Stellar Rotation, Mixing, and the Chronology of the Galaxy

随着类太阳恒星的老化，它们最初的快速自转会通过磁化风的刹车作用减慢。结合角动量损失和内部角动量输运的恒星演化理论有力地预测，在晚些时候，所有特定质量的恒星都应该以相同的速度自转，而不考虑初始条件。原则上，这种速率的收敛和随后的下降可以用作确定场主序星年龄的时钟。然而，这幅图景存在两个根本问题。首先，根本没有足够的数据来判断会聚是否如预测的那样发生，以及它如何依赖于质量。其次，角动量输运有许多不同的可能过程。甚至有来自理论和观测的一些迹象表明，会聚可能不会发生，特别是如果角动量损失停止在阈值自转速率以下的话。为了解决这些问题，平松尼奥博士和他的合作者将对选定的疏散星团中恒星的自转演化进行一项联合的观测和理论研究。MDM天文台的时间序列测光将高精度地挑选出星系团成员，并将发现所有自转亮度调制大于0.3%、周期小于30天的恒星。将选择目标集群，以填补先前研究在年龄和质量方面的重要差距。恒星自转代码中的物理成分将被扩展到包括由波或磁驱动的角动量传输过程。不同的输运现象将根据它们对年轻恒星自转分布和轻元素(如锂、铍或硼)破坏速度的影响来区分。恒星演化代码结合了磁不稳定性或波的角动量转移的新方法，将提供给其他研究人员。该项目将确定旋转时钟可以校准的质量和时间范围，新时钟的早期应用将是分析NASA开普勒任务发现的大量旋转周期。在此过程中，目标星系团的新距离和金属度将通过重新校准造父周期-光度关系来改进河外距离尺度。现有研究中的轻元素丰度将被置于共同的温度和丰度尺度上，新的理论模型将被用于通过研究旧晕星中的锂丰度来创建更好的大爆炸核合成测试。该项目还将被用于为哥伦布COSI科学中心的中学项目创造关于恒星演化的新内容。作为实地考察的一部分，Pinsonneault博士和合作者每年将进行大约五次讲座，并与COSI团队合作，帮助创建教师专业发展材料、经验前和经验后测试、为学生提供的网络和光盘材料等。

As Sun-like stars age, their initial rapid rotation slows through the braking action of magnetized winds. Stellar evolution theory incorporating angular momentum loss and internal angular momentum transport strongly predicts that at late times all stars of a particular mass should spin at the same rate, regardless of initial conditions. This convergence of rates and subsequent decline could, in principle, be used as a clock for determining the ages of field main-sequence stars. There are, however, two fundamental problems with this picture. First, there are simply not enough data to tell whether the convergence takes place as predicted and how it depends on mass. Second, there are many different possible processes for angular momentum transport. There are even some indications from both theory and observation that the convergence might not happen, particularly if the loss of angular momentum stops below a threshold rotation rate.To address these issues, Dr. Pinsonneault and collaborators will undertake a combined observational and theoretical investigation of the rotational evolution of stars in selected open clusters. Time-series photometry at the MDM observatory will pick out cluster members with high accuracy and will find all stars with rotational brightness modulation above 0.3% and periods less than 30 days. Targeted clusters will be selected to fill important gaps in age and mass from previous studies. The physical ingredients in stellar rotation codes will be extended to include wave- or magnetic-driven angular momentum transport processes. The various transport phenomena will be distinguished by their effect on the distribution of rotation and on the rate of destruction of light elements such as lithium, beryllium, or boron in young stars. The stellar evolution codes incorporating new treatments of angular momentum transfer from magnetic instabilities or waves will be made available to other researchers. The project will establish the range of mass and time over which a rotation clock can be calibrated, and an early application of the new clock will be to analyze the vast number of rotation periods to be found by NASA's Kepler mission. Along the way, new distances and metallicities for the targeted clusters will refine the extragalactic distance scale through recalibration of the Cepheid period-luminosity relation. Light element abundances in existing studies will be placed on a common temperature and abundance scale, and the new theoretical models will be used to create better tests of nucleosynthesis in the Big Bang through studies of lithium abundances in old halo stars.This project will also be used to create new content on stellar evolution for the middle school program at the COSI Science Center in Columbus. Dr. Pinsonneault and collaborators will give approximately five lectures per year as part of the field trip experience and help create teacher professional development materials, pre- and post-experience testing, web and CD-ROM materials for students, etc., in collaboration with the COSI team.