Composition of Stars in the Galactic Bulge and Inner Disk

银河系凸起/星棒的起源和化学演化仍然是一个悬而未决的问题。凸起径向速度分析(BRAVA)发现，恒星穿过凸起的径向速度符合N体杆模型。然而，在相同的体积内，可以观察到恒星化学成分的梯度。如果沙洲是通过纯动态过程形成的，则不应存在丰度梯度。也许该凸起是一种假性凸起，并具有较长的形成历史，但来自该凸起中恒星的关闭年龄和丰度比的证据都与早期快速形成的旧凸起相一致。现在有几项研究表明，这个凸起由不同年龄、不同金属丰度、运动学和空间范围的多个恒星亚群组成。对于球体中恒星的丰度和运动学是否相关，研究仍然存在分歧。也有可能是大质量原星系团的解体形成了银河系的凸起。在过去的几年里，高度放大的透镜状凸起亚矮星的高分辨率光谱揭示了一个似乎是富金属的恒星群，年龄为2-7 Gyr--比从颜色-星等图分析得出的凸起场群的年龄要年轻得多。最近的两项光谱研究也表明，这个凸起类似于厚盘，也可能是厚盘的一部分。虽然没有运动学证据表明银河系纬度低于8度时存在“经典的”凸起，但红外色星等图和一些光谱研究发现，在银河系高纬度存在类似凸起的群体。里奇博士和他的合作者对内部凸起和凸起球状星团中的重元素进行了调查，以确定凸起与厚盘或光晕之间的联系。比较场状星团和球状星团的组成有助于解决球状星团是否由解体形成的凸起问题。这项工作的另一部分是对银河系纬度20度以上的红巨星进行高色散光谱测量，以评估该星系群的形成历史是否与内部凸起的历史不同。研究成果通过经评审的论文、座谈、科学会议和公开演讲来传播。

The origin and chemical evolution of the Galactic bulge/bar remains an unsolved problem. The Bulge Radial Velocity Assay (BRAVA) has found that the radial velocities of stars across the bulge are consistent with an N-body bar model. Yet over the same volume, a gradient in the chemical composition of stars is observed. If the bar was created via a purely dynamical process, no abundance gradient should be present. Maybe the bulge is a pseudo-bulge, and has an extended formation history, but evidence from both the turnoff age and abundance ratios of stars in the bulge are consistent with an old bulge that formed early and rapidly. Several studies now suggest that the bulge consists of multiple stellar subpopulations of differing age, metallicity, kinematics, and spatial extent. Studies continue to disagree on whether the abundances and kinematics of stars in the bulge are correlated. There is also the possibility that the disintegration of massive proto-clusters built the Galactic bulge. Within the last few years, high resolution spectroscopy of highly magnified, lensed bulge subdwarfs revealed what appears to be a stellar population that is metal rich, and 2-7 Gyr old- considerably younger than is inferred for the bulge field population from the analysis of color-magnitude diagrams. Two recent spectroscopic studies also suggest that the bulge is similar to and perhaps part of, the thick disk. Although there is no kinematic evidence for a 'classical' bulge at galactic latitudes below 8 degrees, infrared color-magnitude diagrams and some spectroscopic studies find evidence for a bulge-like population at high galactic latitude. Dr. Rich and his collaborators conduct a survey of the heavy elements across the inner bulge and bulge globular clusters to securely relate the bulge to the thick disk or the halo. Comparison of the composition of field and globular cluster stars helps to address whether the bulge formed from disintegrated globular clusters. Another part of this work is to undertake high dispersion spectroscopy of red giants at galactic latitudes above 20 degrees to assess whether the formation history of this population differs from that of the inner bulge. The research results are disseminated via refereed papers, colloquia, scientific meetings, and public lectures.