[论文解读] SiO masers in TX Cam: Simultaneous VLBA observations of two 43 GHz masers at four epochs

Jiyune Yi, Richard A Booth+2 more|ArXiv.org|Nov 24, 2004

Stellar, planetary, and galactic studies参考文献 21被引用 35

一句话总结

本研究对TX Cam在43 GHz频段的v=1和v=2，J=1-0 SiO脉泽进行了四次观测历元的同步VLBA观测，覆盖一个恒星周期。研究揭示了具有系统速度梯度的辐条状特征的环状脉泽结构，表明存在减速的喷流，并发现v=2脉泽形成的半径小于v=1脉泽，且在空间和速度上存在显著重叠，暗示非辐射泵浦机制的存在。

ABSTRACT

We present the results of simultaneous high resolution observations of v=1 and v=2, J=1-0 SiO masers toward TX Cam at four epochs covering a stellar cycle. Near maser maximum (Epochs III and IV), the individual components of both masers are distributed in ring-like structures but the ring is severely disrupted near stellar maser minimum (Epochs I and II). In Epochs III and IV there is a large overlap between the radii at which the two maser transitions occur. However in both epochs the average radius of the v=2 maser ring is smaller than for the v=1 maser ring, the difference being larger for Epoch IV. The observed relative ring radii in the two transitions, and the trends on the ring thickness, are close to those predicted by the model of Humphreys et al. (\cite{humphreys02}). In many individual features there is an almost exact overlap in space and velocity of emission from the two transitions, arguing against pure radiative pumping. At both Epochs III and IV in many spectral features only 50% of the flux density is recovered in our images, implying significant smooth maser structure. For both transitions we find that red- and blue-shifted masers occur in all parts of the rings, with relatively few masers at the systemic velocity. Thus there is no evidence for rotation, although the blue-shifted masers are somewhat more prominent to the west. At all four epochs red-shifted components are generally brighter than blue-shifted ones. At Epochs III and IV, we see many filamentary or spoke-like features in both v=1 and v=2 masers, especially in the red-shifted gas. These spokes show systematic velocity gradients consistent with a decelerating outward flow with increasing radius. We outline a possible model to explain why, given the presence of these spokes, there is a deficit of maser features at the systemic velocity.

研究动机与目标

• 研究TX Cam在恒星周期不同相位下的43 GHz SiO脉泽的空间与运动学结构。
• 确定v=1和v=2脉泽跃迁的相对半径与形态，并评估其与理论模型的一致性。
• 检查红移和蓝移脉泽的分布，检验是否存在旋转或非对称喷流的证据。
• 通过形态与运动学分析，探讨系统速度脉泽特征缺失的成因。
• 通过比较两种脉泽跃迁在空间与速度上的重叠情况，评估辐射泵浦与非辐射泵浦机制的作用。

提出的方法

• 在43 GHz频段进行同步甚长基 baseline 干涉仪（VLBA）观测，以解析TX Cam中v=1和v=2 SiO脉泽跃迁。
• 在恒星周期内选择四个历元进行观测：两个接近脉泽最大值（历元III和IV），两个接近最小值（历元I和II）。
• 采用高角分辨率干涉测量技术，绘制脉泽特征的空间分布与径向速度结构。
• 分析脉泽特征的形态，识别出环状结构、辐条及速度梯度。
• 将观测到的脉泽半径与径向速度分布与Humphreys等人（2002）提出的脉动喷流模型预测结果进行比较。
• 评估图像中的流量恢复情况，以推断是否存在平滑、未解析的脉泽发射结构。

实验结果

研究问题

• RQ1 在TX Cam的恒星周期中，v=1和v=2 SiO脉泽的空间分布如何演化？
• RQ2 v=1和v=2脉泽环的相对径向范围如何？与理论模型相比有何差异？
• RQ3 两种脉泽跃迁在位置和速度上的重叠程度如何？这对泵浦机制有何含义？
• RQ4 为何在存在辐条状结构的情况下，系统速度附近的脉泽特征出现缺失？
• RQ5 在脉泽辐射气体中是否存在旋转运动或非对称喷流的证据？

主要发现

• 在脉泽最大值附近（历元III和IV），v=1和v=2脉泽均形成环状结构，但v=2环始终位于比v=1环更小的半径处，且在历元IV中该差异进一步增大。
• 观测到的相对环半径及环厚度变化趋势与Humphreys等人（2002）提出的脉动喷流模型预测结果高度一致。
• 在许多单个特征中，两种脉泽跃迁的发射在空间和速度上几乎完全重叠，这与纯辐射泵浦机制相矛盾。
• 在历元III和IV中，约50%的总通量密度未在图像中恢复，表明存在显著的平滑、未解析的脉泽结构。
• 红移和蓝移脉泽在环的所有区域均存在，无明显旋转证据，尽管蓝移脉泽在西侧略为显著。
• 两种跃迁中均观测到丝状或辐条状特征，尤其在红移气体中更为明显，其系统速度梯度与减速向外流动一致。