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  • 1
    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
    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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  • 3
    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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  • 4
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    In:  Other Sources
    Publication Date: 2011-08-24
    Description: The origin of tektites has been obscure because of the following dilemma. The application of physical principles to the data available on tektites points strongly to origin from one or more lunar volcanoes; but few glasses of tektite composition have hitherto been reported from the lunar samples. Instead, the lunar silicic glasses consist chiefly of a material very rich in K2O and poor in MgO. The ratio of K2O/MgO is higher in these glasses than in any tektites reported. The solution of the dilemma seems to come from the study of some recently discovered terrestrial deposits of tektite glass with high values of K2O/MgO at the Cretaceous Tertiary boundary. These glasses are found to be very vulnerable to crystallization into sandine or to alteration to smectite. These end products are known and are more abundant than any terrestrial deposits of tektite glass. It seems possible that, in fact, the moon produces tektite glass, mostly of the high K2O-low MgO type; but on Earth these deposits are destroyed. The much less abundant deposits with lower K and higher Mg are observed because they survive. Other objections to the lunar origin hypothesis appear to be answerable.
    Keywords: ASTROPHYSICS
    Type: Meteoritics (ISSN 0026-1114); 29; 1; p. 73-78
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  • 5
    Publication Date: 2011-08-24
    Description: Statistical uncertainties in determining the temperatures of hot (0.5-10 keV) coronal plasmas are investigated. The statistical presicion of various spectral temperature diagnostics is established by analyzing synthetic ASCA solid-state imaging spectrometer (SIS) CCD spectra. The diagnostics considered are the ratio of hydrogen-like to helium-like line complexes of Z greater than or = 14 elements, line-free portions of the continuum, and the entire spectrum. While fits to the entire spectrum yield the highest statistical precision, it is argued that fits to the line-free continuum are less susceptible to atomic data uncertainties but lead to a modest increase in statistical uncertainty over full spectral fits. Temperatures deduced from line ratios can have similar accuracy, but only over a narrow range of temperatures. Convenient estimates of statistical accuracies for the various temperature diagnostics are provided which may be used in planning ASCA SIS observations.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 435; 2; p. L149-L152
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  • 6
    Publication Date: 2011-08-24
    Description: We have investigated the influence of the color temperature of the illuminating radiation field on the chemical and thermal structure of photon-dominated regions (PDRs). We present the results of a study of the photoelectric efficiency of heating by large molecules such as polycyclic aromatic hydrocarbons (PAHs) and very small grains for radiation fields characterized by different effective temperatures. We show that the efficiency for cooler (T(sub eff) approximately = 6000-10,000 K) stars is at most an order of magnitude smaller than that for hotter (T(sub eff) approximately = 20,000-30,000 K) stars. While cooler radiation fields result in less ultraviolet photons capable of heating, the efficiency per absorbed photon is higher, because the grains become less positively charged. We also present detailed calculations of the chemistry and thermal balance for generic PDRs (n(sub 0) approximately = 10(exp 3), G(sub 0) approximately = 10(exp 3)). For cooler radiation fields, the H/H2 and C(+)/C/CO transition layers shift toward the surface of the PDR, because fewer photons are available to photodissociate H2 and CO and to ionize C. The dominant cooling lines are the (C II) 158 micron and the (O I) 63 micron lines for the hotter radiation fields, but cooling by CO becomes dominant for a color temperature of 6000 K or lower. The (C II)/CO and (O I)/CO ratios are found to be very good diagnostics for the color temperature of the radiation field.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 437; 1; p. 270-280
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  • 7
    Publication Date: 2011-08-24
    Description: The properties and evolution of the major outburst experienced by Comet Halley at 14 AU during February-March 1991 are studied. It is found that the observed halo is a segment of a conical surface populated by solid particles ejected from a localized, temporarily activated region on the sunlit hemisphere of the spinning nucleus. The total mass of the ejecta was at least 10 exp 12 g and CO was probably the prime driver, accelerating the smallest grains to a terminal velocity of about 45 m/s. The mass loading of the gas flow by particulate matter is enormous, with the mass production rate exceeding the expected production rate of CO by a factor of several tens.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 263; 1-2; p. 367-386.
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  • 8
    Publication Date: 2011-08-24
    Keywords: AERODYNAMICS
    Type: American Helicopter Society, Journal (ISSN 0002-8711); 38; 2; p. 53-60.
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  • 9
    Publication Date: 2011-08-24
    Description: We have constructed high resolution IRAS maps of the B209 star forming region in the Taurus complex. By combining survey scans with pointed observations using advanced techniques specially developed to handle IRAS data in crowded fields, we have constructed images that reveal two new premain-sequence (PMS) objects. One of these, IRAS 04114+2757G, has FIR colors close to those of typical T Tauri stars and is probably associated with a faint stellar object visible on the POSS prints. The other new PMS source, IRAS 04111+2800G, is a deeply embedded object not detected at 12 microns and not optically visible. The object is projected close to the center of an ammonia core. Our detection of compact blue and red shifted (C-12)O emission peaks around the position of IRAS 04111+2800G confirms the PMS nature and youth of this object. We have examined the reasons for the exclusion of IRAS 04111+2800G and IRAS 04114+2757G from the IRAS Point Source Catalog and estimated the incompleteness of the currently known sample of embedded PMS stars in Taurus. The implications for the determination of evolutionary time scales in low mass star formation are discussed.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 259; 2; p. 537-542.
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  • 10
    Publication Date: 2011-08-24
    Description: A new CFD potential code, FPX (eXtended Full-Potential), has been developed for application to both helicopters and tilt-rotors. The code solves the unsteady, three-dimensional full potential equation and is an extension of the rotor code, FPR. Both entropy and viscosity corrections are included to enhance the physical modeling capabilities. A number of efficiency related modifications have yielded a factor of two speed-up in the code. An axial flow capability has been added to treat tilt-rotor in forward flight (cruise mode). In order to employ streamwise periodicity and accurately solve for the propagation of acoustic signals in the tip region, an H-H topology has been added to the basic O-H grid system. Computations are performed for the XV-15 Standard and ATB blades at high-speed conditions. Comparisons are made for the blade aerodynamics and the induced fuselage cabin pressure for a range of Mach numbers. Grid generation, wake treatment, and far-field wall treatment are identified as problem areas with recommendations for future research.
    Keywords: AERODYNAMICS
    Type: In: AHS and Royal Aeronautical Society, Technical Specialists' Meeting on Rotorcraft Acoustics(Fluid Dynamics, Philadelphia, PA, Oct. 15-17, 1991, Proceedings (A93-29401 10-71); 15 p.
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