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  • ASTROPHYSICS  (943)
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  • 1
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    Nucleus of comet Halley as a torque-free rigid rotator (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A model for the rotation of the nucleus of comet Halley is proposed on the assumption that the nucleus is homogeneous and a torque-free rigid rotator. It spins about its long axis with a period of 7.4 days, while this axis precesses about a fixed direction with a period of 2.2 days. To satisfy the moments of inertia, the precession angle must be 77 deg. This model settles the major problems associated with the recent controversy about two rotation periods.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Nature (ISSN 0028-0836); 325; 326-328
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  • 2
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    Pioneer 10 and 11 observations of waves upstream of interplanetary corotating shocks (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: An extended region of enhanced magnetic field fluctuations is found upstream of some of the corotating shocks observed by Pioneer 10 and Pioneer 11 between 1 and 5 AU. This perturbed region is present when the corotating shock, generally quasi-perpendicular, becomes oblique or quasi-parallel due to a temporary out-of-spiral direction of the upstream magnetic field. The observed waves are almost not compressional. Their amplitude is a large fraction of the ambient field, and their frequency is around 1 mHz in the spacecraft frame. A brief discussion of the possible mechanisms of generation is given.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 92; 285-290
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  • 3
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    Collisionless coupling in the AMPTE artificial comet (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Analysis of previously reported observations of the solar wind-barium interaction associated with the AMPTE artificial comet release of Dec. 27, 1984, is presented. On the basis of these results it is argued that solar wind couples momentum (and energy) to the barium ions through both laminar and turbulent processes. The laminar forces acting on the particles are the laminar electric and magnetic fields; the turbulent forces are associated with the intense electrostatic wave activity. This wave activity is shown to be caused by a cross-field solar wind proton-barium ion streaming instability. The observed wave frequencies and saturated amplitudes are consistent with the theoretical analysis.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 92; 47-54
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  • 4
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    Are small diamonds thermodynamically stable in the interstellar medium? (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Very small diamond particles (50 A diameter) are shown to be thermodynamically stable with respect to similar sized graphite particles for reasonable values of the surface free energies of diamond and graphite. Small diamonds are likely to be stable against both thermal evaporation and chemical attack in the general interstellar medium. A few of the consequences of these conclusions are examined.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysics and Space Science (ISSN 0004-640X); 139; 1, De
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  • 5
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    The ultraviolet variability of AY Ceti (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: AY Cet is a single-line binary comprised of a spotted G5 III primary and a white dwarf secondary. A series of UV spectra have been obtained with the IUE satellite on five different dates covering a substantial part of the optical cycle of the primary star. No evidence that the continuum or the Ly-alpha absorption line of the secondary star varied is found. There were significant changes in the strengths of the UV emission lines, but the variations were only weakly correlated with either the orbital phase of the binary or the rotational phase of the primary. The UV emission lines were especially strong near maximum visual brightness at a time when the starspot(s) on the primary was least visible. The enhanced line emission is attributed to a flare event on the primary, most likely at a high-latitude site close to the pole of this star. The UV radiative losses of this flare were comparable with those of flares previously observed on the RC CVn variables Lambda And and HR 1099.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomical Journal (ISSN 0004-6256); 94; 1657-166
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  • 6
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    On the saturation of the refractive index structure function. II - Influence of the correlation length on astronomical 'seeing' (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A physical length scale in the wavefront corresponding to the parameter (r sub 0) characterizing the loss in detail in a long exposure image is identified, and the influence of the correlation scale of turbulence as r sub 0 approaches this scale is shown. Allowing for the effect of 2-point correlations in the fluctuations of the refractive index, Venkatakrishnan and Chatterjee (1987) proposed a modified law for the phase structure function. It is suggested that the departure of the phase structure function from the 5/3 power law for length scales in the wavefront approaching the correlation scale of turbulence may lead to better 'seeing' at longer wavelengths.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 229; 379-382
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  • 7
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    Microturbulence in the upper photosphere of Alpha Persei (F5 Ib) derived from ultraviolet spectral observations (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: High-resolution ultraviolet spectra of the moderate supergiant Alpha Per (F5 Ib) were studied to determine the dynamic state of its upper photosphere. It was found that the line-of-sight microturbulent velocity component in the region of origin of the UV spectrum is about 5 km/s, and is slightly smaller than the value derived from the visual spectrum. This is ascribed to dissipation of mechanical energy between the higher and lower layers where, respectively, the ultraviolet and visual light lines originate. Between these two levels, which are one scale height apart, the mechanical energy flux decreases to about 0.3 of its photospheric value. The consequent value for the (outward directed) turbulent acceleration is 24 cm/sec-squared, more than one half the observationally determined effective acceleration of gravity.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 185; 1-2,
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  • 8
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    Deconvolution of a pre-outburst picture of SN 1987A (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The block iterative method (Young 1971) of algebraic image restoration is applied to a photographic plate of the field around SN 1987A obtained 4 yr before outburst by Chu at the CTIO 4-m telescope. By setting appropriate constraints for the solution on a pixel-by-pixel basis, the image of a starfield centered on Sk -69 202, the precursor to the supernova, is restored. It is found that this star has two companions, which others (e.g., Walborn et al., 1987 and West et al., 1987) have also noted and designated as Star 2 and Star 3. Also found is marginal evidence for Star 4, a weak source at theta = 222 deg, r = 1.9 arcsec. Correction for the contributions of these companions yields an estimate of the magnitude for Sk -69 202 alone of V = 12.37.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 185; 1-2,
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  • 9
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    Helium diffusion in stars and the value of the cosmological constant (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The effect of He diffusion on the evolution of stars is to speed it up by increasing the mean atomic weight at the center, thereby minimizing the apparent conflict between the great ages of globular clusters and the age of the universe, as arrived at via the Hubble constant. Attention is presently given to the consequences of work on this effect by Noerdlinger and Arrigo (1980) and Stringfellow et al. (1983), implying that Tayler's (1986) estimate for globular clusters of 16 Gyr should be reduced to 13 Gyr.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Royal Astronomical Society, Quarterly Journal (ISSN 0035-8738); 28; 345
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  • 10
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    Particle acceleration and production of energetic photons in SN1987A (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A pulsar wind model for the acceleration of particles in SN1987A is discussed. The expected photon flux is investigated in terms of the spectrum of parent protons and electrons, the nature of the region in which they propagate after acceleration, and the magnetic field and radiation environment which determines the subsequent fate of produced photons. The model is found to produce observable signals if the spin period of the pulsar is 10 ms or less.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Nature (ISSN 0028-0836); 329; 314-316
    Format: text
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