Star Formation at Zero and Very Low Metallicities

We describe how star formation is expected to proceed in the early metal-free Universe, focusing on the very first generations of stars. We then discuss how the star formation process may change as the effects of metallicity, external radiative feedback, and magnetic and turbulent support of the gas become more important. The very first stars (Pop III.1) have relatively simple initial conditions set by cosmology and the cooling properties of primordial gas. We describe the evolution of these stars as they grow in mass by accretion from their surrounding gas cores and how the accretion process is affected and eventually terminated by radiative feedback processes, especially HII region expansion and disk photoevaporation. The ability of the protostar and its disk to generate dynamically important magnetic fields is reviewed and their effects discussed. Pop III.1 star formation is likely to produce massive (~100-200Msun) stars that then influence their surroundings via ionization, stellar winds, and supernovae. These processes heat, ionize and metal-enrich the gas, thus altering the initial conditions for the next generation of star formation. Stars formed from gas that has been altered significantly by radiative and/or mechanical feedback, but not by metal enrichment (Pop III.2) are expected to have significantly smaller masses than Pop III.1 stars because of more efficient cooling from enhanced HD production. Stars formed from gas that is metal-enriched to levels that affect the dynamics of the collapse (the first Pop II stars) are also expected to have relatively low masses. We briefly compare the above star formation scenarios to what is known about present-day star formation.Comment: 16 pages, including 11 figures, Review paper to appear in "First Stars III", eds. B. O'Shea, A. Heger and T. Abe

Mass Limits to Primordial Star Formation from Protostellar Feedback

How massive were the first stars? This question is of fundamental importance for galaxy formation and cosmic reionization. Here we consider how protostellar feedback can limit the mass of a forming star. For this we must understand the rate at which primordial protostars accrete, how they and their feedback output evolve, and how this feedback interacts with the infalling matter. We describe the accretion rate with an ``isentropic accretion'' model: the rate is initially very large (~0.03 M_sun/yr when m_* =1 M_sun) and declines as m_*^{-3/7}. Protostellar evolution is treated with a model that tracks the total energy of the star. A key difference compared to previous studies is allowance for rotation of the infalling envelope. This leads to photospheric conditions at the star and dramatic differences in the feedback. Two feedback mechanisms are considered: HII region breakout and radiation pressure from Lyman-alpha and FUV photons. Radiation pressure appears to be the dominant mechanism for suppressing infall, becoming dynamically important around 20 M_sun.Comment: 4 pages; To appear in proceedings of the 13th Annual Astrophysics Conference in Maryland: The Emergence of Cosmic Structure, eds. S. Holt and C. Reynolds, (AIP

Turbopump thermodynamic cooling

System for cooling turbopumps used in cryogenic fluid storage facilities is described. Technique uses thermodynamic propellant vent to intercept pump heat at desired conditions. Cooling system uses hydrogen from outside source or residual hydrogen from cryogenic storage tank

A study of morphology, provenance, and movement of desert sand seas in Africa, Asia, and Australia

A description and classification of major types of sand seas on the basis of morphological pattern and lineation are discussed. The steps involved in analyzing the patterns of deposits on ERTS-1 imagery, where the visible forms are mostly dune complexes rather than individual dunes are outlined. After completion of thematic maps portraying the pattern and lineation of the sand bodies, data on directions and intensity of prevailing and other winds are plotted on corresponding bases, as a preliminary to determination of internal structures through ground truth

Trans-Relativistic Supernovae, Circumstellar Gamma-Ray Bursts, and Supernova 1998bw

Supernova (SN) 1998bw and gamma-ray burst (GRB) 980425 offer the first direct evidence that supernovae are the progenitors of some GRBs. However, this burst was unusually dim, smooth and soft compared to other bursts with known afterglows. Whether it should be considered a prototype for cosmological GRBs depends largely on whether the supernova explosion and burst were asymmetrical or can be modeled as spherical. We address this question by treating the acceleration of the supernova shock in the outermost layers of the stellar envelope, the transition to relativistic flow, and the subsequent expansion (and further acceleration) of the ejecta into the surrounding medium. We find that GRB 980425 could plausibly have been produced by a collision between the relativistic ejecta from SN 1998bw and the star's pre-supernova wind; the model requires no significant asymmetry. This event therefore belongs to a dim subclass of GRBs and is not a prototype for jet-like cosmological GRBs.Comment: 5 pages, 2 figures, to appear in Gamma 2001, eds. S. Ritz, N. Gehrels, and C. Shrade

An inverse problem of reconstructing the electrical and geometrical parameters characterising airframe structures and connector interfaces

This article is concerned with the detection of environmental ageing in adhesively bonded structures used in the aircraft industry. Using a transmission line approach a forward model for the reflection coefficients is constructed and is shown to have an analytic solution in the case of constant permeability and permittivity. The inverse problem is analysed to determine necessary conditions for a unique recovery. The main thrust of this article then involves modelling the connector and then experimental rigs are built for the case of the air-filled line to enable the connector parameters to be identified and the inverse solver to be tested. Some results are also displayed for the dielectric-filled line