Observations of Late-Type Stars with mm-VLBI

用鹿岛34-m和Nobeama 45-m射电望远镜对三颗晚型恒星的两条SiO脉泽线Nu=1和Nu=2，J=1-0进行了VLBI观测。我们发现Mu Cep和W Hya的脉泽辐射来自于直径相当于中心恒星的环状结构。在VY CMA周围看到的环形结构的直径比估计的中央恒星的直径大好几倍。脉泽斑的空间分布与其径向速度之间没有明显的关系。这些事实表明，脉泽辐射来自一个扩展的大气层，在那里物质可能几乎是静止的，脉泽放大路径是朝向边缘，而不是沿着到达中央恒星的视线。我们的观测还发现，nu=1和nu=2脉泽的相应速度分量具有相同的分布。我们的结果有力地表明，红外线重叠在SiO脉泽机制中起着重要的作用。我们的结果还表明，利用频率间隔为300 MHz的振动态nu=1和nu=2的SiO J=1-0脉泽线，用带宽合成技术测量群延迟是可能的。这种新方法使我们能够以毫角秒的绝对位置精度确定这些恒星的位置，然后确定它们的自行。我们曾试图分析这些恒星的群体延迟，但一些技术问题使我们无法如此准确地进行测量。

VLBI observations of the two SiO maser lines, the nu=1 and nu=2,J=1-0,for three late-type stars have been made with the Kashima 34-m and Nobeyama 45-m radio telescopes. We found that the maser emissions come from a ring-like structure with diameters comparable to those of a central star for mu Cep and W Hya. The diameter of the ring structure seen around VY CMa was several times larger than that estimated for the central star. No clear relations have been found between the spatial distributions of maser spots and their radial velocities. These facts suggest that the maser emission arises from an extended atmosphere where material may be nearly stationary and that maser amplification paths are toward the limb rather than along the line of sight to the central star. Our observations also revealed that the corresponding velocity components of the nu=1 and nu=2 masers have the same distributions. Our results strongly suggest that an infrared line-overlapping plays an important role in the SiO maser mechanism. Our results also imply that it is possible to measure a group delay with a bandwidth synthesis technique using the SiO J=1-0 maser lines in the vibrational states nu=1 and nu=2, which are separated by 300 MHz in frequency. This new method allows us to determine the positions, and then the proper motions, of these stars with absolute positional accuracies of milliarcseconds. We have tried to analyze group delays for these stars, but some technical problems prevented us to measure with such an accuracy.