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  • Other Sources  (7)
  • 1980-1984  (7)
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  • 1
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    On the '3 kiloparsec arm' - Resonance excitation of linear and nonlinear waves by an oval distortion in the central region (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The mechanism responsible for the massive rapidly expanding Galactic feature known as the 3-kpc arm (Rougoor and Oort, 1960) is investigated theoretically in the framework of a density-wave model of the Galaxy. The arm is attributed to outgoing waves excited at the outer Lindblad resonance (OLR) of a small rotating oval distortion near the Galactic center. Linear and nonlinear models are developed, and it is shown that a density crest at 3.5 kpc with expansion velocity 53 km/s can be generated by a distortion turning at 118 km/s kpc with a 10-percent perturbation of the main gravitational field at the OLR (located at 3 kpc). The wave character of the nonlinear solution is found to be the same as that of the linear solution and to give good agreement with the observations in terms of expansion velocity, size, shape, and mass; only the observed asymmetry of the arm is not reproduced.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 281; 600-613
    Format: text
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  • 2
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    Vertical extensions of galactic spiral arms (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The vertical structure of the large scale galactic shock along spiral arms is studied in order to understand the extended features or the wings of spiral arms observed by Kepner (1970) and recently checked by Soukup (1979). Numerical calculations of two-dimensional gas flow were performed for two models of the gas. The results show that the shock front, standing perfectly perpendicular to the galactic plane, extends impressively above the scale height of the interstellar gas. In the extreme case of isothermal gas, the shock solution exists at any distance from the plane. In a realistic model in which the kinetic temperature of the gas is stratified, increasing with height, the shock can extend up to 700 pc. For both models, although the velocity component perpendicular to the galactic plane is small, the solutions corresponding to layered one-dimensional flow contain no shocks at such heights. In order to study the geometry of the shock front and the flow pattern near it, a simplified model is adopted, in which the Coriolis force is neglected. However, estimates show that including it will enhance the shock strength. The compression at the shock front makes the gas observable at high latitudes.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 246
    Format: text
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  • 3
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    Surface photometry of spiral galaxies. I - Theoretical color variation and surface brightness across spiral arms (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A method for the construction of color variations and surface brightness across a galactic spiral arm, using the concepts and results of the density-wave theory, is developed. The color variation measured by the index Q varies with the ratio of the light of young stars to that of the old stars, the initial mass function, and the pattern speed. The fact that the variation of brightness and color across a spiral arm is relatively smooth and symmetrical and not sharp as in the galactic shock picture is due to the smearing-out effects of star formation inside a cloud in the regions behind the galactic shock, and the tendency of new stars to fall back to small galactocentric distances after their formation because of their lower angular velocity at birth. Both effects arise naturally in the context of the density-wave theory.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 243
    Format: text
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  • 4
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    Motions of the cloud medium behind large scale galactic shocks (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Mechanisms of decelerating the cloud medium in the large-scale galactic shock are studied. It is shown that the process of cloud-cloud collisions, which results in diffusive momentum transport and hence gives rise to the turbulent viscosity, is very effective in slowing down the cloud medium so that the postshock velocity of the intercloud medium can be matched within a short distance behind the shock front. The drag force exerted by the slow-moving intercloud medium alone is simply not enough to effectively decelerate the cloud medium in the shock. By the use of the results of Shu et al (1972), the internal structure of the shock of the cloud medium is analyzed by including turbulent viscous effects. The thickness of the shock is found to be on the order of 100 pc if the turbulent viscosity is taken proportional to the mean free path of the cloud-cloud collisions. The phase transition takes place in an even thinner layer on the order of 10 pc immediately after the viscous shock front of the intercloud medium.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 252
    Format: text
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  • 5
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    On the local standard of rest (1982)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Under the influence of a spiral gravitational field, there should be differences among the mean motions of different types of objects with different dispersion velocities in a sipral galaxy. The old stars with high dispersion velocity should have essentially no mean motion normal to the galactic rotation. On the other hand, young objects and interstellar gas may be moving relative to the old stars at a velocity of a few kilometer per second in both the radial (galacto-centric), and circular directions, depending on the spiral model adopted. Such a velocity is usually referred as the systematic motion or the streaming motion. The conventionally adopted local standard of rest is indeed co-moving with the young objects of the solar vicinity. Therefore, it has a net systematic motion with respect to the circular motion of an equilibrium galactic model, defined by the old stars.
    Keywords: ASTROPHYSICS
    Type: NASA-CR-170238 , NAS 1.26:170238
    Format: application/pdf
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  • 6
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    On the local standard of rest (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Under the influence of a spiral gravitational field, there should be differences among the mean motions of different types of objects with different dispersion velocities in a spiral galaxy. The old stars with high dispersion velocity should have essentially no mean motion normal to the galactic rotation. On the other hand, young objects and interstellar gas may be moving relative to the old stars at a velocity of a few kilometer per second in both the radial (galacto-centric), and circular directions, depending on the spiral model adopted. Such a velocity is usually referred as the systematic motion or the streaming motion. The conventionally adopted local standard of rest is indeed co-moving with the young objects of the solar vicinity. Therefore, it has a net systematic motion with respect to the circular motion of an equilibrium galactic model, defined by the old stars. Previously announced in STAR as N83-24443
    Keywords: ASTROPHYSICS
    Type: Kinematics, dynamics and structure of the Milky Way; Workshop on the Milky Way; May 17, 1982 - May 19, 1982; British Columbia; Canada
    Format: text
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  • 7
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    Theoretical problems of spiral galaxies (1982)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Three theoretical problems concerning the large scale structure of disk galaxies in general, and the Milky Way System, in particular, were proposed to study. They are, namely, modes of spiral density waves, evolutionary change of the abundance distribution of the gas in the Milky Way System and the motions of the cloud medium behind the large scale galactic shock.
    Keywords: ASTROPHYSICS
    Type: NASA-CR-170232 , NAS 1.26:170232
    Format: application/pdf
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