An overview of the discussions of the working group on Low-Z stars is presented. Key questions addressed include how the abundances of lithium observed in these stars should be compared to that produced in the Big Bang. Evidence for and against a small star-to-star variation in Li abundances is reviewed, and whether such a variation, if real, necessarily indicates that stellar depletion has occurred, necessitating correction to the value compared to primordial nucleosynthesis calculations. A second key question concerns how and where the light elements are produced. Taken together, their abundance ratios strongly suggest that in low-Z stars the light elements other than 7Li are produced by cosmic ray spallation. The most recent evidence suggests that a minority of this spallation happens in the general interstellar medium, and that a larger fraction might happen in the immediate vicinity of Supernovae, possibly producing observable star-to-star variation. Finally, the question of the overall metallicity of the Galaxy is discussed. How homogeneous in space and time is its evolution? Can we identify subsystems or individual stars which indicate a pregalactic contribution to the galactic metallicity?
Standard solar models, although they are free from the influence of much of the fluid motion that is bound to be present in the Sun, have been shown by helioseismology to represent the spherically averaged structure of the Sun amazingly well. This state of affairs has come about after painstaking refinements by a great many people of the pertinent microphysics, including that which controls the equation of state, the opacity, the nuclear reaction rates and the diffusion that inhibits gravitational segregation of chemical elements. It has instilled confidence in the modellers in being able to predict the composition of the solar interior. But there are consequences of the flow, related particularly to redistribution of chemical species, that can be difficult to identify observationally, yet which may degrade any inferences we might make. Their potential presence must at least be acknowledged by anyone who tries to asses the reliability of the models. This report summarizes the discussions in the preceding pages of this volume of the current theoretical and observational status of the subject, pointing to many of the caveats that have been raised, and attempting at the same time to put them into a seemingly coherent discourse in the context of our present understanding of the workings of the solar interior. This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
Charge exchange(CEX) ions could inflict severe damages on the ion thruster optical system. This article is aimed at investigating the characteristics of the CEX ions and their influences upon the optical system by means of particle-in- cell(PIC) ion simulation and Monte Carlo collision(MCC) methods. The results from numerical simulation indicate that despite the fact that CEX ions appear in the entire beamlet region near the ion optical system, the ones that present themselves downstream of the accelerator grid have good reason for attracting more attention. As their trajectories are significantly affected by the local electric field, a great number of CEX ions are accelerated toward grids resulting in sputtering erosion. When the influences of the CEX ions are considered in the numerical simulation, there could hardly be observed augments in the screen grid current, but the accelerator grid current increases from zero to 1. 4% of the beamlet current. It can be understood from the numerical simulation that the CEX ions formed in the region far downstream of the accelerator grid should be blamed for the erosion on the downstream surface of the accelerator grid. 相似文献
