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.
The fragmentation of small asteroids in the atmosphere
The fragmentation of a small asteroid in the atmosphere greatly increases its cross-sections for aerodynamic braking and energy dissipation. The differential pressure across a meteoroid disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with meteoroid density. At a typical impact velocity of 22 km/s, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, D M < 220 M AND IRON METEOROIDS WITH D M < 80 m. The corresponding diameter for comets with impact velocity 50 km/s is D M < 1600 m. Most of this energy dissipation occurs in a fraction of a scale height, which causes large meteoroids to appear to »explode« or »flare« at the end of their visible paths