97 10 25 8 v 1 2 3 O ct 1 99 7 Metal systems of the Lyman forest
An extremely careful analysis of the forest of weak metal-line systems at z ∼ 2 has allowed the separation into individual 'single-phase ionization' components with accurate parameters. The systems typically span several hundred km s −1 and within each the components show a strong coherence in their ioniza-tion properties. A general feature of the observed systems is the presence of high ionization broad components which co-exist in velocity space with more numerous, narrow, lower ionization components. From a sample of components extending to z ∼ 3.5, taken either individually or as system totals, no rapid evolution is found in the column density ratio N(Si IV)/N(C IV), contrary to previous indication of a large change near z = 3.1. Comparisons of component ion ratios including Si IV, C IV, C II and Si II from the sample at z ∼ 2 with CLOUDY derived values using several model ionizing spectra show the general dominance of the metagalactic ionizing background over local stellar radiation and give Si/C distributed 1–3 times the solar value. Indications of the evolution of structure are highlighted.
Cold Gas at High Redshift Netherlands Foundation for Research in Astronomy
We discuss the current observational and theoretical issues concerning cold gas at high redshift and present simulations showing how a number of observational issues can be resolved with planned future instrumentation.
Mass Loss History of Evolved Stars
We provide high-resolution maps of the circumstellar dust shells of several dozen Asymptotic Giant Branch (AGB) stars using data from the Spitzer space telescope Multiband Imaging Photometer (MIPS) imaged at 70μm. AGB stars are the major contributors of chemical elements such as carbon, oxygen and silicon, which are essential to the existence of life in the universe, through mass loss processes that take place at the surface of the star. We probe the spatial distribution of cold (∼40 K) dust grains in order to trace the history of mass loss from the observed radial density variation in these shells. Our images illustrate that different morphologies may evolve because AGB stars can already interact with the interstellar medium (ISM) that surrounds them even at these early stages of the AGB mass loss history. Relative motion of the star as well as the ISM with respect to the local environment may be of particular importance for shell structure evolution.