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?
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
We have studied the loss of O+ and O+2 ions at Mars with a numerical model. In our quasi-neutral hybrid model ions (H+, He++, O+, O+2) are treated as particles while electrons form a massless charge-neutralising fluid. The employed model version does not
include the Martian magnetic field resulting from the crustal magnetic anomalies. In this study we focus the Martian nightside
where the ASPERA instrument on the Phobos-2 spacecraft and recently the ASPERA-3 instruments on the Mars Express spacecraft
have measured the proprieties of escaping atomic and molecular ions, in particular O+ and O+2 ions. We study the ion velocity distribution and how the escaping planetary ions are distributed in the tail. We also create
similar types of energy-spectrograms from the simulation as were obtained from ASPERA-3 ion measurements. We found that the
properties of the simulated escaping planetary ions have many qualitative and quantitative similarities with the observations
made by ASPERA instruments. The general agreement with the observations suggest that acceleration of the planetary ions by
the convective electric field associated with the flowing plasma is the key acceleration mechanism for the escaping ions observed
at Mars. 相似文献
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