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  • 1
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    The waiting point approximation in R-process calculations (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The steady flow approximation to the r-process has been used to evaluate the validity of the waiting point assumption (that, for a given charge number Z, neutron captures become balanced by neutron photodisintegrations) under a variety of conditions. The criterion for the accuracy of the approximation for any set of nuclei with the same value of Z is that the ratio of the total rate of neutron photodisintegration in those nuclei to the total rate of neutron captures should be close to unity. No set of conditions involving neutron production by helium thermonuclear reactions (at temperatures up to 10 to the 9th K) could be found for which the waiting point approximation is useful for all values of Z, and it is recommended that all such r-process calculations not use this approximation. At higher temperatures, such as those characteristic of explosive carbon burning (2 x 10 to the 9th K), the waiting point approximation is much better.
    Keywords: ASTROPHYSICS
    Type: Astrophysics and Space Science (ISSN 0004-640X); 91; 2 Ap; April 19
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  • 2
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    Steady flow approximations to the helium r-process (1983)
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    Publication Date: 2019-06-28
    Description: A steady flow approximation to the r-process is presented and used for numerical experiments with physical quantities to determine the sensitivity of the process to variations in those quantities. The effect of neutron capture cross sections along the capture path and of recently available improved beta decay rates on the r-process are discussed. The peaks in the observed r-process yield curve near mass numbers 80 and 130 are roughly characterized by a neutron number density of 10 to the 20th per cu/cm; the mean beta decay rates are about 10/s, and the freezing time is comparable to or less than 0.1 s. The peak near mass number 195 is roughly characterized by a neutron number density of 10 to the 21st/cm, the mean beta decay rates are about 100/s, and the freezing time is comparable to or less than 0.01 s. The flow path of the steady state r-process is sensitively dependent upon the neutron capture cross sections in the flow network and on the values of the beta decay rates.
    Keywords: ASTROPHYSICS
    Type: Astrophysics and Space Science (ISSN 0004-640X); 91; 2 Ap; April 19
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  • 3
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    Dissipation of thick accretion disks (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A thick accretion disk which is isentropic cannot have simple laminar flow because fluid elements follow orbits which intersect the orbits of other fluid elements, leading to turbulence in astrophysical disks which have very large Reynolds numbers. The turbulence in such disks is estimated using molecular analogies for the behavior of the fluid elements. The usual empirical dissipation parameter 'alpha' is found to be equal to 0.25 under normal circumstances. Characteristic local disk parameters are calculated for a variety of conditions at different distances from a central star of one solar mass. Circumstances involving low midplane optical depths or external heating which can lead to large reductions in the turbulence are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysics and Space Science (ISSN 0004-640X); 93; 2 Ju; June 198
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  • 4
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    Evolution of giant gaseous protoplanets embedded in the primitive solar nebula (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: In contrast to earlier evolutionary calculations, which assumed that protoplanets are isolated, the present spherically symmetric approximation for a protoplanet of one Jovian mass during the early phase of quasi-static contraction invokes a time-dependent surface boundary condition that simulates physical conditions in an evolving primitive solar nebula. Assuming in a first set of calculations that the protoplanet is surrounded by a thermal bath whose temperature varies with time, and whose pressure is small and constant, results show evaporation and complete dispersal of the object. A second set of calculations varies both temperature and pressure at the surface with time, according to solar nebula models, and yields an acceleration or retardation of evolution, relative to that of an isolated protoplanet, depending on the relative entropy of the nebula and the protoplanet's outer layers. Processes by which terrestrial planets can form in the cores of giant gaseous protoplanets are discussed.
    Keywords: ASTROPHYSICS
    Type: Icarus; 49; Mar. 198
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  • 5
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    The heavy element yields of neutron capture nucleosynthesis (1982)
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    Publication Date: 2019-06-28
    Description: Consideration of the contribution made to the abundances of the heavy element isotopes by the S- and R-processes of nucleosynthesis has led to the determination that the previous assumption concerning the exclusive alignment of isobars to one or the other of these processes is probably in error. If the relatively small odd and even mass number abundance fluctuations characterizing R-process abundances are always the case, as assumed by this study, S-process contributions to the abundances of R-process isobars are substantial, consistent with transient flashing episodes in the S-process neutron production processes. A smooth and monotonically-decreasing curve of the abundance of the S-process yields times the neutron capture cross-section versus mass number is therefore the primary tool for the separation of the abundances due to the two processes.
    Keywords: ASTROPHYSICS
    Type: Astrophysics and Space Science; 82; 1, Fe; Feb. 198
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  • 6
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    Star formation and extinct radioactivities (1984)
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    Publication Date: 2019-06-28
    Description: An assessment is made of the evidence for the existence of now-extinct radioactivities in primitive solar system material, giving attention to implications for the early stages of sun and solar system formation. The characteristics of possible disturbances in dense molecular clouds which can initiate the formation of cloud cores is discussed, with emphasis on these disturbances able to generate fresh radioactivities. A one-solar mass red giant star on the asymptotic giant branch appears to have been the best candidate to account for the short-lived extinct radioactivities in the early solar system.
    Keywords: ASTROPHYSICS
    Type: Icarus (ISSN 0019-1035); 60; 416-427
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  • 7
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    Predicted magnitudes and colors from cool-star model atmospheres (1981)
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    Publication Date: 2019-06-28
    Description: An intercomparison of model stellar atmospheres and observations of real stars can lead to a better understanding of the relationship between the physical properties of stars and their observed radiative flux. In this spirit we have determined wide-band and narrow-band magnitudes and colors for a subset of models of K and M giant and supergiant stars selected from the grid of 40 models by Johnson, Bernat and Krupp (1980) (hereafter referred to as JBK). The 24 models selected have effective temperatures of 4000, 3800, 3600, 3400, 3200, 3000, 2750 and 2500 K and log g = 0, 1 or 2. Emergent energy fluxes (erg/ sq cm s A) were calculated at 9140 wavelengths for each model. These computed flux curves were folded through the transmission functions of Wing's 8-color system (Wing, 1971; White and Wing, 1978) and through Johnson's (1965) wide-band (BVRIJKLM) system. The calibration of the resultant magnitudes was made by using the absolute calibration of the flux curve of Vega by Schild, et al. (1971).
    Keywords: ASTROPHYSICS
    Type: Smithsonian Astrophysics Observatory 2nd Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, Vol. 1; p 239-244
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