High-temperature thermal storage systems for advanced solar receivers materials selections

Advanced space power systems that use solar energy and Brayton or Stirling heat engines require thermal energy storage (TES) systems to operate continuously through periods of shade. The receiver storage units, key elements in both Brayton and Stirling systems, are designed to use the latent heat of fusion of phase-change materials (PCMs). The power systems under current consideration for near-future National Aeronautics and Space Administration space missions require working fluid temperatures in the 1100 to 1400 K range. The PCMs under current investigation that gave liquid temperatures within this range are the fluoride family of salts. However, these salts have low thermal conductivity, which causes large temperature gradients in the storage systems. Improvements can be obtained, however, with the use of thermal conductivity enhancements or metallic PCMs. In fact, if suitable containment materials can be found, the use of metallic PCMs would virtually eliminate the orbit associated temperature variations in TES systems. The high thermal conductivity and generally low volume change on melting of germanium and alloys based on silicon make them attractive for storage of thermal energy in space power systems. An approach to solving the containment problem, involving both chemical and physical compatibility, preparation of NiSi/NiSi2, and initial results for containment of germanium and NiSi/NiSi2, are presented

Catching VY Sculptoris in a low state

Context. In the context of a large campaign to determine the system parameters of high mass transfer cataclysmic variables, we found VY Scl in a low state in 2008. Aims. Making use of this low state, we study the stellar components of the binary with little influence of the normally dominating accretion disc. Methods. Time-resolved spectroscopy and photometry of VY Scl taken during the low state are presented. We analysed the light-curve and radial velocity curve and use time-resolved spectroscopy to calculate Doppler maps of the dominant emission lines. Results. The spectra show narrow emission lines of Halpha, Hbeta, HeI, NaID, and FeII, as well as faint TiO absorption bands that trace the motion of the irradiated secondary star, and Halpha and HeI emission line wings that trace the motion of the white dwarf. From these radial velocities, we find an orbital period of 3.84 h, and put constraints on binary parameters such as the mass ratio M2/M1 of 0.43 and the inclination of 15 deg. With a secondary's mass between 0.3 and 0.35 Msol, we derive the mass for the white dwarf as M1 = 0.6-0.1 Msol.Comment: 8 pages, 9 figures, accepted for publication in A&

Fall back accretion and energy injections in gamma-ray bursts

Intense flares that occur at late times relative to the prompt phase have been observed by the $Swift$ satellite in the X-ray afterglows of gamma-ray bursts (GRBs). Here, we present a detailed analysis on the fall back accretion process to explain the intense flare phase in the very early X-ray afterglow light curves. To reproduce the afterglow at late times, we resort to the external shock by engaging energy injections. By applying our model to GRBs 080810, 081028 and 091029, we show that their X-ray afterglow light curves can be reproduced well. We then apply our model to the ultra-long $Swift$ GRB 111209A, which is the longest burst ever observed. The very early X-ray afterglow of GRB 111209A showed many interesting features, such as a significant bump observed at around 2000 s after the $Swift$/BAT trigger. We assume two constant energy injection processes in our model. These can explain the observed plateau at X-ray wavelength in the relatively early stage ($8.0\times10^{3}$ s) and a second X-ray plateau and optical rebrightening at about $10^{5}$ s. Our analysis supports the scenario that a significant amount of material may fall back toward the central engine after the prompt phase, causing an enhanced and long lived mass accretion rate powering a Poynting-flux-dominated outflow.Comment: 2 tables, 3 figures, accepted by MNRA

Challenges in the development of the orbiter radiator system

Major technical challenges which were met in the design and development of the Space Shuttle Orbiter Radiator System are discussed. This system rejects up to 30 kW of waste heat from eight individual radiators having a combined surface area of 175 sq m. The radiators, which are deployable, are mounted on the inside of the payload bay doors for protection from aerodynamic heating during ascent and re-entry. While in orbit the payload bay doors are opened to expose the radiators for operation. An R21 coolant loop accumulates waste heat from various components in the Orbiter and delivers the heat to the radiators for rejection to space. Specific challenges included high acoustically induced loads during lift-off, severe radiating area constraints, demanding heat load control requirements, and long life goals. Details of major design and analysis efforts are discussed. The success of the developed hardware in satisfying mission objectives showed how well the design challenge was met

On A Battlefield In France

https://digitalcommons.library.umaine.edu/mmb-vp/4809/thumbnail.jp

The Kepler Light Curves of AGN: A Detailed Analysis

We present a comprehensive analysis of 21 light curves of Type 1 AGN from the Kepler spacecraft. First, we describe the necessity and development of a customized pipeline for treating Kepler data of stochastically variable sources like AGN. We then present the light curves, power spectral density functions (PSDs), and flux histograms. The light curves display an astonishing variety of behaviors, many of which would not be detected in ground-based studies, including switching between distinct flux levels. Six objects exhibit PSD flattening at characteristic timescales which roughly correlate with black hole mass. These timescales are consistent with orbital timescales or freefall accretion timescales. We check for correlations of variability and high-frequency PSD slope with accretion rate, black hole mass, redshift and luminosity. We find that bolometric luminosity is anticorrelated with both variability and steepness of the PSD slope. We do not find evidence of the linear rms-flux relationships or lognormal flux distributions found in X-ray AGN light curves, indicating that reprocessing is not a significant contributor to optical variability at the 0.1-10% level.Comment: 39 pages including 2 appendices. Accepted for Publication in the Astrophysical Journal, with higher resolution figure

Radiation-Hydrodynamic Simulations of Collapse and Fragmentation in Massive Protostellar Cores

We simulate the early stages of the evolution of turbulent, virialized, high-mass protostellar cores, with primary attention to how cores fragment, and whether they form a small or large number of protostars. Our simulations use the Orion adaptive mesh refinement code to follow the collapse from ~0.1 pc scales to ~10 AU scales, for durations that cover the main fragmentation phase, using three-dimensional gravito-radiation hydrodynamics. We find that for a wide range of initial conditions radiation feedback from accreting protostars inhibits the formation of fragments, so that the vast majority of the collapsed mass accretes onto one or a few objects. Most of the fragmentation that does occur takes place in massive, self-shielding disks. These are driven to gravitational instability by rapid accretion, producing rapid mass and angular momentum transport that allows most of the gas to accrete onto the central star rather than forming fragments. In contrast, a control run using the same initial conditions but an isothermal equation of state produces much more fragmentation, both in and out of the disk. We conclude that massive cores with observed properties are not likely to fragment into many stars, so that, at least at high masses, the core mass function probably determines the stellar initial mass function. Our results also demonstrate that simulations of massive star forming regions that do not include radiative transfer, and instead rely on a barotropic equation of state or optically thin heating and cooling curves, are likely to produce misleading results.Comment: 23 pages, 18 figures, emulateapj format. Accepted to ApJ. This version has minor typo fixes and small additions, no significant changes. Resolution of images severely degraded to fit within size limit. Download the full paper from http://www.astro.princeton.edu/~krumholz/recent.htm

Transmission Electron Study of Heteroepitaxial Growth in the BiSrCaCuO System

Films of Bi$\rm _2$Sr$\rm _2$CaCu$\rm _2$O$\rm _8$ and Bi$\rm _2$Sr$\rm _2$CuO$\rm _6$ have been grown using Atomic-Layer-by-Layer Molecular Beam Epitaxy (ALL-MBE) on lattice-matched substrates. These materials have been combined with layers of closely-related metastable compounds like Bi$\rm _2$Sr$\rm _2$Ca$\rm _7$Cu$\rm _8$O$\rm _{20}$ (2278) and rare-earth-doped compounds like Bi$\rm _2$Sr$\rm _2$Dy$\rm _x$Ca$\rm _{1-x}$Cu$\rm _2$O$\rm _8$ (Dy:2212) to form heterostructures with unique superconducting properties, including superconductor/insulator multilayers and tunnel junctions. Transmission electron microscopy (TEM) has been used to study the morphology and microstructure of these heterostructures. These TEM studies shed light on the physical properties of the films, and give insight into the growth mode of highly anisotropic solids like Bi$\rm _2$Sr$\rm _2$CaCu$\rm _2$O$\rm _8$.Comment: 17 pages, submitted to J. Materials Research. Email to [email protected] if you want to receive copies of the figure

Optically excited states in positronium

Optical excitation are reported of the 1 3S-2 3P transition in positronium, and a second excitation from n=2 to higher n states. The experiment used light from two pulsed dye lasers. Changes in the positronium annihilation rate during and after the laser pulse were used to deduce the excited state populations. The n=2 level was found to be saturable and excitable to a substantial fraction of n=2 positronium to higher levels. Preliminary spectroscopic measurements were performed on n=14 and n=15 positronium

Observations of Binary Stars with the Differential Speckle Survey Instrument. II. Hipparcos Stars Observed in 2010 January and June

The results of 497 speckle observations of Hipparcos stars and selected other targets are presented. Of these, 367 were resolved into components and 130 were unresolved. The data were obtained using the Differential Speckle Survey Instrument at the WIYN 3.5 m Telescope. (The WIYN Observatory is a joint facility of the University of Wisconsin-Madison, Indiana University, Yale University, and the National Optical Astronomy Observatories.) Since the first paper in this series, the instrument has been upgraded so that it now uses two electron-multiplying CCD cameras. The measurement precision obtained when comparing to ephemeris positions of binaries with very well known orbits is approximately 1-2 mas in separation and better than 0°.6 in position angle. Differential photometry is found to be in very good agreement with Hipparcos measures in cases where the comparison is most relevant. We derive preliminary orbits for two systems