Laser Light Scattering, from an Advanced Technology Development Program to Experiments in a Reduced Gravity Environment

Recent advancements in laser light scattering hardware are described. These include intelligent single card correlators; active quench/active reset avalanche photodiodes; laser diodes; and fiber optics which were used by or developed for a NASA advanced technology development program. A space shuttle experiment which will employ aspects of these hardware developments is previewed

Chemistry in Disks. II. -- Poor molecular content of the AB Aur disk

We study the molecular content and chemistry of a circumstellar disk surrounding the Herbig Ae star AB Aur at (sub-)millimeter wavelengths. Our aim is to reconstruct the chemical history and composition of the AB Aur disk and to compare it with disks around low-mass, cooler T Tauri stars. We observe the AB Aur disk with the IRAM Plateau de Bure Interferometer in the C- and D- configurations in rotational lines of CS, HCN, C2H, CH3OH, HCO+, and CO isotopes. Using an iterative minimization technique, observed columns densities and abundances are derived. These values are further compared with results of an advanced chemical model that is based on a steady-state flared disk structure with a vertical temperature gradient, and gas-grain chemical network with surface reactions. We firmly detect HCO+ in the 1--0 transition, tentatively detect HCN, and do not detect CS, C2H, and CH3OH. The observed HCO+ and 13CO column densities as well as the upper limits to the column densities of HCN, CS, C2H, and CH3OH are in good agreement with modeling results and those from previous studies. The AB Aur disk possesses more CO, but is less abundant in other molecular species compared to the DM Tau disk. This is primarily caused by intense UV irradiation from the central Herbig A0 star, which results in a hotter disk where CO freeze out does not occur and thus surface formation of complex CO-bearing molecules might be inhibited.Comment: Accepted by A&

Magnetically Controlled Spasmodic Accretion During Star Formation. I. Formulation of the Problem and Method of Solution

We formulate the problem of the late accretion phase of the evolution of an isothermal magnetic disk surrounding a forming star. The evolution is described by the six-fluid MHD equations, accounting for the presence of neutrals, atomic and molecular ions, electrons, and neutral, positively, and negatively charged grains. Only the electron fluid is assumed to be attached to the magnetic field, in order to investigate the effect of the detachment of the ions from the magnetic field lines that begins at densities as low as 10^8 cm^-3. The "central sink approximation" is used to circumvent the problem of describing the evolution inside the opaque central region for densities greater than 10^11 cm^-3. In this way, the structure and evolution of the isothermal disk surrounding the forming star can be studied at late times without having to implement the numerically costly radiative transfer required by the physics of the opaque core. The mass and magnetic flux accumulating in the forming star arecalculated, as are their effects on the structure & evolution of the surrounding disk. The numerical method of solution first uses an adaptive grid and later, after a central region a few AU in radius becomes opaque, switches to a stationary but nonuniform grid with a central sink cell. It also involves an implicit time integrator, an advective difference scheme that possesses the transportive property, a second-order difference approximation of forces inside a cell, an integral approximation of the gravitational and magnetic fields, and tensor artificial viscosity that permits an accurate investigation of the formation and evolution of shocks in the neutral fluid.Comment: Astrophysical Journal, in press. 32 page

Calibrative approaches to protein solubility modeling of a mutant series using physicochemical descriptors

A set of physicochemical properties describing a protein of known structure is employed for a calibrative approach to protein solubility. Common hydrodynamic and electrophoretic properties routinely measured in the bio-analytical laboratory such as zeta potential, dipole moment, the second osmotic virial coefficient are first estimated in silico as a function a pH and solution ionic strength starting with the protein crystal structure. The utility of these descriptors in understanding the solubility of a series of ribonuclease Sa mutants is investigated. A simple two parameter model was trained using solubility data of the wild type protein measured at a restricted number of solution pHs. Solubility estimates of the mutants demonstrate that zeta potential and dipole moment may be used to rationalize solubility trends over a wide pH range. Additionally a calibrative model based on the protein’s second osmotic virial coefficient, B22 was developed. A modified DVLO type potential along with a simplified representation of the protein allowed for efficient computation of the second viral coefficient. The standard error of prediction for both models was on the order of 0.3 log S units. These results are very encouraging and demonstrate that these models may be trained with a small number of samples and employed extrapolatively for estimating mutant solubilities

Control of star formation by supersonic turbulence

Understanding the formation of stars in galaxies is central to much of modern astrophysics. For several decades it has been thought that stellar birth is primarily controlled by the interplay between gravity and magnetostatic support, modulated by ambipolar diffusion. Recently, however, both observational and numerical work has begun to suggest that support by supersonic turbulence rather than magnetic fields controls star formation. In this review we outline a new theory of star formation relying on the control by turbulence. We demonstrate that although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28 figures, in pres

CRITICAL OPALESCENCE OF PERFLUOROMETHYLCYCLOHEXANE IN CARBON TETRACHLORIDE

Un spectromètre homodyne laser (6 328 Å) couvrant un domaine angulaire de 20° à 150°, stabilisé en température à ± 0,001 °C, a été raccordé à un ordinateur digital IBM 1800 ; l'interface permet l'introduction directe et simultanée des distributions angulaire et spectrale de la lumière diffusée par une solution près de son point critique de démixion. Le perfluorocyclohexane et le tétrachlorure de carbone utilisés furent soigneusement purifiés par chromatographie puis par distillation sous vide. Les données expérimentales furent obtenues à l'aide d'une cellule cylindrique de 8 mm (dia. int.) et d'une cellule parallépépidique ayant une longueur de 1 mm. Les mesures furent effectuées avec les ouvertures angulaires suivantes : intensité 1,4°, largeur de raie 0,5°. De ces mesures, nous déduisons que (1) la relation liant (Ic intensité de la lumière diffusée)-1 à K2 est vérifiée. (2) La variation avec la température de l'intensité à l'angle zéro (extrapolée) et la distance de corrélation (ζs) ont la forme : [MATH] I-1c αεγ où ε = (T- Tc)/Tc, et γ est l'exposant critique. Pour une solution de concentration critique, nous obtenons γ = 1,15 ± 0,02. La largeur de la raie Rayleigh du mélange critique a été mesurée entre 0,006 et 0,715 °C au-dessus de Tc, à des angles 30° ≤θ≤ 150°, correspondant à 0,704 105 ≤ K ≤ 2,63 105 cm-1. Dans le domaine hydrodynamique où ζs K ≪ 1 nous obtenons Ɖ = DK2 où D = D0 εγ* ; D0 = (1,24 ± 0,04) 10-5 cm2/s et γ* = 0,665 ± 0,018. Lorsque ζs K est petit mais non négligeable, nos résultats concordent avec une équation modifiée pour la largeur de la raie Rayleigh : Ɖ = DK2(1 + b ζ2 K2) où ζ = ζ0 ε-ν. Avec b = 3/5 nous trouvons ζ0 = 4,13 ± 0,22 Å et v = 0,540 ± 0,023. Dans la région critique ζs K > 1, nous démontrons que Ɖ = AK3. Pour 0,076 ≤ Kζs ≤ 3,75, nos résultats sont en accord avec la théorie de Kawasaki sur le couplage « mode-mode ». Toutefois, nos résultats démontrent que la relation γ* (= γ - Ψ) = νr or s (Kadanoff-Swift-Kawasaki) n'est pas vérifiée. Malgré tout, si nous prenons en considération les variations en fonction de la température et les anomalies de la viscosité à haute fréquence, γ - Ψ est voisin de v et dans le cas de l'acide isobutyrique dans l'eau γ - Ψ≈ν. Des considérations portant sur la « loi d'échelle » nous amènent à remettre en question la validité de la relation (sv = 2 - α) ou des relations équivalentes sv = γ + 2 β et sv = µ + ν.A laser homodyne spectrometer with a measurable angular range of 20°-150° and a temperature control of better than ± 0.001 °C was interfaced with an IBM 1800 digital computer for direct on-line data acquisition of the angular and the spectral distribution of the scattered intensity from a two-component critical liquid system, perfluoromethylcyclohexane-carbon tetrachloride, near its critical mixing point. Both components were purified by preparative gas chromatography and subsequent vacuum distillation. Special care was taken to select appropriate columns for the separation of impurities and to prevent contamination of the system from moisture. Scattering data were obtained by means of a cylindrical light-scattering cell of 8 mm i. d. and a flat cell with a 1 mm light path. The intensity measurements were obtained using an angular aperture of 1.4° while the linewidth measurements were obtained using an angular aperture of 0.5°. From our intensity studies we have observed that (1) the K2 dependence in the reciprocal scattered intensity due to concentration fluctuations (I-1c) is obeyed (2) the temperature dependence of the extrapolated zeroangle scattered intensity which is related to the osmotic coefficient (δµ/δC)P, T, has the form : [MATH]I-1c α εγ, where ε = (T - Tc)/Tc ; and γ is a critical exponent. We have obtained γ = 1.15 ± 0.02 at the critical solution concentration. The Rayleigh linewidth of the critical mixture was measured at temperatures between 0.006° and 0.715 °C above Tc, for scattering angles 30° ≤θ≤ 150° so that 7.04 x 104 cm-1 ≤ K ≤ 2.63 x 105 cm-1. In the hydrodynamic region where ζs K ≪ 1, we find Ɖ = DK2 with D = D0 εγ* ; D0 = (1,24 ± 0,04) 10-5 cm2/s where D0 εγ* ; D0 = (1,24 ± 0,04) 10-5 cm2/s and γ* = 0,665 ± 0,018 0.018. In the non-local hydrodynamic region where ζs K is small but not negligible when compared with unity, our results agree with a modified Rayleigh linewidth equation Ɖ = DK2(1 + b ζ2 K2) where ζ = ζ0 ε-ν. With b = 3/5 we find ζ0 = 4,13 ± 0,22 Å et v = 0,540 ± 0,023. In the critical region where ζs K > 1, we show that Ɖ approaches AK3. In the region 0,076 ≤ Kζs ≤ 3,75, our data agree quite well with the Kawasaki extended mode-mode coupling theory. Our results show that a direct application of the Kadanoff-Swift-Kawasaki result γ* (= γ - Ψ) = ν is invalid. However, if we take into account the temperature dependence and anomaly of the high-frequency shear viscosity, then γ - Ψ approaches v and, in the case of isobutyric acid in water, γ - Ψ≈ν. Scaling-law considerations also lead us to question the validity of the relation (sv = 2 - α) or its equivalents sv = γ + 2 β and sv = µ + ν

Multiaxial yield behaviour of polypropylene

In order to characterize the yield behavior of polypropylene as a function of pressure and to verify the applicability of the Drucker-Prager yield function, various tests were conducted to cover a wide range of stress states from uniaxial tension and compression to multiaxial tension and confined compression. Tests were performed below and above the glass transition temperature, to study the combined effect of pressure and temperature. The pressure sensitivity coefficient as an intrinsic material parameter was determined as a function of temperature. Increasing pressure sensitivity values were found with increasing temperature, which can be related to the change in the free volume and thus, to the enhanced molecular mobility. A best-fit Drucker-Prager yield function was applied to the experimental yield stresses and an average error between the predictions and the measurements of 7 % was obtained

Eine Regelung fuer Antrieb und Schlupf bei Lokomotiven

Available from TIB Hannover: RN 7879(9612) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman