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  • 1
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    Differential rotation and turbulent convection: A new Reynolds stress model and comparison with solar data (1994)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: In most hydrodynamic cases, the existence of a turbulent flow superimposed on a mean flow is caused by a shear instability in the latter. Boussinesq suggested the first model for the turbulent Reynolds stresses bar-(u(sub i)u(sub j)) in which the mean shear S(sub ij) is the cause (or source) of turbulence represented by the stress bar-(u(sub i)u(sub j)). In the case of solar differential rotation, exactly the reverse physical process occurs: turbulence (which must pre-exist) generates a mean flow which manifests itself in the form of differential rotation. Thus, the Boussinesq model is wholly inadequate because in the solar case, cause and effect are reversed. Since the Boussinesq model is inadequate, one needs an alternative model for the Reynolds stresses. We present a new dynamical model for the Reynolds stresses, convective fluxes, turbulent kinetic energy, and temperature fluctuations. The complete model requires the solution of 11 differential equations. We then introduce a set of simplifying assumptions which reduce the full dynamical model to a set of algebraic Reynolds stress models. We explicitly solve one of these models that entails only one differential equation. The overall agreement with the data is obtained with a model that is neither phenomenological nor one that requires a full numerical simulation, since it is algebraic in nature. The new model can play an important role in understanding the complex physics underlying the interplay between solar differential rotation and convection, as many physical processes can naturally be incorporated into the model.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 425; 1; p. 303-325
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  • 2
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    EX Hydrae in outburst (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Photometry and red spectroscopy of the intermediate polar EX Hya in its rare outburst state are presented. Photometry during the declining phase of the July-August 1986 outburst shows the 67-min (spin) modulation to be present with similar characteristics to that in quiescence. In contrast, photometry from near the peak of the 1987 May outburst shows little evidence of the 67-min modulation, while spectroscopy obtained nearly simultaneously is similarly lacking in such evidence, despite its presence in quiescent spectroscopic data. Near the beginning of the May 1987 outburst the H alpha emission line develops a broad, high velocity base component whose velocity is modulated with the orbital cycle. The velocity and phase of the broad base component suggest that it is produced near the magnetosphere of the white dwarf at a point along the projected trajectory of the gas stream from the companion. The feature disappears later in the outburst and is not present during quiescence. It is suggested that the outbursts in EX Hya are caused by an increase in the rate of mass transfer from the companion, and that part of this enhanced mass-transfer stream skims over the top of the accretion disk to strike the magnetosphere directly. The interaction of the stream with the magnetosphere gives rise to the broad-base component observed.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 238; 1107-111
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  • 3
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    Some models of the Geminids meteor stream formation (1987)
    In:  CASI
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    Publication Date: 2013-08-31
    Description: Further development of methods of investigations of meteor shower structure and a great deal of observational data have made it possible to obtain a precise flux density profile along the Earth's orbit for the Geminids meteor shower. This curve proved to be adequately described by a exponential law. Examined are: (1) which ejection model will fit the observed shower structure; and (2) to what extent the subsequent process of evolution modifies the formed structure. The following conclusions are drawn: (1) When studying the mechanism of formation of meteor streams, it is necessary to take into consideration the velocity distribution of particles that have been ejected from the parent body; (2) On the basis of the observed density variations, it is possible to determine what kind of ejection takes place (a single ejection or that around an orbital arch); and (3) The original structure of the Geminids cross section persists for no more than 1,500 to 2,000 years.
    Keywords: ASTROPHYSICS
    Type: International Council of Scientific Unions, Middle Atmosphere Program. Handbook for MAP, Volume 25; p 344-350
    Format: application/pdf
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  • 4
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    The contribution of the preparation of the Hipparcos mission to solving the evolutionary problems of LPV's (1990)
    In:  Other Sources
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    Publication Date: 2019-07-27
    Description: The pulsation modes, the long-term behavior, and the evolutionary status of some long-period variable stars are discussed with reference to the results of investigations carried out as part of the preparation for the Hipparcos mission. In particular, attention is given to three different methods used to predict the brightness variations of large-amplitude (greater than 2 mag) LPVs. The discussion also covers statistical results relating to the evolutionary problems of LPVs and preliminary data on the pulsation mode.
    Keywords: ASTROPHYSICS
    Type: International Colloquium; Sept. 4-7, 1989; Montpellier; France
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  • 5
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    Spectrophotometry of Bowen resonance fluorescence lines in three planetary nebulae (1992)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The results are presented of a uniquely complete, carefully reduced set of observations of the O III Bowen fluorescence lines in the planetary nebulae NGC 6210, NGC 7027, and NGC 7662. A detailed comparison with the predictions of radiative excitation verify that some secondary lines are enhanced by selective population by the charge exchange mechanism involving O IV. Charge exchange is most important in NGC 6210, which is of significantly lower ionization than the other nebulae. In addition to the principal Bowen lines arising from Ly-alpha pumping of the O III O1 line, lines arising from pumping of the O3 line are also observed. Comparison of lines produced by O1 and O3 with the theoretical predictions of Neufeld indicate poor agreement; comparison with the theoretical predictions of Harrington show agreement with NGC 7027 and NGC 7662.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 390; 1 Ma
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  • 6
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    Far-ultraviolet and X-ray emission of the long period RS CVn star Sigma Geminorum (1988)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Observations of the RS CVn binary Sigma Gem were obtained in 1984 with the IUE and Exosat satellites. The longitudes of two starspot regions relative to the cental meridian are derived from the photometric data using an improved value of the orbital period. The observations support the theory that large active regions associated with starspots are responsible for the enhanced line emission commonly seen in RS CVn systems.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 192; 1-2; 234-248
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  • 7
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    Evidence for turbulence in H II regions (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The results of extensive velocity mapping across the face of Galactic H II regions are used to make an accurate determination of velocity dispersions and structure functions for a number of nebulae. A dependence of random velocities upon scale, indicating the presence of turbulence, is found. These relationships are compared with the predictions of models for Kolmogorov turbulence in H II regions, and poor agreement is found. A more general formulation shows that there is no single power law for the velocity-distance relation that fits the observations. From the variations in the shape of the observed structure function, it is concluded that the turbulent energy enters the nebulae at several different scales. It is also found that velocity broadening of the observed lines is significantly greater than would be inferred from the velocity scatter across the face of the nebulae.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 317; 686-692
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  • 8
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    Fine-scale motion in NGC 6514 and NGC 6523 (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Velocity maps of the inner regions of the bright H II regions NGC 6514 and NGC 6523 were made with unprecedented spatial and spectral resolution in the 5007 A line of forbidden O III. In addition to the advantages of an instrumental full width at half-maximum intensity of only 5.4 km/s, the small thermal width of the heavy oxygen ion also allows determination of accurate line widths and velocities. The CCD spectra were numerically fitted to Gaussian line profiles and revealed two separate velocity systems in NGC 6523. The data sets of radial velocities were used to derive the dependence of the most probable turbulent velocities upon the sample sizes, and the spatial dependence of the structure function. These relationships are the basic functions for comparison with the predictions of the models for turbulence in H II regions.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 317; 676-685
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  • 9
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    Fine-scale motion in the central Orion nebula (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The inner region of M42 has been mapped in the O III N1 forbidden line at an unprecedented combination of velocity and spatial resolution and photometric accuracy. Three separate velocity systems along the line of sight are identified. Differential mass flow is present in two of these systems. All three systems show random fluctuations in velocity well above their measurement errors. The point-by-point velocities are used to determine the diagnostic statistical parameter, the structure function. The random motions show correlation with spatial separation, as expected for turbulence; but comparison with the predictions of classical theory for nebular turbulence shows poor agreement. If Kolmogorov (1941, 1942) theory applies, then the energy must be input at many scales.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 315; L55-L59
    Format: text
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  • 10
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    Ultraviolet emission lines and optical photometry of the flare star AT Microscopii (1988)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: UV spectra of the dwarf flare star binary AT Mic were obtained with the IUE spacecraft over the course of three days in September 1985. A high-resolution short-wavelength spectrum was exposed for 25 h. Line wavelengths, widths, and fluxes were derived from the observed high-resolution spectra, and the effects of the binary structure of AT Mic on the spectral lines were evaluated. The data show that the UV emission-line spectrum is similar in nature to that of flare regions on the sun and other stars. The width of the Mg II k-line shows a good fit with the Wilson-Bappu relation, which thus seems to extend over a very large range of absolute magnitudes.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 193; 1-2; 211-221
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