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  • Other Sources  (10)
  • COMPUTERS  (4)
  • ASTROPHYSICS  (3)
  • Numerical Analysis  (3)
  • 2005-2009  (3)
  • 1970-1974  (7)
  • 1
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    The interaction of interstellar gas with stellar density wave packets (1974)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: We have numerically studied the interaction between interstellar gas and a gravitational perturbation produced by a fluctuation in the stellar mass distribution. For a simple equation of state, shock waves develop even if the gravitational perturbation is aperiodic and transient. In a medium which can change thermal phase, shocks do not form and the gas makes a transition directly to the cold, dense phase. The cold material has an anomalous velocity imparted to it by the gravitational perturbation and it becomes an accretion front moving through the hot gas and capturing it. The accretion fronts trigger thermal phase change on a large scale and can reproduce the extent of observed cold clouds. Furthermore their high mass is favorable to gravitational instability and subsequent star formation.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics; 36; 2, No; Nov. 197
    Format: text
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  • 2
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    Fine-scale structure of a cold hydrogen layer (1974)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: Neutral-hydrogen absorption-line interferometer observations are reported for five continuum radio sources situated behind an extensive layer of cold interstellar H I gas. In contrast to low-spatial-resolution emission-line observations reported earlier which show the large-scale uniformity of the layer, the high-spatial-resolution absorption-line observations reveal variations in the velocity and density within the layer. The new results together with the old indicate the apparent coexistence of a large-scale mechanism for triggering the formation of cold hydrogen and an instability mechanism leading to small-scale irregularities.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; 194; Dec. 1
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  • 3
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    An experimental test of the homogeneity of the interstellar medium (1974)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: Observations of nonthermal continuum radio emission at 750 MHz and hydrogen-line emission at 1420 MHz have been compared in order to test the extent to which cosmic-ray electrons, magnetic fields, and hydrogen gas coexist in the interstellar medium. If the interstellar medium is homogeneous in the sense that its constituents are mixed together and can interact, then regions of line and continuum emission should be spatially correlated. The measurements indicate that at most 28% of continuum emission in the Galaxy comes from such a homogeneous medium. The remaining nonthermal emission agrees in magnitude with the residual emission found by Berkhuijsen after subtraction of the polarized component of nonthermal emission. In addition, the data discussed here show that the major filamentary structures found in the two forms of emission rarely coincide spatially. Consequently, both the average and the exceptional structure in the medium indicate that the synchrotron-radiating magnetoplasma and the neutral hydrogen gas are not closely interacting in the present state of the medium.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; 194; Dec. 1
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  • 4
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    Arbitrary grammars generating context-free languages (1972)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: If G is a grammar such that in each noncontext-free rule of G, the right side contains a string of terminals longer than any terminal string appearing between two nonterminals in the left side; then the language generated by G is context-free. Six previous results follow as simple corollaries of this theorem.
    Keywords: COMPUTERS
    Type: NASA-CR-127764 , REPT-11-72
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  • 5
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    A spacecraft computer repairable via command. (1971)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The MULTIPAC is a central data system developed for deep-space probes with the distinctive feature that it may be repaired during flight via command and telemetry links by reprogramming around the failed unit. The computer organization uses pools of identical modules which the program organizes into one or more computers called processors. The interaction of these modules is dynamically controlled by the program rather than hardware. In the event of a failure, new programs are entered which reorganize the central data system with a somewhat reduced total processing capability aboard the spacecraft. Emphasis is placed on the evolution of the system architecture and the final overall system design rather than the specific logic design.
    Keywords: COMPUTERS
    Type: IEEE Transactions on Aerospace and Electronic Systems; AES-7; Sept
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  • 6
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    Non-context-free grammars generating context-free languages (1974)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: If G is a grammar such that in each non-context-free rule of G, the right side contains a string of terminals longer than any terminal string appearing between two nonterminals in the left side, then the language generated by G is context free. Six previous results follow as corollaries of this theorem.
    Keywords: COMPUTERS
    Type: Information and Control; 24
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  • 7
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    Geometric aspects in digital analysis of Multi-Spectral Scanner (MSS) data (1973)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Present automated systems of interpretation which apply pattern recognition techniques on MSS data do not fully consider the geometry of the acquisition system. In an effort to improve the usefulness of the MSS data when digitally treated, geometric aspects are analyzed and discussed. Attempts to correct for scanner instabilities in position and orientation by affine and polynomial transformations, as well as by modified collinearity equations are described. Methods of accounting for panoramic and relief effects are also discussed. It is anticipated that reliable area as well as position determinations can be accomplished during the process of automatic interpretation. A concept for a unified approach to the treatment of remote sensing data, both metric and nonmetric is presented.
    Keywords: COMPUTERS
    Type: NASA-CR-133641 , LARS-042573
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  • 8
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    LISA Parameter Estimation using Numerical Merger Waveforms (2008)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Coalescing supermassive black holes are expected to provide the strongest sources for gravitational radiation detected by LISA. Recent advances in numerical relativity provide a detailed description of the waveforms of such signals. We present a preliminary study of LISA's sensitivity to waveform parameters using a hybrid numerical/analytic waveform describing the coalescence of two equal-mass, nonspinning black holes. The Synthetic LISA software package is used to simulate the instrument response and the Fisher information matrix method is used to estimate errors in the waveform parameters. Initial results indicate that inclusion of the merger signal can significantly improve the precision of some parameter estimates. For example, the median parameter errors for an ensemble of systems with total redshifted mass of 10(exp 6) deg M solar mass at a redshift of z is approximately 1 were found to decrease by a factor of slightly more than two when the merger was included.
    Keywords: Numerical Analysis
    Type: 7th International LISA Symposium/Space Sciences Institute of California (IEEC); Jun 16, 2008 - Jun 20, 2008; Barcelona; Spain
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  • 9
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    Spherical Harmonic Decomposition of Gravitational Waves Across Mesh Refinement Boundaries (2005)
    In:  CASI
    Details
    Publication Date: 2019-07-11
    Description: We evolve a linearized (Teukolsky) solution of the Einstein equations with a non-linear Einstein solver. Using this testbed, we are able to show that such gravitational waves, defined by the Weyl scalars in the Newman-Penrose formalism, propagate faithfully across mesh refinement boundaries, and use, for the first time to our knowledge, a novel algorithm due to Misner to compute spherical harmonic components of our waveforms. We show that the algorithm performs extremely well, even when the extraction sphere intersects refinement boundaries.
    Keywords: Numerical Analysis
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  • 10
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    Parameter Estimation using Numerical Merger Waveforms (2008)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Results: Developed parameter estimation model integrating complete waveforms and improved instrumental models. Initial results for equal-mass non-spinning systems indicate moderate improvement in most parameters, significant improvement in some Near-term improvement: a) Improved statistics; b) T-channel; c) Larger parameter space coverage. Combination with other results: a) Higher harmonics; b) Spin precession; c) Instrumental effects.
    Keywords: Numerical Analysis
    Type: 7th International LISA Symposium/Space Sciences Institute of Catalonia; Jun 16, 2008 - Jun 20, 2008; Barcelona; Spain
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