剖析AGB恒星L2船尾座：一个正在形成的环面
Dissecting the AGB star L2 Puppis: a torus in the making
摘要：
对富氧半规则变星L2 Pup的星周环境进行了观测，以了解恒星演化后期质量损失的演化和喷出物的形成。利用NACO/VLT的近红外（1.64、2.30和3.74 $\rmμm$）孔径掩模，在成像和偏振模式下，从一台8米望远镜获得高角分辨率观测。L2 Pup在2.30 $\rmμm$处的孔径掩蔽图像显示了一个解析结构，该结构类似于长轴约为140毫弧秒（mas）且为东西方向的环形结构。在恒星的两侧可以看到两个团块，距离恒星约65质量，超出星周包层的边缘（估计直径约为27质量），而东北方向则出现一个微弱的钩状结构。这些图案在成像和偏振模式下都是可见的，尽管后者仅用于测量总强度（斯托克斯I）。在3.74 $\rmμm$伪连续体（尘埃排放）处，该结构的整体形状相似，其中团块似乎嵌入在黑暗的尘埃通道中。在这个波长上也可以看到微弱的钩状图案，向东北和西南方向延伸，中央的黑暗通道是明显的对称轴。我们解释的结构作为一个拱星环面的内半径为4.2 au。根据SiO脉泽剖面的建议，$^{-1}$的自转速度为10 km/s，我们估计恒星质量为0.7 M $_\odot$。
英文摘要：
The circumstellar environment of L2 Pup, an oxygen-rich semiregular variable, was observed to understand the evolution of mass loss and the shaping of ejecta in the late stages of stellar evolution. High-angular resolution observations from a single 8 m telescope were obtained using aperture masking in the near-infrared (1.64, 2.30 and 3.74 $\rmμm$) on the NACO/VLT, both in imaging and polarimetric modes. The aperture-masking images of L2 Pup at 2.30 $\rmμm$ show a resolved structure that resembles a toroidal structure with a major axis of ~140 milliarcseconds (mas) and an east-west orientation. Two clumps can be seen on either side of the star, ~65 mas from the star, beyond the edge of the circumstellar envelope (estimated diameter is ~27 mas), while a faint, hook-like structure appear toward the northeast. The patterns are visible both in the imaging and polarimetric mode, although the latter was only used to measure the total intensity (Stokes I). The overall shape of the structure is similar at the 3.74 $\rmμm$ pseudo-continuum (dust emission), where the clumps appear to be embedded within a dark, dusty lane. The faint, hook-like patterns are also seen at this wavelength, extending northeast and southwest with the central, dark lane being an apparent axis of symmetry. We interpret the structure as a circumstellar torus with inner radius of 4.2 au. With a rotation velocity of 10 km s$^{-1}$ as suggested by the SiO maser profile, we estimate a stellar mass of 0.7 M$_\odot$.}
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