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  • 1
    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
    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
    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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  • 4
    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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  • 5
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    In:  Other Sources
    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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  • 6
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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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  • 7
    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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  • 8
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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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  • 9
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    In:  Other Sources
    Publication Date: 2019-07-12
    Description: General relativistic hydrodynamical calculations of the collapse of O + Ne + Mg cores of a 9 solar mass star are reported. Collapse is induced by rapid electron captures as the O + Ne + Mg is burned to nuclear statistical equilibrium. The high entropy in the core after burning leads to a large abundance of free protons which readily capture electrons. This leads to large neutrino losses and a correspondingly small infalling homologous core. The hydrodynamic shock thus forms at a small mass point. The shock stalls before reaching the edge of the O + Ne + Mg core and thereby fails to produce a successful supernova explosion by the direct mechanism. No enhancement in the shock energy due to nuclear burning is found.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 320; 300-303
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  • 10
    Publication Date: 2019-07-12
    Description: Evidence is cited that supernova 1987A involved a large explosion energy, of about (2-3) x 10 to the 51st ergs. Such large explosion energy has not come from delayed shocks to date, nor is it likely to. Improved physics in the presupernova evolution, especially the inclusion of Coulomb interactions, has brought the iron-core mass down by less than about 0.1 solar mass in the 13 solar mass star which has recently been evolved. It is found that supernova explosion energies up to 3 x 10 to the 51st ergs can be obtained by the prompt-explosion mechanism, provided that a somewhat soft equation of state is used at supranuclear densities.
    Keywords: ASTROPHYSICS
    Type: Physical Review Letters (ISSN 0031-9007); 59; 736-739
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