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  • ASTROPHYSICS  (16,581)
  • Biochemistry and Biotechnology  (13,095)
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  • 1
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    Can comet clouds around neutron stars explain gamma-ray bursts? (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The proposal of Harwit and Salpeter (1973) that gamma-ray bursts are due to impacts of comets onto neutron stars is examined further. It is assumed that most stars are formed with comet clouds similar to the Oort comet cloud which surrounds the sun, and it is suggested that there are at least four mechanisms by wich neutron stars may be formed while retaining their comet clouds: a spherically symmetric supernova explosion in an isolated star, accretion-induced collapse of a white dwarf in a cataclysmic variable with a very low mass secondary, accretion-induced collapse of a white dwarf in a wide binary with a low-mass giant companion, and coalescence of a close binary composed of two white dwarfs. Estimates are given of the cometary impact rates for such systems. It is suggested that if the wide binary scenario is correct, optical bursts may arise from the impact of comets onto the white dwarf remnant of the giant companion.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 301; 155-163
    Format: text
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  • 2
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    Flux-limited diffusion in a scattering medium (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A diffusion equation (FDT) is presented with a coefficient that reduces to the appropriate limiting form in the streaming and near thermodynamic limits for a moving fluid in which the dominant source of opacity is Thomson scattering. The present results are compared to those obtained with the corresponding equations for an absorptive medium. It is found that FDT for a scattering medium is accurate to better than less than about 17 percent over the range of optical depths of tau in the range of about 0 to 3.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 374; 732-740
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  • 3
    Electronic Resource
    Electronic Resource
    Proliferation of anchorage-dependent contact-inhibited cells: I. Development of theoretical models based on cellular automata (1991)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 38 (1991), S. 459-470 
    Details
    ISSN: 0006-3592
    Keywords: cell culture ; contact inhibition phenomena ; discrete mathematical model ; cell proliferation ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: We report the development of new class of discrete models that can accurately describe the contact-inhibited proliferation of anchorage-dependent cells. The models are based on cellular automata, and they quantitatively account for contact inhibition phenomena occurring during all stages of the proliferation process: (a) the initial stage of “exponential” growth of cells without contact inhibition; (b) the second stage where cell colonies form and grow with few colony mergings; and (c) the final stage where proliferation rates are dominated by colony merging events. Model prediction are presented and analyzed to study the complicated dynamics of large cell populations and determine how the initial spatial cell distribution, the seeding density, and the geometry of the growth surface affect the observed proliferation rates. Finally, we present a model variant that can simulate contact-inhibited proliferation of asynchronous cell populations with arbitrary cell cycle-time distribution. The latter model can also compute the percentage of cells that are in a specific phase of their division cycle at a given time.
    Additional Material: 17 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bit.260380504
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  • 4
    Electronic Resource
    Electronic Resource
    Proliferation of anchorage-dependent contact-inhibited cells. II: Experimental results and validation of the theoretical modelsParts of this work were presented at the 1988 annual meeting of the AUChE, Washington, DC, November 27-December 2 (1988); 1989 annual meeting of the AIChE, San Francisco, CA, November 5-10 (1989); 1990 FASEB Meeting, Washington, D.C., April 2-6 (1990); and 199th national meeting of the American chemical society, Boston, MA, April 22-27 (1990). (1991)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 38 (1991), S. 471-479 
    Details
    ISSN: 0006-3592
    Keywords: endothelial cells ; cell culture contact inhibition ; mathematical model ; experimental data ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: This study establishes that the cellular automata models developed in an earlier article capture the essential features of the proliferation process for anchorage-dependent contact-inhibited cells. Model predictions are in excellent agreement with experimental data obtained with bovine pulmonary artery endothelial cells. The models are particularly suitable for predictive purposes since they have no adjustable parameters. All model parameters can be easily obtained from a priori measurements. Our studies also show that proliferation rates are very sensitive to the spatial distributions of seed cells. The adverse effects of seeding heterogeneities become more pronounced as a cell population approaches confluency and they should be accounted for in experimental studies attempting to assess the response of cells to external stimuli.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bit.260380505
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  • 5
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    X-ray refelection from photoionized media in active galactic nuclei (1994)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We calculate the spectrum of X-ray radiation and reprocessed by a partly ionized optically thick medium in an active galactic nucleus. We self-consistently calculate the ionization balance and thermal balance in the medium along with the distribution of X-ray intensity with optical depth. In addition to absorption or scattering of the incident X-rays, we also compute the spectrum of X-rays emitted by the material, including lines, edges, and bremsstrahlung. The albedo of the medium depends primarily on the X-ray ionization parameter (ratio of incident flux to gas density, zeta(sub Chi), and secondarily on the UV flux generated by dissipation inside the disk; we locate the critical range of zeta(sub Chi) over which the albedo increases from small to nearly unity. While the continuum reflection is very weak below 10 keV when zeta(sub Chi) is small, significnat fluxes are emitted in atomic lines and edges in this energy range. In the limit of large zeta(sub Chi), the albedo below 10 keV increases, but reflection in this band is never gray: some photoelectric absorption remains up to rather large values of zeta(sub Chi), while at still higher values, inverse Compton scattering amplifies the soft X-ray flux. These features are sufficiently sharp that current and near-future X-ray experiments should permit diagnostic measures of zeta(sub Chi).
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 437; 2; p. 597-610
    Format: text
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  • 6
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    COMPTEL neutron response at 17 MeV (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The Compton imaging telescope (COMPTEL) instrument of the Gamma Ray Observatory was exposed to 17 MeV d,t neutrons prior to launch. These data were analyzed and compared with Monte Carlo calculations using the MCNP(LANL) code. Energy and angular resolutions are compared and absolute efficiencies are calculated at 0 and 30 degrees incident angle. The COMPTEL neutron responses at 17 MeV and higher energies are needed to understand solar flare neutron data.
    Keywords: ASTROPHYSICS
    Type: NASA. Goddard Space Flight Center, The Compton Observatory Science Workshop,; p 109-115
    Format: application/pdf
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  • 7
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    Low-energy and medium-energy gamma rays from PSR 0531 + 21 (1985)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Results are presented from the Crab Pulsar PSR 0531 + 21 for energies of 0.3-30 MeV. For energies of 1-30 MeV, the absolute phase and separation of the first and second pulses 12.9 + or - 0.3 ms, widths of the pulses 2.2 + or - 0.5 ms FWHM, and 1.6 + or - 0.4 ms FWHM, ratio of the counts in the second to the first pulse 0.64 + or - 0.33 and ratio of counts in the interpulse region to the total pulsed counts 0.17 + or - 0.30 are compared to te results vor energies, E greater than 50 MeV and their variations with time. Values from the phase plots for E greater than 0.3 MeV are compared with low-energy gamma rays during 1970 through 1980 from several experiments with energies from about 20 to 360 keV. The above suggest different production mechanisms for low- and high-energy gamma rays with a transition at about 1 MeV. Derived fluxes from 0.3 to 30 MeV confirm a previously derived power law.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 299; L23-L27
    Format: text
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  • 8
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    Meteoroid/space debris impacts on MSFC LDEF experiments (1991)
    In:  Other Sources
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    Publication Date: 2019-01-25
    Description: A number of unusual effects are presented that were observed on the TCSE test samples, front cover, and structural components. These include induced UV fluorescence in some materials, the migration and degradation of KRS-5 materials, atomic oxygen (AO), and contaminant texturing and discoloration, and meteoroid and debris impacts. These effects and their causes are discussed.
    Keywords: ASTROPHYSICS
    Type: NASA, Langley Research Center, First LDEF Post-Retrieval Symposium Abstracts; p 33
    Format: text
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  • 9
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    Electron velocity distributions near interplantary shocks (1983)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: Previous work on electron heating mechanisms operating at the supercritical earth's bow shock is extended to the generally lower Mach number interplanetary shocks. This is done by studying electron velocity distributions observed both upstream and downstream of interplanetary shocks observed by ISEE 3 between August 1978 and December 1979. It is found that perpendicular heating accompanied by little or no parallel heating occurs at the weaker shocks but that parallel heating is dominant for the stronger shocks. When perpendicular heating is dominant, downstream velocity distributions are for the most part Gaussian at low energies (core) and Lorentzian at high energies (halo). When parallel heating is dominant, however, these distributions are flat-topped at low energies, having power law extensions to higher energies. At the weaker shocks, the observations are consistent with electron heating resulting from conservation of the magnetic moment, whereas at the stronger shocks the heating is quite similar to that observed at the earth's bow shock.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 88; 9949-995
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  • 10
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    An overview of STIP interval 18: September 1985, the G-Z encounter (1987)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The primary period of interest during September 1985 is the few days surrounding the ICE encounter with comet Giacobini-Zinner(G-Z) which occurred at 1102 UT on 11 September. To place into perspective the actual in situ observations measured during the comet encounter it is necessary to understand the prevailing solar and interplanetary conditions. Starting two solar rotations prior to and extending through the rotation after the encounter the interplanetary stream structure was very uniform, similar to the 1973-4 long-lived structure. Prior to the arrival of the corotating high-speed stream at 5 UT on 11 September, ICE was already measuring the effects of G-Z on the surrounding interplanetary medium. An overview of available solar, interplanetary, and ICE data for the cometary interval is presented.
    Keywords: ASTROPHYSICS
    Type: Alabama Univ., Huntsville. STIP Symposium on Physical Interpretation of Solar(Interplanetary and Cometary Intervals; p 37
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