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  • Other Sources  (12)
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  • 1
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    Pressure-confined Lyman-alpha clouds (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Results are presented of numerical models of pressure-confined spherical gas clouds which produce absorption resembling the low to intermediate atomic column density lines found in high-redshift QSO spectra. One-dimensional hydrodynamical models including electron conduction are described, and the rate equations are solved to find ionization and excitation states. Results are presented for both static and adiabatically expanding confining media covering a range of initial pressures. It is found that Ly-alpha lines are very similar over a wide range of conditions and that the most promising diagnostic of pressure is to compare the column density in H I to that in He I and He II. No single-pressure model can explain the wide range of observed H I column densities.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 337; 609-616
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  • 2
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    Modeling and interpretation of the optical and HST UV spectrum of SN 1993J (1994)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We present spherically symmetric, non-local thermodynamic equilibrium (non-LTE), expanding model atmosphere synthetic spectra of SN 1993J and compare them to the ultraviolet and optical spectra obtained simultaneously on 1993 April 15 by the Hubble Space Telescope (HST) and at Lick Observatory. We are able to fit the optical and HST near-UV spectra with a variety of compositions, but find that our best fits are obtained with an enhanced helium abundance (Y = 0.8) and significant nonthermal ionization due to gamma rays produced by the decay of Ni-56. We find the effects of X-rays produced by thermal bremsstrahlung in the circumstellar material to have little effect on the spectrum. Our current models are unable to fit the HST far-UV region, predicting too little flux and strong, unseen Fe II features.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 426; 1; p. 334-339
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  • 3
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    Flux-limited diffusion in hydrodynamics (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The radiation transport equation, correct to order v/c, is used to derive a flux limiter valid in high-velocity hydrodynamic flows. In the limit v = 0 the widely used result of Levermore and Pomraning (LP) (1982) is recovered. In the case of homologous flow (v varies as R) the LP result remains valid, but in the presence of shock waves and other nonhomologous flows differences may be large. We calculate the Eddington factors, which are significantly altered, and discuss the importance of having them possess the correct form in the presence of a shock wave, e.g., on the neutrino transport during the formation of a supernova.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 398; 2; p. 531-543.
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  • 4
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    Spectrum synthesis of the Type Ia supernovae SN 1992A and SN 1981B (1995)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We present non-local thermodynamic equilibrium (non-LTE) synthetic spectra for the Type Ia supernovae SN 1992A and SN 1981B, near maximum light. At this epoch both supernovae were observed from the UV through the optical. This wide spectral coverage is essential for determining the density structure of a SN Ia. Our fits are in good agreement with observation and provide some insight as to the differences between these supernovae. We also discuss the application of the expanding photosphere method to SNe Ia which gives a distance that is independent of those based on the decay of Ni-56 and Cepheid variable stars.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 441; 1; p. L33-L36
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  • 5
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    Non-LTE spectral analysis and model constraints on SN 1993J (1995)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We present non-Local Thermodynamic Equilibrium (LTE) synthetic spectra for a time series of observations of SN 1993J obtained on 1993 March 30-31, April 7, April 13-15, and June 13 UT. The spectra are dominated by hydrogen Balmer lines; neutral helium lines, which have been nonthermally excited; and Fe II features. The density profile evolves from an extremely steep 'brick wall' structure with an equivalent power-law index of about 50 on March 30 to a more typical SN II profile with a power law index of about 10. The early spectra are well fitted by a solar composition of metals, although an enhanced abundance of helium is required in order to fit the neutral helium lines. By June 13, the photosphere has receded deep into the helium layer, although there appears to be a layer of hydrogen at higher velocity. The distance is estimated for each epoch. While consistent results are found for spectra obtained in the month of April, the spread in distances from March to June is quite large. Our value for April is mu = 28.0 +/- 0.3 mag, consistent with the recent Cepheid distance to the host galaxy M81. We also compare our results to other implementations of the expanding photosphere method.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 441; 1; p. 170-181
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  • 6
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    Shock waves and nucleosynthesis in type II supernovae (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: In the study of nucleosynthesis in type II SN, shock waves are initiated artificially, since collapse calculations do not, as yet, give self-consistent shock waves strong enough to produce the SN explosion. The two initiation methods currently used by light-curve modelers are studied, with a focus on the peak temperatures and the nucleosynthetic yields in each method. The various parameters involved in artificially initiating a shock wave and the effects of varying these parameters are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 370; 630-642
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  • 7
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    The collapse of white dwarfs to neutron stars (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The observable consequences of an accreting white dwarf collapsing directly to a neutron star are considered. The outcome depends critically upon the nature of the wind that is driven by neutrino absorption in the surface layers as the dwarf collapses. Unlike previous calculations which either ignored mass loss or employed inadequate zoning to resolve it, a characteristic mass-loss rate of about 0.005 solar mass/s and an energy input of 5 x 10 exp 50 ergs/s is found. Such a large mass-loss rate almost completely obscures any prompt electromagnetic display and certainly rules out the production by this model of gamma-ray bursts situated at cosmological distances. The occurrence of such collapses with the Milky Way Galaxy might, however, be detected and limited by their nucleosynthesis and gamma-ray line emission. To avoid the overproduction of rare neutron-rich isotopes heavier than iron, such events must be very infrequent, probably happening no more than once every thousand years.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 391; 1 Ma; 228-235
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  • 8
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    The appearance of primeval galaxies (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The appearance of protogalaxies expected by current theories of structure formation is discussed. A review of when and how galaxies form is given, and the results of two numerical simulations of inhomogeneous galaxy formation are presented. These models are used to predict the abundance of primeval galaxies in observational surveys.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 322; 585-596
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  • 9
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    Neutrino pair energy deposition in supernovae (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The role of neutrino-antineutrino annihilation (producing an electron-positron pair) in delayed type II supernova explosions is investigated theoretically. The assumptions and analysis of Goodman et al. (1987) are examined in detail, with a focus on the need for a large density gradient at the edge of the neutrinosphere. It is found that the neutrino annihilation mechanism cannot contribute significantly to the energy of the explosion.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 321; L129-L13
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  • 10
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    Collapsing white dwarfs (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The results of the hydrodynamic collapse of an accreting C + O white dwarf are presented. Collapse is induced by electron captures in the iron core behind a conductive deflagration front. The shock wave produced by the hydrodynamic bounce of the iron core stalls at about 115 km, and thus a neutron star formed in such a model would be formed as an optically quiet event.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 320; 304-307
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