Laboratory Spectroscopy for Studies of Metal-Poor Halo Stars

对贫金属晕星中重元素丰度的研究有助于更定量地理解恒星核合成和银河系计时。研究了稀土元素丰度的奇偶锯齿图，以区分r过程和S过程(快中子俘获和慢中子俘获合成)在银河系演化中的作用。放射性计时器Th和U的丰度与其他邻近的r过程元素如Os、Ir和Pt的丰度一起被测量，以确定恒星年龄，从而为银河系年龄设定下限。丰度的准确性是这些研究的关键，然而，许多重元素的丰度测定所依赖的转变概率数据库严重缺乏。天文学家通常必须利用太阳gf值，它有许多不确定的来源，特别是与太阳光谱中的铁族相比相对较弱的重元素。旧的实验室测量和半经验结果也经常被使用，尽管已知它们对于较弱的线有很大的误差。利用现代光束激光技术，gf值可以作为太阳和恒星丰度测定的主要不确定源而被消除。威斯康星大学的詹姆斯·劳勒和他的同事们将对大量准确的、绝对的转变概率进行实验室测量。跃迁概率将通过将辐射寿命与分支分数相结合来确定。辐射寿命将使用慢束原子/离子上的时间分辨激光诱导荧光来测量。分支分数将使用傅里叶变换光谱辐射测量法进行测量。将优先考虑镧系元素光谱ND II，以及重要的计时器Th II和U II的光谱。这些光谱的gf值已被确定为最需要改进的，并且在晕星的研究中至关重要。在时间和资源允许的情况下，将继续对光谱Os i、Pti和Ir i进行测量，这些光谱也被认为需要改进gf值，在晕星研究中同样重要。

AST 0205124LawlerThe study of abundances of the heavy elements in metal-poor halo stars is contributing to a more quantitative understanding of stellar nucleosynthesis and Galactic chronometry. The odd-even sawtooth pattern of abundances throughout the lanthanides is examined to separate out thecontributions of the r- and s- processes (synthesis by rapid and slow neutron capture) throughoutthe evolution of the Galaxy. The abundances of the radioactive chronometers Th and U aremeasured along with abundances of other near-lying r-process elements such as Os, Ir and Pt todetermine stellar age and thereby put a lower limit on the Galactic age. Abundance accuracy is key in these studies, and yet the database of transition probabilities on which the abundance determination depends is sorely lacking for many of the heavy elements. Often astronomers must make use of solar gf-values which have many sources of uncertainty, particularly for heavy elements which are relatively weak in comparison to the Fe group in the solar spectrum. Old laboratory measurements and semi-empirical results are also often used, although they are known to have large errors for weaker lines. With modern beam-laser techniques, gf-values can be eliminated as the major source of uncertainty in solar and stellar abundance determinations.Dr. James Lawler and colleagues, at the University of Wisconsin, will conduct laboratory measurements of large sets of accurate, absolute transition probabilities. Transition probabilities will be determined by combining radiative lifetimes with branching fractions. The radiative lifetimes will be measured using time-resolved laser-induced fluorescence on a slow beam of atoms/ions. Branching fractions will be measured using Fourier transform spectroradiometry. Priority will be given to the lanthanide spectrum Nd II, as well as spectra of important chronometers Th II and U II. The gf-values of these spectra have been identified as the most in need of improvement, and as critical in studies of halo stars. As time and resources allow, measurements will continue on the spectra Os I, Pt I and Ir I, which have also been identified as needing improved gf-values, and as important in studies of halo stars.****