Angular Momentum Evolution and X-ray Production in Low-Mass Pre-Main-Sequence Stars

在这个项目中，Stassun博士将与合作者和学生一起沿着执行一个多方面的观测研究计划，目标是解决与行星形成时期（1-10百万年）低质量恒星物理学有关的三个基本问题：（1）是什么支配着低质量前主序星的角动量演化？(2)低质量前主序星的X射线是由什么物理机制产生的？(3)X射线如何影响原行星盘的性质和演化？通过利用猎户座星云团（100万年）和猎户座OB 1协会（1000万年）中数百颗低质量主序前恒星的几个大型光学，红外和X射线数据库，他们将：1）探索年轻，低质量恒星角动量损失的机制：经验上已知，在恒星演化的主序前阶段，角动量不守恒。自转数据显示，在~ 100万年和主序（~ 1亿年）之间，低质量恒星的角动量含量会减少一个数量级或更多。不知道这是如何发生的。Stassun博士将研究前主序星角动量演化的两种可能机制。首先，所谓的“盘锁定”理论的关键预测，其中认为，恒星的磁耦合到其拱星盘调节星星的角动量，将直接测试。第二，由于在猎户座星云团恒星中发现了极其强大的X射线“超级耀斑”，将开发和改进一个通过按比例放大的太阳型日冕物质抛射而损失角动量的新模型。现在已经确定，低质量的前主序星产生的X射线是现在太阳的104倍。然而，目前还不清楚他们是如何做到这一点的。虽然主序星上的X射线产生被理解为旋转驱动的发电机，但到目前为止，对前主序星的X射线观测未能找到像主序星上那样清晰的“旋转-活动关系”。前主序星的潜在旋转-活动关系可能确实存在，但由于天体物理学的偏差，使得已知旋转周期的恒星系统地更具X射线亮度，因此它被忽略了。为了验证这一假设，并探索旋转活动联系的演变，Stassun博士将对猎户座星云团和猎户座OB 1恒星进行敏感的v sin i和旋转周期测量，并将这些测量与恒星的X射线光度相关联。3)研究恒星X射线对星周环境的影响：X射线被认为在许多微观物理学中起着核心作用，这些微观物理学控制着磁性恒星-磁盘相互作用，吸积过程，物质流出和年轻恒星的角动量演化。然而，迄今为止，还没有直接的观测证据表明恒星X射线与星周气体相互作用并加热星周气体。在这里，Stassun博士将进行一项实验，首次建立恒星X射线与前主序星大样本中星周气体加热之间的直接联系。特别是，他将在800颗主序前恒星中寻找H-α和X射线的时间相关变化，我们有一个独特的同步H-α和X射线光变曲线数据库。该项目将继续并扩大与菲斯克大学合作开发的成功的本科生和研究生课程：菲斯克天文学和空间科学培训（FASST）计划和菲斯克-范德比尔特硕士到博士桥梁计划。本研究计划将形成一个范德比尔特博士生的博士论文的基础上，与桥梁研究生和一个或多个FASST本科生的积极参与。该项目还将为寻求整合研究，教学和学生指导经验的博士后研究人员提供绝佳的专业发展机会。

In this project, Dr. Stassun, along with collaborators and students, will execute a multi-faceted, observational research program, with the goal of addressing three fundamental questions relating to the physics of low-mass stars during the planet-building era (1-10 Myr): (1) What governs the angular momentum evolution of low-mass pre-main sequence stars? (2) By which physical mechanisms are X-rays in low-mass pre-main sequence stars generated? (3) How do X-rays influence the properties and evolution of protoplanetary disks?By harnessing several large optical, infrared, and X-ray databases of hundreds of low-mass pre-main sequence stars in the Orion Nebula Cluster (1 Myr old) and the Orion OB1 association (10 Myr old), they will:1) Explore mechanisms for angular momentum loss in young, low-mass stars: It is empirically known that angular momentum is not conserved during the pre-main sequence phase of stellar evolution. Rotation data show that low-mass stars deplete their angular momentum content by an order of magnitude or more between ~1 Myr and the main sequence (~100 Myr). What is not known is how this happens. Dr. Stassun will examine two possible mechanisms for angular momentum evolution of pre-main sequence stars. First, the key predictions of so-called "disk locking" theory, in which it is thought that magnetic coupling of stars to their circumstellar disks regulates the star's angular momentum, will be directly tested. Second, motivated by the discovery of extremely powerful X-ray "super-flares" in the Orion Nebula Cluster stars, a new model of angular momentum loss via scaled-up solar-type coronal mass ejections will be developed and refined.2) Elucidate the origins of X-ray production in young, low-mass stars: It is now well established that low-mass pre-main sequence stars produce X-rays at up to ~104 times that of the present-day Sun. Yet, it is still unclear how they do this. While X-ray production on the main sequence is well understood in terms of a rotation-driven dynamo, X-ray observations of pre-main sequence stars have so far failed to find a clear "rotation-activity relationship" such as that found on the main sequence. An underlying rotation-activity relation for pre-main sequence stars may indeed exist, but it has been missed due to an astrophysical bias that makes stars with known rotation periods systematically more X-ray luminous. To test this hypothesis, and to explore the evolution of the rotation-activity connection, Dr. Stassun will make sensitive v sin i and rotation-period measurements of Orion Nebula Cluster and Orion OB1 stars, and correlate these measures with the stars' X-ray luminosities. 3) Investigate the influence of stellar X-rays on the circumstellar environment: X-rays are thought to play a central role in much of the microphysics that governs magnetic star-disk interaction, accretion processes, outflows of material, and angular momentum evolution of young stars. However, to date there has been no direct observational proof of stellar X-rays interacting with and heating circumstellar gas. Here, Dr. Stassun will conduct an experiment to establish for the first time a direct link between stellar X-rays and the heating of circumstellar gas in a large sample of pre-main sequence stars. In particular, he will search for time-correlated variability in H-alpha and X-rays among 800 pre-main sequence stars for which we have a unique database of simultaneous H-alpha and X-ray light curves. This project will continue and expand upon the successful undergraduate and graduate programs developed in collaboration with Fisk University: The Fisk Astronomy and Space Science Training (FASST) program, and the Fisk-Vanderbilt Masters-to-PhD Bridge program. This research program will form the basis of a PhD thesis for a Vanderbilt PhD student, with active participation of a Bridge graduate student and one or more FASST undergraduates. The project will also result in an excellent professional development opportunity for a postdoctoral researcher seeking experience in integrating research, teaching, and student mentoring.