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  • Other Sources  (6)
  • ASTROPHYSICS  (6)
  • Analytical Chemistry and Spectroscopy
  • Biochemistry and Biotechnology
  • 2015-2019
  • 1990-1994
  • 1985-1989  (6)
  • 1970-1974
  • 1945-1949
  • 1989  (3)
  • 1985  (3)
  • 1
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    Significant variation from a 1/R-squared potential in the cumulative flux determined from the Giotto Comet Halley Dust Impact Experiment (1989)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The Dust Impact Detection System (DIDSY) aboard the Giotto spacecraft provided the information on the dust flux, mass spectrum, and cumulative mass distribution flux in the coma of Comet Halley. Analysis of discrete pulse height data of cometary particles for the mass range of particles between 4.0 x 10 to the -10th g and 6.0 x 10 to the -6th g registered by the Giotto DIDSY detectors 2, 3, and 4 has been completed, and a cumulative flux has been determined for this size range of particles. Inside the cometopause, anomalous peaks have been identified as deviation from a 1/R-squared curve in both pre- and postencounter measurements.
    Keywords: ASTROPHYSICS
    Type: Lunar and Planetary Science Conference; Mar 14, 1988 - Mar 18, 1988; Houston, TX; United States
    Format: text
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    NASA TECHNICAL REPORTS
  • 2
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    r-process nucleosynthesis in dynamic helium-burning environments (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The results of an extended examination of r-process nucleosynthesis in helium-burning enviroments are presented. Using newly calculated nuclear rates, dynamical r-process calculations have been made of thermal runaways in helium cores typical of low-mass stars and in the helium zones of stars undergoing supernova explosions. These calculations show that, for a sufficient flux of neutrons produced by the C-13 neutron source, r-process nuclei in solar proportions can be produced. The conditions required for r-process production are found to be 10 to the 20th-10 to the 21st neutrons per cubic centimeter for times of 0.01-0.1 s and neutron number densities in excess of 10 to the 19th per cubic centimeter for times of about 1 s. The amount of C-13 required is found to be exceedingly high - larger than is found to occur in any current stellar evolutionary model. It is thus unlikely that these helium-burning environments are responsible for producing the bulk of the r-process elements seen in the solar system.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 294; 656-662
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    NASA TECHNICAL REPORTS
  • 3
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    Accumulation of a swarm of small planetesimals (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The present gasdynamic study of the planetesimal-accumulation stage in which 10-km bodies in the neighborhood of 1 AU grow to 10 to the 25th-10 to the 27th g mass, or 'planetary embryo' size, attempts to identify the circumstances under which runaway growth forms a small number of massive embryos in the terrestrial-planet region on a 0.1-1.0 million year time-scale. No runaways are found, however, unless more plausible physical processes are invoked; in that case, runaways in the terrestrial planet region are probable on a 0.1 million-year time-scale, and the final stage of planetary accumulation may involve the growth of these embryos into the present planets on a 10-100 million-year time-scale.
    Keywords: ASTROPHYSICS
    Type: Icarus (ISSN 0019-1035); 77; 330-357
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    NASA TECHNICAL REPORTS
  • 4
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    Further evidence for a circumstellar disk around PV Cephei (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Spectra of PV Cephei from 2 to 4 microns reveal a moderately strong 3 micron ice-absorption feature. Normally seen in objects with high visual obscuration, A(V) greater than 10, this absorption in an object with A(V) = 3.5-5.0 suggests the existence of a dense circumstellar disk. This paper discusses the evidence for a disk from this data and other wavelength regions, the possible origin of the 3-micron feature in PV Cephei, and the relationship between this object and other young stellar objects.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 98; 1382-138
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    NASA TECHNICAL REPORTS
  • 5
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    On bipolar ejection (1985)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Observations of bipolar outflows, including jets often with clumpy concentrations of matter, have been made for a wide variety of astronomical systems. In most but not all of the systems, an accretion disk is present. It is proposed that the general process responsible for bipolar ejection involves the conversion of rotational energy into magnetic energy, usually in the form of a polar magnetic torus, deep in the interiors of the systems involved. If the buoyancy of the torus resullts in draining the field lines of most of the matter which they thread, then the acceleration of the remaining matter in the toroidal bubble may produce velocities in excess of the escape velocity from the surface of the system. It is contemplated that this process will be repeated many times in most systems. A discussion is given of the application of these ideas to protostars, to stars evolved beyond the main sequence, to neutron stars, and to black holes on both stellar and galactic scales.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 299; L83-L86
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    NASA TECHNICAL REPORTS
  • 6
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    The range of validity of the two-body approximation in models of terrestrial planet accumulation. II - Gravitational cross sections and runaway accretion (1985)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The validity of the two-body approximation in calculating encounters between planetesimals has been evaluated as a function of the ratio of unperturbed planetesimal velocity (with respect to a circular orbit) to mutual escape velocity when their surfaces are in contact (V/V-sub-e). Impact rates as a function of this ratio are calculated to within about 20 percent by numerical integration of the equations of motion. It is found that when the ratio is greater than 0.4 the two-body approximation is a good one. Consequences of reducing the ratio to less than 0.02 are examined. Factors leading to an optimal size for growth of planetesimals from a swarm of given eccentricity and placing a limit on the extent of runaway accretion are derived.
    Keywords: ASTROPHYSICS
    Type: Icarus (ISSN 0019-1035); 63; 290-303
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