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  • 1
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    Numerical studies of collapsing interstellar clouds (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Numerical simulation of the structure and evolution of interstellar clouds was conducted. Steps were taken toward an integrated treatment of the dynamical, thermal, and chemical processes entering model calculations, and a detailed study was made of radiative transfer in molecular lines to allow model predictions to be tested against empirical data. It is shown that the shapes of molecular lines are sensitive to details of the cloud structure and evolutionary state and are thus useful in inferring the cloud density, temperature, chemical composition, age, and initial conditions. The calculations have successfully reproduced and explained several observed cloud properties, including abundances of complex molecular species and the apparent depletion of CO in dense cores.
    Keywords: ASTROPHYSICS
    Type: NASA-CR-180933 , NAS 1.26:180933
    Format: application/pdf
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  • 2
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    Thermal instabilities in diffuse molecular clouds - Formation of molecular cloud cores (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-08-13
    Description: The stability of diffuse clouds to thermal instabilities is examined using the semiempirical cooling function derived by Tarafdar et al. (1985) for these clouds. It is found that diffuse clouds which obey such a cooling function are susceptible to thermal instability at densities n of less than about 70-80/cu cm. The growth rate for instability is large and the mass contained in unstable regions ranges from about 0.001 to 1 solar mass. It is suggested that such instabilities may trigger formation of molecular cloud cores of the type found in low-mass molecular clouds (e.g., TMC-2). Criteria for thermal instability in self-gravitating systems are also derived.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Letters (ISSN 0004-6388); 25; 4 19; 235-245
    Format: text
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  • 3
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    Brown dwarfs as close companions to white dwarfs (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The influence of the radiation flux emitted by a white dwarf primary on the evolution of a closely orbiting brown dwarf (BD) companion is investigated. Full stellar evolutionary calculations are presented for both isolated and thermal bath cases, including effects of large variations in the atmospheric grain opacities. High grain opacities significantly increase the radii of the BDs, but the thermal bath does not. The major influence of the thermal bath is to increase substantially the surface temperature and luminosity of the BD at a given age. These results are compared with the observational properties of the possible BD companion of the white dwarf G29-38. Inclusion of both physical effects, high grain opacities and thermal bath, increases the mass range (0.034-0.063 solar masses) of viable models significantly, yet the final determination of whether the object is indeed a BD requires improvements in the observations of the system's properties.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 349; L59-L62
    Format: text
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