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执行一个多方面的观测研究计划，其目标是解决与行星形成时代（1-10兆r）低质量恒星物理学相关的三个基本问题：(1)是什么控制了低质量前主序恒星的角动量演化？(2)低质量前主序星的x射线是通过哪些物理机制产生的？(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星团（1 Myr）和猎户座OB1星团（10 Myr）中数百颗低质量主序前恒星的大型光学、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：经验表明，在恒星演化的主序前阶段，角动量是不守恒的。旋转数据显示，在~1 Myr和主序星（~100 Myr）之间，低质量恒星的角动量含量消耗了一个数量级或更多。目前尚不清楚这是如何发生的。Stassun博士将研究主序前恒星角动量演化的两种可能机制。首先，所谓的“盘面锁定”理论的关键预测将被直接测试。该理论认为，恒星与其星周盘的磁耦合调节了恒星的角动量。其次，由于猎户座星云星团中发现了极其强大的x射线“超级耀斑”，一种新的角动量损失模型将被开发和完善，这种模型是通过按比例放大的太阳型日冕物质抛射来实现的。2)阐明年轻的低质量恒星产生x射线的起源：现在已经确定，低质量的主序前恒星产生的x射线高达今天太阳的104倍。然而，目前尚不清楚他们是如何做到这一点的。虽然主序上的x射线产生可以很好地理解为旋转驱动的发电机，但对主序前恒星的x射线观测到目前为止还没有发现一个明确的“旋转-活动关系”，就像在主序上发现的那样。主序前恒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发出更多的x射线，因此错过了它。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星团和猎户座OB1恒星进行敏感的v sin i和旋转周期测量，并将这些测量结果与恒星的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星盘相互作用、吸积过程、物质流出和年轻恒星的角动量演化。然而，到目前为止，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。在这里，斯塔松博士将进行一项实验，首次在大量主序前恒星样本中建立恒星x射线与恒星周围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射线的时间相关变异性，我们有一个独特的同时h - α和x射线光曲线数据库。该项目将继续并扩展与菲斯克大学合作开发的成功的本科和研究生课程：菲斯克天文学和空间科学培训（FASST）计划，以及菲斯克-范德比尔特硕士到博士的桥梁计划。该研究项目将成为范德比尔特大学博士生博士论文的基础，一名桥梁研究生和一名或多名FASST本科生积极参与。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员提供良好的专业发展机会。

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.