早期光譜型恆星的分佈與運動：古德帶分析

Abstract

亮星在天空中的分佈並不均勻。可以發現亮星主要分佈在銀河盤面上，但是另外仍然可以找到一部分星星，它們屬於太陽附近的帶狀結構，這個結構被稱為古德帶(Gould Belt)。 本研究是由Hipparcos星表中，取出光譜型O、B及A0到A5的恆星研究古德帶的分佈與運動性質。 由於銀河盤面上A型星數量眾多，古德帶的A型星分佈不像OB型星一般顯而易見，只能約略看出一個窄帶狀，若以太陽為中心，這些星所處的環狀區域和銀河盤面的夾角約為16.5度，長軸指向l = 38∘與218∘的方向。除此之外，結合三角視差、自行與徑向速度等觀測數據，可以求得太陽相對於OB型星及A0到A5型恆星的特有運動之速度分量。不論古德帶成因和高速雲或是爆炸事件相關，在速度分佈圖中，應該都可以依據古德帶恆星和銀河盤面恆星運動性質，區分出部分古德帶的成員，改善位置分佈圖中成員星和銀河盤面星星相混雜而難以區別的困境。雖然OB型星能明確的反應古德帶的空間分佈，但在可見光波段也確實可以使用至A5型星來研究。如果早期星有助於研究古德帶邊緣區域，那麼，使用至晚期星應該更能了解古德帶大尺度的分佈及特性。此外，根據運動特性、空間速度及自行速度的差異，能有效突顯古德帶特質。依此，本研究發現O型星和B型星在自行的分佈圖中可用肉眼看見兩個群體，其中一群應該與古德帶相關。The distribution of the brightest stars on the sky looks like a great circle, known as Gould belt from Gould’s research about one century ago. From Hipparcos catalogue and the database enclosed, some early type stars, O, B, and A0 - A5, are selected to analyze to fit the Gould Belt. The distribution and kinematics of these selected stars are used to find information of Gould Belt. Because the amounts of A type stars spreading on the galactic plane are much more than those O and B type stars, the Gould Belt is not obviously seen. From the distribution of the stars, the Gould Belt likely forms a ring region which is a flat inclining about 16.5 degree to the galactic plane and whose major axis is in the direction of l = 38 degree and 218 degree. In addition, the parallax, proper motion, and radial velocity are combined to find the solar motion with respect to OB stars and A0-A5 stars. Whether the Gould Belt was formed in the explosive event or in the impact event of a high velocity cloud, the members of the Gould Belt should still reserve several information of the velocity, which is different from that of the disk stars. Therefore, the property of the Gould Belt could be found out according to the different kinematical character. The Gould belt could be reflected by space position usually for OB stars, however, the Gould belt is also reflected roughly for main sequence stars with spectral down to A5. If the edge of the Gould Belt could be indexed by early type stars, the late type stars might be used to index larger scale understanding of the Gould Belt. Furthermore, the kinematical characteristic, proper motion or space velocity, is helpful to stand out the Gould Belt in the research. Two groups are seen clearly by eyes from the proper motion vector point diagrams of OB stars, and one could be with relation to the Gould Belt.