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  • 1
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    Radio structure at 8.4 GHz in Sagittarius A, the compact radio source at the Galactic center (1989)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-19
    Beschreibung: VLBI observations of the compact, nonthermal radio source at the Galactic center show it to be elongated at 8.4 GHz along a position angle of 82 + or - 6 deg. The source has an axial ratio of 0.53 + or - 0.10 with a major axis of 17.4 + or - 0.5 mas. Examination of VLA maps of the Galactic center region indicate no obvious alignment with this smaller-scale elongation of the nuclear region, nor is the nuclear position angle aligned with the axis of Galactic rotation. Comparison with the size measured at frequencies from 1 to 22 GHz shows that the size follows very closely the lambda-squared dependence expected from interstellar scattering. The alongated nature of the source implies either that the scattering medium is anisotropic or that some remnant of the intrinsic structure remains visible through the scattering medium.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomical Journal (ISSN 0004-6256); 98; 44-48
    Format: text
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  • 2
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    The high-resolution structure of the Centaurus A nucleus at 2.3 and 8.4 GHz (1989)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-19
    Beschreibung: VLBI observations of the nucleus of Centaurus A have been made at two frequencies with an array of five Australian radio telescopes as part of the Southern Hemisphere VLBI Experiment. Observations were made at 2.3 GHz with all five antennas, while only two were employed at 8.4 GHz. At 2.3 GHz seven tracks in the (u,v) plane with coverage of 6-8 hr each were obtained, yielding significant information on the structure of the nuclear jet. At 8.4 GHz a compact unresolved core was detected as well. It is found that the source consists of the compact self-absorbed core, a jet containing a set of three knots extending from 100 to 160 mas from the core, and a very long, narrow component elongated along the same position angle as the knots. The allowable range for the position angle of the jet is 51 + or - 3 deg, in agreement with that of the radio and X-ray structure on arcsecond and arcminute scales. The jet has brightened at 2.3 GHz by about 4 Jy, a factor of nearly 3, since the early 1970s, 1.8 Jy of which has occurred in the last 2 yr with no discernable changes in structure.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomical Journal (ISSN 0004-6256); 98; 27-35
    Format: text
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  • 3
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    Evidence for a radio flare in the nucleus of Virgo A (1988)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-19
    Beschreibung: An increase in radio emission has been observed in the nucleus of the galaxy Virgo A by use of VLBI measurements. These 2.29 GHz observations show that the correlated flux density of the milliarcsec structure increased by 30 percent during a four month period in 1977, and eventually returned to the preflare level of about 0.6 Jy. Previous evidence had indicated that the nuclear radio emission was stable, a surprising result for an active galactic nucleus.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomical Journal (ISSN 0004-6256); 95; 1037-103
    Format: text
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  • 4
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    Nucleosynthesis, neutrino bursts and gamma-rays from coalescing neutron stars (1989)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-19
    Beschreibung: It is pointed out here that neutron-star collisions should synthesize neutron-rich heavy elements, thought to be formed by rapid neutron capture (the r-process). Furthermore, these collisions should produce neutrino bursts and resultant bursts of gamma rays; the latter should comprise a subclass of observable gamma-ray bursts. It is argued that observed r-process abundances and gamma-ray burst rates predict rates for these collisions that are both significant and consistent with other estimates.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Nature (ISSN 0028-0836); 340; 126-128
    Format: text
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  • 5
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    On relative supernova rates and nucleosynthesis roles (1989)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-19
    Beschreibung: It is shown that the Ni-56-Fe-56 observed in SN 1987A argues that core collapse supernovae may be responsible for more than 50 percent of the iron in the galaxy. Furthermore it is argued that the time averaged rate of thermonuclear driven Type I supernovae may be at least an order of magnitude lower than the average rate of core collapse supernovae. The present low rate of Type II supernovae (below their time averaged rate of approx. 1/10 yr) is either because the past rate was much higher because many core collapse supernovae are dim like SN 1987A. However, even in this latter case they are only an order of magnitude dimmer that normal Type II's due to the contribution of Ni-56 decay to the light curve.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 339; L25-L27
    Format: text
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  • 6
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    Pre-perihelion observations of Comet Halley at 2-13 microns (1986)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: Between Nov. 8, 1985 and Jan. 13, 1986 comet Halley brightness was obtained as a function of time at 2.3, 3.6, 4.6, and 10.3 microns. Brightness outbursts were noted, an especially significant one beginning Jan. 10, 1986 and persisting for several days during which the IR brightness more than doubled. The heliocentric brightness indices for 2.3, 3.6, 4.6, and 10.3 microns are -5.7, -7.1, -8.5, and -6.2, respectively. Comet Halley's absolute magnitudes at these wavelengths are 7.3, 4.4, 2.8, and -1.8. The 4.6 to 10.3 micron color temperature dependence on heliocentric distance (R) is slightly less steep than the Rsup -1/2 expected theoretically. The albedo of the comet is 8% to 9%. Based on spectral differences between outbursts on Nov. 8, 1985 and Jan. 10 to 13, 1986, it is concluded that the comet displays composition inhomogeneities.
    Schlagwort(e): ASTROPHYSICS
    Materialart: ESA, Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 3: Posters; p 479-484
    Format: text
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  • 7
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    The shocking development of lithium (and boron) in supernovae (1989)
    In:  CASI
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: It is shown that significant amounts of Li-7 and B-11 are produced in Type 2 supernovae. The synthesis of these rare elements occurs as the supernova shock traverses the base of the hydrogen envelope burning He-3 to masses 7 and 11 via alpha capture. The yields in this process are sufficient to account for the difference in lithium abundance observed between Pop 2 and Pop 1 stars. Since lithium (and boron) would, in this manner, be created in the same stars that produce the bulk of the heavy elements, the lithium abundance even in old Pop 1 stars would be high (as observed). The B-11 production may remedy the long-standing problem of the traditional spallation scenario to account for the observed isotopic ratio of boron. Observational consequences of this mechanism are discussed, including the evolution of lithium and boron isotope ratios in the Galaxy and the possible use of the boron yields to constrain the number of blue progenitor Type 2 supernovae.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NASA-CR-183330 , NAS 1.26:183330 , FERMILAB-CONF-89/50-A
    Format: application/pdf
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  • 8
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    On relative supernova rates and nucleosynthesis roles (1988)
    In:  CASI
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: It is shown that the Ni-56-Fe-56 observed in SN 1987A argues that core collapse supernovae may be responsible for more that 50 percent of the iron in the galaxy. Furthermore it is argued that the time averaged rate of thermonuclear driven Type I supernovae may be at least an order of magnitude lower than the average rate of core collapse supernovae. The present low rate of Type II supernovae (below their time averaged rate of approx. 1/10 yr) is either because the past rate was much higher because many core collapse supernovae are dim like SN 1987A. However, even in this latter case they are only an order of magnitude dimmer that normal Type II's due to the contribution of Ni-56 decay to the light curve.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NASA-CR-183373 , NAS 1.26:183373 , FERMILAB-PUB-88/118-A
    Format: application/pdf
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  • 9
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    The area of isodensity contours in cosmological models and galaxy surveys (1989)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-12
    Beschreibung: The contour crossing statistic, defined as the mean number of times per unit length that a straight line drawn through the field crosses a given contour, is applied to model density fields and to smoothed samples of galaxies. Models in which the matter is in a bubble structure, in a filamentary net, or in clusters can be distinguished from Gaussian density distributions. The shape of the contour crossing curve in the initially Gaussian fields considered remains Gaussian after gravitational evolution and biasing, as long as the smoothing length is longer than the mass correlation length. With a smoothing length of 5/h Mpc, models containing cosmic strings are indistinguishable from Gaussian distributions. Cosmic explosion models are significantly non-Gaussian, having a bubbly structure. Samples from the CfA survey and the Haynes and Giovanelli (1986) survey are more strongly non-Gaussian at a smoothing length of 6/h Mpc than any of the models examined. At a smoothing length of 12/h Mpc, the Haynes and Giovanelli sample appears Gaussian.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 340; 647-660
    Format: text
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  • 10
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    Numerical simulations of magnetized jets (1989)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-12
    Beschreibung: The present axisymmetric numerical simulations of light hypersonic jets allow unmagnetized jets and jets carrying a dynamically important magnetic field to be contrasted. After decelerating a weakly magnetized jet through a series of weak, oblique shocks, a Mach disk and a strong annular shock are encountered near the outer edges of the contact discontinuity separating the shocked fluid from the shocked ambient gas. Upon passing the annular shock, the gas quickly expands and enters a backflowing cocoon surrounding the jet. The overall speed of advance of the jet is reduced; matter near the jet axis which passes through the terminal Mach disk accumulates in a plug, and gas is discharged into the cocoon by the intermittent shedding of vortices. When magnetic stresses dominate, however, the jet is rapidly decelerated via a Mach disk and strong annular shock.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 344; 89-103
    Format: text
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