Rotational Evolution of Massive Stars

这个项目的动机是需要更好地了解大质量恒星的演变，通过电离和风的星际介质的塑造者，银河系历史上重元素的来源，以及超新星的祖先。特别是，大质量早期OB型恒星的旋转特性在它们的演化中起着关键作用，因为快速旋转会引发强烈的混合，从而延长它们的核心氢燃烧寿命，显著增加它们的亮度，并改变它们表面的化学成分。事实上，一些大质量恒星在接近终端主序时（由内部结构的变化引起）甚至可能经历短暂的快速旋转，这可能解释了快速旋转的Be星，具有巴尔默线发射的b型恒星和由于环星盘而产生的红外过量。对恒星自转的观测研究是有问题的，因为在大多数情况下，很难确定自转轴相对于我们视线的倾角。但是，在对大样本目标进行统计分析时，可以克服这一缺点。这项研究的目的是获得年轻疏散星团中一个重要的大质量b型恒星样本的预测旋转速度的数据。模型线合成代码将用于确定基本的恒星参数，如有效温度和重力，以显示旋转特性如何从主序前阶段变化，通过主序生命周期的持续时间，直到终末年龄的主序。还将分析光谱，以确定旋转对CNO处理的气体混合到表层的影响。这些结果将为检验现代演化密码提供关键手段，并帮助我们解释年轻星团和星团的特性。该计划的第二个方面是研究恒星的物理过程，这些恒星旋转得如此之快，以至于离心加速度和重力加速度在恒星赤道几乎相互平衡。这项工作将集中在威尔逊山天文台乔治亚州立大学高角分辨率天文阵列中心（CHARA）的新观测上。这种六元光学长基线干涉仪将解析这些超快旋转器的形状和尺寸，并将构建数值模型来匹配干涉条纹数据和互补的高信噪光谱。目的是确定一颗恒星必须旋转多快才能将物质抛射到恒星的星周盘中。CHARA阵列对Be盘的观测将有助于确定它们的大小、方向和质量流出率。通过该奖项，博士后研究员将得到支持和培训。该博士后还将参与hara阵列的仪器支持和开发，开发电光学和计算方面的专业知识，并支持光学干涉仪。研究生也希望参与并通过工作得到教育。这项工作的结果将包括在天文学入门课程和GSU的讲座中，并通过新闻稿和公开讲座向公众传播。

This project is motivated by the need to better understand the evolution of massive starsas shapers of the interstellar medium through ionization and winds, sources of heavyelements throughout Galactic history, and as the progenitors of supernovae. In particular,the rotation properties of massive early-type OB stars play a key role in their evolution asrapid rotation can trigger strong mixing which can in turn extend their core-hydrogenburning lifetimes, significantly increase their luminosities, and change the chemicalcompositions of their surfaces. Indeed, some massive stars may even experience a shortepisode of faster rotation during their approach to the terminal main sequence (caused bychanges in internal structure) which may account for the occurrence of the rapidlyrotating Be stars, B-type stars with Balmer line emission and an infrared excess due to acircumstellar disk.Observational studies of stellar rotation are problematic because in most cases it is difficult to determine the inclination of the spin axis relative to our line of sight. However, this shortcoming can be overcome in statistical analyses of large samples of targets. The purpose of this investigation is to obtain such data on the projected rotational velocities of a significant sample of massive B-type stars in young open clusters. Model line synthesis codes will be used to determine fundamental stellar parameters such as effective temperature and gravity in order to show how the rotational properties vary from the pre-main sequence phase, through the duration of the main sequence lifetime, and up to the terminal age main sequence. The spectra will also be analyzed to determine the influence of rotation on mixing of CNO processed gas into the surface layers. The results will provide the crucial means to test modern evolutionary codes and to aid in our interpretation of the properties of young stellar clusters and associations.A second aspect of the program is a study of the physical processes in stars that arespinning so fast that the centrifugal and gravitational accelerations nearly balance eachother at the stellar equator. This work will center on new observations with the GeorgiaState University Center for High Angular Resolution Astronomy Array (CHARA) atMount Wilson Observatory. This six-element, optical long baseline interferometer willresolve the shapes and sizes of these ultrafast rotators, and numerical models will beconstructed to match both the interferometric fringe data and complementary high signalto-noise spectroscopy. The goal is to determine how fast a star must rotate in order toeject matter out into a circumstellar disk of the kind found in Be stars. CHARA Arrayobservations of Be disks will help determine their size, orientation, and mass outflowrate.Through this award, a postdoctoral research fellow will be supported and trained. Thepostdoctoral fellow will also participate in instrument support and development at theCHARA Array, developing expertise in electro-optics and computation and supportingthe optical interferometer. Graduate students are also expected to be involved andeducated through the work. The results of this work will be included in introductoryastronomy course lectures and GSU and disseminated to the public through press releasesand public talks.