[论文解读] Cometary shaped sources at the Galactic Center - Evidence for a wind from the central 0.2 pc

K. Mužić, A. Eckart+3 more|arXiv (Cornell University)|Jun 4, 2010

Astrophysical Phenomena and Observations参考文献 42被引用 34

一句话总结

该论文提供了证据表明，银河系中心附近天体X3和X7的彗星状结构是由来自银心的高速、准直喷流形成的弓形激波，而非仅由恒星风造成。建模显示，该喷流必须起源于大质量恒星群或Sgr A*本身，其动量通量与半径依赖的、低效的吸积流一致，该吸积流以高速抛射物质，从而解释了激波形态的错位及高速自行运动（约300 km s⁻¹）。

ABSTRACT

In 2007 we reported two cometary shaped sources in the vicinity of Sgr A* (0.8" and 3.4" projected distance), named X7 and X3. The symmetry axes of the two sources are aligned to within 5 degrees in the plane of the sky and the tips of their bow-shocks point towards Sgr A*. Our measurements show that the proper motion vectors of both features are pointing in directions more than 45 deg away from the line that connects them with Sgr A*. This misalignment of the bow-shock symmetry axes and their proper motion vectors, combined with the high proper motion velocities of several 100 km/s, suggest that the bow-shocks must be produced by an interaction with some external fast wind, possibly coming from Sgr A*, or stars in its vicinity. We have developed a bow-shock model to fit the observed morphology and constrain the source of the external wind. The result of our modeling allows us to estimate the velocity of the external wind, making sure that all likely stellar types of the bow-shock stars are considered. We show that neither of the two bow-shocks (one of which is clearly associated with a stellar source) can be produced by influence of a stellar wind of a single mass-losing star in the central parsec. Instead, an outflow carrying a momentum comparable to the one contributed by the ensemble of the massive young stars, can drive shock velocities capable of producing the observed cometary features. We argue that a collimated outflow arising perpendicular to the plane of the clockwise rotating stars (CWS), can easily account for the two features and the mini-cavity. However, the collective wind from the CWS has a scale of >10''. The presence of a strong, mass-loaded outbound wind at projected distances from Sgr A* of <1'' is in fact in agreement with models that predict a highly inefficient accretion onto the central black hole due to a strongly radius dependent accretion flow.

研究动机与目标

• 解释X3和X7的弓形激波形态与自行运动矢量不一致的原因。
• 确定观测到的特征是否可由单一恒星风产生，或需要外部喷流的共同作用。
• 约束塑造彗星状特征的外部喷流的物理属性。
• 在Sgr A*低效吸积的背景下，探讨喷流的起源及其与小型空腔的关系。
• 评估喷流是由中心黑洞还是顺时针旋转的恒星团（CWS）驱动的。

提出的方法

• 利用VLT/NACO仪器在L’波段（3.8 μm）进行多历元成像，测量2002年至2008年共7个历元中X3和X7的自行运动。
• 通过弓形激波建模拟观测到的形态并估算脱离距离，采用高速喷流与介质相互作用的流体动力学原理。
• 通过将观测到的激波几何结构与外部喷流产生的弓形激波理论模型对比，计算喷流速度和动量通量。
• 通过考虑中心秒差距内所有可能的恒星类型（包括沃尔夫-拉叶星和[WC]型星），评估可能的恒星风源。
• 将喷流的空间尺度（≤1 角秒）与顺时针旋转恒星团（CWS）的集体喷流尺度（约10 角秒）进行比较。
• 结合半径依赖的吸积流理论模型，评估其与观测到的质量损失率（约10⁻³ M⊙ yr⁻¹）和喷流抛射的一致性。

实验结果

研究问题

• RQ1 X3和X7的观测弓形激波形态能否由单一恒星风解释，还是必须依赖外部喷流？
• RQ2 能够产生观测到的激波特征的外部喷流的速度和动量通量是多少？
• RQ3 该喷流是否与中心黑洞Sgr A*或顺时针旋转恒星团中大质量恒星的集体喷流一致？
• RQ4 X3、X7与小型空腔的空间对齐关系，是否支持其起源于同一准直喷流？
• RQ5 在Sgr A*附近投影距离小于1 角秒处存在强且富含质量的喷流，是否与低效吸积的理论模型一致？

主要发现

• X3和X7的对称轴彼此之间仅相差5°，且均指向Sgr A*，但其自行运动矢量之间的夹角超过45°，表明存在外部喷流，而非在静态介质中运动。
• 这些特征的自行速度约为300 km s⁻¹，过高，无法用单颗恒星在静态星际介质中运动来解释。
• X3和X7均无法由单个质量损失恒星形成；所需喷流动量远超中心秒差距内任何单颗恒星所能提供的动量。
• 观测到的弓形激波需要一个集体喷流，其动量通量与中心秒差距内全部大质量年轻恒星群的总动量通量相当。
• 垂直于顺时针旋转恒星群平面的准直喷流（喷流张角约30°）可同时解释X3、X7和小型空腔。
• 在Sgr A*投影距离小于1 角秒处存在强且富含质量的喷流，与半径依赖的、低效吸积流的理论模型一致，该模型可将大部分吸积物质以高速抛射。