G45．07＋0．13羟基分子脉泽的VLBI观测

VLBI OBSERVATIONS OF HYDROXYL MOLECULAR MASERS IN G45.07+0.13

利用甚长基线干涉仪，在１６６５ＭＨｚ频率上，观测超致密氢ＩＩ区Ｇ４５．０７＋０．１３的左右圆偏振羟基（ＯＨ）分子脉泽辐射．利用多条纹率成图方法，获得了这个区域２０ｍａｓ相对位置精度的脉泽结构图．除两个弱子源以外，所有脉泽子源都位于彗状结构氢ＩＩ区“彗头”的前沿．脉泽团离致密氢ＩＩ区中心投影距离为０＂．４．在完全饱和辐射的假设下，估计脉泽活动区具有氢分子数密度约为４．５×１０８（Ｈ２）ｃｍ－３．在脉泽源中，发现一对Ｚｅｅｍａｎ对，从它们的ＬＳＲ速度差，导出脉泽区的磁场约为３ｍＧ，方向为远离地球方向．在致密氢ＩＩ区Ｇ４５．０７＋０．１３彗头附近的脉泽团的直径为６×１０１６ｃｍ．这与Ｇ３４．０３＋０．２的观测结果很类似．这给解释彗状结构的氢ＩＩ区的各种模型提供了一个可靠的观测约束条件．

Both the left-and right-handed circularly polarized radiations of the masers in the ultra-compact hydrogen II region G45.07+0.13, due to the transition from the OH ground state 2II3/2, J=3/2, F=1→1, have been observed with the US VLBI array. Using the multiplepoint fringe rate method, we have constructed the map of the maser structure with a relative position accuracy of 20 mas. With the fringe rate of the calibration source and that of the maser component with a velocity of 56.76km s-1, taken to be the phase reference point, the absolute position of the phase reference point has been calculated as given by a (1950.0) =19h11m00s 46,δ(1950.0) =10°45' 43'. 2. We have found that most maser components are located in the front of the comet-shaped HII region and that their distance to the center of hydrogen II region is approximately 0' 4. This agrees well with the distance from the vertex of the parabolic ionization wavefront to the young celestial body, as estimated with the bow shock model. By use of the tube-shaped model and under the assumption of fully saturated radiation, the number density of the hydrogen molecules in maser regions has been derived to be 4.5×108cm-3. Such high density regions are very possibly the thick clouds of neutral molecules swept up by shock waves and are located in the front side of ionization wavefront. In this maser active region a pair of Zeeman-split lines have been found and from the difference of their radial velocities a magnetic field of 3mG has been inferred. Its direction is away from the Earth.Similar to the observation of G34.3±0.2, the diameter of the maser cluster nearthe compact hydrogen 11 region has the order of magnitude of 1016cm. This is 50 times larger than the thickness of a neutral gas layer possibly swept up by shock waves. It is still not clear how this can be explained.