Collaborative Research: Fundamental Stellar Physics from Large-sample Globular Cluster Photometry

在一项合作研究项目中，圣地亚哥州立大学的埃里克·桑奎斯特博士和加州大学圣克鲁斯分校的迈克尔·博尔特博士将对球状星团中大量（数以万计）进化的低质量恒星进行光度观测，以探测恒星内部的物理条件。因为一颗恒星一旦消耗完核心的氢，其可观测的特性就会迅速演化，而且因为一颗恒星的寿命主要是由它的质量决定的，所以质量稍有不同的恒星可能处于生命中非常不同的阶段。通过简单地计算恒星的数量作为亮度和温度的函数，这些研究人员可以估计出大约80%太阳质量的恒星在内部过程的影响下变化的速度。研究人员将对恒星物理进行一系列明确的测试，这些测试可以通过这些观测来进行。在一些缺乏金属的星团中，在亚巨星演化阶段观察到过剩的恒星，这表明可能存在一种异常有效的方式将能量从这些恒星的核心转移出去。长期以来，人们一直知道红巨星的肿块可以测试对流运动渗透到巨大分支恒星的深度，研究人员将测试这种特征和各种光度下红巨星的数量所能提供的混合过程。他们还描述了几种新的诊断方法，可以应用于恒星生命的氦聚变阶段（水平分支和渐近巨星分支）。特别是渐近巨支，由于其持续时间短，一直是研究的难点。这些诊断提供了检验燃烧氦的恒星是否按照理论预测的方式演化的实用方法。最后，他们描述了通过对蓝离散星的观测，可以对整个星团的动态演化施加的限制。蓝离散星似乎是由恒星碰撞或双星演化产生的。这项研究将包括对已经从一些人口稠密的球状星团中收集到的CCD数据进行分析，然后对光度测量的不完全性进行彻底研究——这是一个计算密集型的步骤，迄今为止已经进行了少量的大样本研究。为了获得可靠的恒星数量，需要详细了解星团恒星在其他恒星的光线下或在噪音中消失的概率，这是恒星亮度和与星团中心距离的函数。这类光度学研究是非常重要的垫脚石，有助于我们对恒星物理和恒星群的理解进行新的测试

AST 0098454BolteIn a collaborative research project, Dr. Eric Sandquist at San Diego State University and Dr. Michael Bolte at the University of California at Santa Cruz will use photometric observations of large numbers (tens of thousands) of evolved low-mass stars in globular clusters to probe the physical conditions found inside the stars. Because the observable properties of a star evolve quickly once it finishes consuming its core hydrogen and because a star's lifetime is primarily determined by its mass, stars of very slightly different masses can be in very different phases of their lives. By simply counting the numbers of stars present as a function of luminosity and temperature, these researchers can estimate how quickly stars of approximately 80% of the mass of the Sun change under the influence of internal processes.The investigators will pursue a number of definite tests of stellar physics that can be conducted with these kinds of observations. In several metal-poor clusters, a surplus of stars has been observed in the subgiant evolutionary phase, which is an indication that there may be an unusually efficient means of transporting energy away from the cores of those stars. The red giant bump has long been known to test how deeply convective motions penetrate into giant branch stars, and the investigators will test mixing processes that this feature and counts of red giant stars at all luminosities can provide. They also describe several new diagnostics that can be applied to the helium fusion phases of a star's life ( the horizontal branch and asymptotic giant branch). The asymptotic giant branch in particular has been difficult to study because of its short duration. The diagnostics provide practical methods of testing whether helium-burning stars evolve in ways predicted by theory. Finally, they describe the constraints that can be placed on the dynamical evolution of entire clusters by observations of blue straggler stars -a population that appears to be created by collisions of stars or the evolution of binary stars.The study will involve the analysis of CCD data already collected from a number of heavily-populated globular clusters, followed by thorough study of the incompleteness of the photometry -- a computationally intensive step that has been responsible for the small number of large-sample studies to date. To derive robust star counts, what is needed is detailed understanding of the probability that cluster stars were lost in the light of other stars or in noise as a function of stellar brightness and distance from the cluster center. Photometric studies of this kind are very important stepping stones toward new tests of our understanding of stellar physics and stellar populations.***