原行星状星云OH 231.8+4中的水汽和一氧化硅脉泽观测
Water vapor and silicon monoxide maser observations in the protoplanetary nebula OH 231.8+4
摘要：
OH 231.8+4.2是一个经过充分研究的行星前星云（pPN），围绕着一个双星系统，显示出显著的双极外流。为了研究星云内部1--0 -80 AU区域的结构和运动学，我们用甚长基线阵列对H $_2$ O 6 $_{1,6}$-5 $_{2,3}$和$^{28}$ SiO $v$ =2，$J$ =1- 0脉泽辐射进行了高分辨率观测。使用相位参考技术恢复了两个发射分布的绝对位置，并在HST光学图像中准确配准。发现H $_2$ O脉泽团分布在两个大小为20 mas的区域内，空间位移为$\sim$ 60毫角秒，轴向接近南北。初步发现SiO脉泽位于两个H $_2$ O脉泽发射区之间，可能指示了系统Mira组分的位置。SiO脉泽辐射跟踪一个直径为12 AU的内赤道分量，可能是一个围绕M型星旋转的圆盘。向外，我们在H $_2$ O数据中检测到一对极冠，相隔80 AU。我们相信，星云的内部区域可能已经被伴星的存在所改变，导致赤道到极点的密度对比，这可能解释了在密度较高的赤道区域缺乏H $_2$ O脉泽和强烈的SiO脉泽发射。
英文摘要：
OH 231.8+4.2 is a well studied preplanetary nebula (pPN) around a binary stellar system that shows a remarkable bipolar outflow. To study the structure and kinematics of the inner 10-80 AU nebular regions we performed high-resolution observations of the H$_2$O 6$_{1,6}$--5$_{2,3}$ and $^{28}$SiO $v$=2, $J$=1--0 maser emissions with the Very Long Baseline Array. The absolute position of both emission distributions were recovered using the phase referencing technique, and accurately registered in HST optical images. H$_2$O maser clumps are found to be distributed in two areas of 20 mas in size spatially displaced by $\sim$60 milli-arcseconds along an axis oriented nearly north-south. SiO masers are tentatively found to be placed between the two H$_2$O maser emitting regions, probably indicating the position of the Mira component of the system. The SiO maser emission traces an inner equatorial component with a diameter of 12 AU, probably a disk rotating around the M-type star. Outwards, we detect in the H$_2$O data a pair of polar caps, separated by 80 AU. We believe that the inner regions of the nebula probably have been altered by the presence of the companion, leading to an equator-to-pole density contrast that may explain the lack of H$_2$O masers and strong SiO maser emission in the denser, equatorial regions.
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