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化，它们通过电离和风塑造星际介质，是整个银河系历史上重元素的来源，也是超新星的前身。特别是，质量较大的早型OB星的自转性质在它们的演化中起着关键作用，因为快速自转可以引发强烈的混合，这反过来可以延长它们的核心氢燃烧寿命，显著增加它们的光度，并改变它们表面的化学成分。事实上，一些大质量恒星甚至可能在接近末端主序列时经历较短的较快自转速度(由内部结构的变化引起)，这可能是快速旋转的Be星、具有Balmer线发射的B型恒星和非累积星盘造成的红外过剩的原因。对恒星自转的观测研究是有问题的，因为在大多数情况下，很难确定自旋轴相对于我们视线的倾斜度。然而，在大样本目标的统计分析中，这一缺点是可以克服的。这项研究的目的是获得关于年轻疏散星团中大质量B型恒星的显著样本的投影自转速度的数据。模型线合成代码将被用来确定基本的恒星参数，如有效温度和重力，以显示从主序前阶段到主序寿命的持续时间，直到末期主序的旋转性质是如何变化的。还将对光谱进行分析，以确定旋转对CNO处理后的气体混合进入表层的影响。这些结果将为测试现代进化密码提供关键手段，并有助于我们解释年轻星团和星团的性质。该计划的第二个方面是研究恒星中的物理过程，这些恒星以如此之快的速度撞击，以至于离心力和引力加速在恒星赤道几乎相互平衡。这项工作将集中在与位于威尔逊山天文台的乔治亚州立大学高角度分辨率天文阵列中心(CHARA)的新观测。这种六元光学长基线干涉仪将确定这些超快旋转体的形状和大小，并将建立数值模型，以匹配干涉条纹数据和互补的高信噪比光谱。目标是确定恒星必须以多快的速度旋转，才能将物质弹出到Be恒星中发现的那种星周圆盘中。脉轮阵列对Be盘的观察将有助于确定它们的大小、取向和质量流出率。通过这个奖项，将支持和培训博士后研究员。博士后研究员还将参与CHARA阵列的仪器支持和开发，发展光电和计算方面的专业知识，并支持光学干涉仪。研究生也将参与到这项工作中，并通过这项工作获得扣减。这项工作的结果将包括在天文学入门课程讲座和GSU中，并通过新闻稿和公开演讲向公众传播。

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.