瓣，較準直且極高速的噴流現象 (EHV) 也在L1448C 原恆星處發現，另外 SiO (8-7) 和 SO (5 -4 ) 兩個譜線也偵測到極高速噴流。SiO 和 SO 的出現顯示在柱狀噴流中有震波 (Shock) 的存在，也代表此處是噴流內部交互作用發生的介面。而L1157 的噴流中， 12CO 譜線在南邊噴流瓣的東側和北邊噴流瓣的西側強度(Intensity)較強。這代表了L1157 最近的質量噴發是沿著西北-東南的方向。這個近期質量噴發現象的位置角可以用噴發軸的進動模型 (Precession model) 來解釋 (進動角度15°)。 C18O (2-1)、CS (7-6) 和 H2CO (5 -4 ) 譜線較為緻密且圍繞在原恆星附近。在L1157 和L1448C 周圍的緻密氣體雲受到噴流的影響相當大。L1448C 的這三個譜線都有延著噴流方向的延展結構，其速度梯度趨勢也與 12CO相同。L1157的C18O譜線也有相同於噴流的速度梯度，然而CS 和 H2CO 譜線則出現相反的速度梯度。L1157 周圍緻密氣體雲特殊的速度結構可解釋為靠近原恆星的氣體雲有一大張角的空腔 (cavity)。我們的結果也支持了噴流會對原恆星週遭物理環境產生極大的影響。另外，兩個原恆星的C18O 譜線也有延著垂直於噴流方向的速度梯度，代表原恆星週遭氣體雲可能也在旋轉。

We present molecular lines and continuum images of two low-mass class 0 protostars, L1157mm and L1448C obtained by the Submillimeter Array (SMA). The angular resolution was 3′′ ∼ 5′′, which corresponds to 650 ∼ 1500 AU. The 12CO(2-1), 13CO(2-1), C18O(2-1), CS (7-6) and H2CO (51;5-41;4) lines were detected in both sources. The 12CO emission with low velocity ( V ≤ ± 6 km s−1 in L1157,and ≤ ± 20 km s−1 in L1448C) show that the outflow lobes near the protostars have a V-shaped structure with an opening angle of 30◦ ∼40◦. The morphologies and kinematics of the low velocity shells are reproduced by the model of wide-opening angle wind. In addition to the V-shaped lobes, a narrow jet with extremely high velocity (EHV) was observed in the L1448C outflow. The EHV jet in the L1448C outflow was also traced by the SiO (8-7) and SO (56-45) lines. The appearance of this EHV jet in SiO and SO suggests that there are shocks in the jet beam, representing the internal working surfaces. In the L1157 outflow, the 12CO intensity is significantly enhanced in the eastern wall of the southern lobe and the western wall of the northern lobe. This suggests that the recent mass ejection of the L1157 outflow is in the NW-SE direction. The position angle of this recent ejection event can be explained by the precession model with a precession angle of 15◦. The C18O(2-1), CS (7-6) and H2CO (51;5-41;4) emission show compact structures around the central stars. These dense gas envelopes around L1157-mm and L1448C are strongly affected by the outflow. In L1448C, these lines show elongated structures along the outflow axis, and velocity gradients with the same sense of 12CO outflow. In L1157, the C18O shows the same velocity gradient as the outflow, while the CS and H2CO exhibit the opposite velocity gradient. The kinematic structure of the L1157 envelope is explained if the envelope had cavities with wide opening angles at their bases. Our results support the idea that the outflows play key roles in changing the physical condition of circumstellar envelopes. On the other hand, the C18O shows a velocity gradient that is perpendicular to the outflow axis in both sources, suggesting that the envelopes around the protostars are rotating. The estimated specific angular momenta are 2.17 × 10−4 km s−1 pc and 5.5 × 10−3 km s−1 pc for L1157 and L1448C, respectively.