Cloud Stability Research Articles

Radiation pressure and the stability of broad-line region clouds

view Abstract Citations (31) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiation pressure and the stability of broad-line region clouds Ferland, G. J. ; Elitzur, M. Abstract Two constraints on the existence and stability of quasar emission-line gas are discussed. The case of a constant pressure photoionized cloud in pressure equilibrium with a surrounding hot gas is considered. It is shown that the radiation pressure of the internally generated line emission makes a significant contribution to the total pressure in standard broad-line region models, and that models with either a higher ionization parameter or lower gas density are dominated by radiation pressure and hence unstable. Further, clouds with ionization parameters lower than the standard value cannot be supported by any hot gas which is heated only by the observed radiation field. The result is a strict limit on the range of parameters which permit the existence of the observed emission-line gas. These results offer a natural explanation for the similarities among emission-line spectra of active galactic nuclei. Publication: The Astrophysical Journal Pub Date: October 1984 DOI: 10.1086/184354 Bibcode: 1984ApJ...285L..11F Keywords: Gas Density; Hydrogen Clouds; Magnetohydrodynamic Stability; Quasars; Radiation Pressure; Spectral Line Width; Active Galactic Nuclei; Emission Spectra; Gas Ionization; Astrophysics full text sources ADS |

Effect of pectic enzymes on cloud stability and soluble limonin concentration in stored orange juice

AbstractAttempts to use pectic enzymes to stabilise the cloud and reduce the limonin content simultaneously in citrus juices were unsuccessful but led to several significant findings. Acid‐catalysed reactions and residual effects of inactivated enzymes were shown to play a more important role in cloud stability than had been generally recognised. The maintenance of resuspended cloud, sometimes used as a measure of cloud stability during storage, was not always related to the actual retention of cloud; and although it was directly related to water‐soluble pectin content, cloud retention was not. Factors controlling the solubility of limonin in orange juice appeared more complicated than previously thought and varied in their relative contribution from juice to juice. Other chemical reactions were involved besides the conversion of a non‐bitter precursor to limonin, and the changes induced by endogenous pectic enzymes in unpasteurized juice had a more important effect on limonin solubility than the changes following the addition of fungal pectic enzymes to pasteurised juice. Finally, the separation of micro‐crystalline limonin complicated limonin analysis of stored juices.

Mass Outflow in Molecular Clouds: A New Phase in the Evolution of Newly-Formed Stars?

Molecular gas with velocity dispersions exceeding 100 km s-1 toward dense, interstellar clouds was discovered by microwave observers about a decade ago. It has now been established that this high velocity gas is a result of violent mass outflows (winds) originating from T-Tau stars, compact infrared sources or ultra-compact HII regions at the cores of the clouds. The mass outflow phenomena occur in regions of star formation covering five orders of magnitude in luminosity, and are of long duration . The observations suggest that violent mass loss is a new, important phase in the pre-main sequence evolution of newly-formed stars more massive than a few solar masses. The impact of the flows on the energy balance and dynamical stability of molecular clouds may be substantial. This review is mainly a summary of the observational facts, and gives a description of the physical conditions in an outflow zone. The impact of the flows on molecular clouds is discussed. Recent reviews of the outflow phenomena can also be found in (1, 2, 3, ).

4388330 Process for the preparation of citrus juice containing beverages with improved cloud stability: Hendrik J Wobben, Hian-Bie Tan, Naarden, Netherlands assigned to Naarden International N V

4388330 Process for the preparation of citrus juice containing beverages with improved cloud stability: Hendrik J Wobben, Hian-Bie Tan, Naarden, Netherlands assigned to Naarden International N V

Tidal stability of gas clouds in the Large Magellanic Cloud and M101

The tidal stability of the giant atomic gas clouds in the Large Magellanic Cloud (LMC) and in M101 is examined. The giant atomic clouds in the Large Magellanic Cloud are found to be unstable against tidal disruption by the gravity of the LMC, but the clouds in M101 are in approximate tidal balance. It is unlikely that there is sufficient unobserved molecular gas to stabilize the LMC clouds, although they may have substantial molecular cores which are tidally stable. The time scale for tidal disruption of the LMC clouds is of the order of 20-30 million years. Because most of the clouds have associated H II regions, the onset of star formation in these clouds must occur rapidly. In order to balance the loss due to tidal disruption, the clouds would have to be forming at a rate of approximately 0.09 solar masses/year per sq kpc, a value comparable to that derived for the Milky Way.

Studies of an Unneutral Electron Cloud in the CFIA

In the Collective Focusing Ion Accelerator (CFIA) scheme, a non-neutral electron cloud is trapped in a multiple mirror toroidal magnetic field. The electrons' radial electrostatic field serves to confine and focus an accelerated ion beam. The ion beam, with a density up to 10% that of the electrons', is accelerated by an induced toroidal electric field. The induced toroidal electric field, however, is not strong enough to overcome the mirror force acting on the electrons. This scheme enables the construction of a very small radius accelerator in the GeV range. As a first stage in the development of the CFIA, it is necessary to demonstrate trapping and stability of a non-neutral electron cloud over a long time (a few msec). It is also necessary to show that the electron cloud remains confined to the mirror cells when an accelerating toroidal electric field is applied. 5 refs.

Stability and Nonlinear Evolution of Plasma Clouds Via Regularized Contour Dynamics.

A model is introduced of an ionospheric plasma cloud (deformable dielectric) with a piecewise-constant ion density and a diffusive-regularized boundary. The linear stability of a single-contour circular cloud is studied and a new evolution equation for modal amplitudes is obtained which has the property that the wave number of maximum amplitude decreases with time (downward cascade). Analytic expressions show that large clouds evolve more slowly and appear more dissipative.

The impairment of cloud stability of orangeade by yeasts

By identifying 27 yeasts isolated from orangeades, orangeade bases, sugar syrups and orange concentrates, the dominating position ofSaccharomyces andCandida yeasts within the soft-drink flora was confirmed.Candida pelliculosa andKloeckera javanica were isolated from this habitat for the first time. Seven yeasts — exclusively belonging to the genusSaccharomyces — excreted polygalacturonase (PG) under favourable conditions, wheares no yeast produced pectinesterase or pectinlyase. Three of the PG-producing yeasts destabilized the cloud of commercial orangeades (pH 3.4) after suitable reproduction. The cloud-destabilizing activity ofSaccharomyces yeasts is primarily based on their PG activity. It may occur in orangeades of pH values between 2.7 and about 4.0, if the degree of esterification of the cloud pectin is low enough to allow degradation. A possible mechanism of cloud coagulation is discussed.

Turbulence and the stability of molecular clouds

Molecular clouds may be stabilized against gravitational collapse by the turbulent velocity field within them. It is suggested that the energy derived from differential galactic rotation can maintain the turbulent flow in the interstellar medium. The characteristic decay time for interstellar turbulence is found to be about 10-billion years. The rate and efficiency of star formation in giant molecular clouds reflect the stochastic nature of turbulence.

Stability and Nonlinear Evolution of Plasma Clouds via Regularized Contour Dynamics

A model is introduced of an ionospheric plasma cloud (deformable dielectric) with a piecewise-constant ion density and a diffusive-regularized boundary. The linear stability of a single-contour circular cloud is studied and a new evolution equation for modal amplitudes is obtained which has the property that the wave number of maximum amplitude decreases with time (downward cascade). Analytic expressions show that large clouds evolve more slowly and appear more dissipative.

Cloud stabilization in citrus beverages by low methoxyl pectin

SummaryCloud in citrus beverages containing 40–65% total soluble solids (TSS) is stabilized by the addition of 0.05–0.1% ammonia deesterified low methoxyl pectin (LMP) containing 4.5% methoxyl, and 2–6 mg of calcium per 100 g of the beverage. The concentration of LMP and calcium required decreased as the TSS increased. The stabilization of cloud is based on the principle that calcium added at concentrations much lower than that required for gel formation results in considerable increase in the viscosity of the LMP solution. Heat inactivation of pectin esterase (PE) in the juice is not essential for cloud stabilization.

The stability of interstellar clouds containing magnetic fields

The stability of interstellar clouds against gravitational collapse and fragmentation in the presence of magnetic fields is investigated. A magnetic field can provide pressure support against collapse if it is strongly coupled to the neutral gas; this coupling is mediated by ion-neutral collisions in the gas. The time scale for the growth of perturbations in the gas is found to be a sensitive function of the fractional ion abundance of the gas. For a relatively large fractional ion abundance, corresponding to strong coupling, the collapse of the gas is retarded. Star formation is inhibited in dense clouds and the collapse time for diffuse clouds can exceed the limit on their lifetime set by disruptive processes. For a small fractional ion abundance, the magnetic fields do not inhibit collapse and the distribution of the masses of collapsing fragments are likely to be quite different in regions of differing ion abundance. The solutions also predict the existence of large-scale density waves corresponding to two gravitational-magnetoacoustic modes. The conditions which best support these modes correspond to those found in the giant molecular clouds.

Stability of fine water droplet clouds

The rate of evaporation of monodisperse water droplets was first evaluated by solving numerically the modified Maxwell equation, assuming the cellular model for a droplet clouds. The results are discussed in comparison with those for a single isolated droplet, which can be obtained analytically. The critical conditions for the droplet cloud to be stable are then evaluated as a function of droplet number concentration, droplet size and initial conditions of the surrounding air. Secondly, the equilibrated system, where a water droplet cloud is steadily mixed with unsaturated air, was analysed on the basis of enthalpy and material balance of the system to evaluate the total volume change of the droplets. Some of these analyses were verified by experiment, using an ultramicroscopic technique which is useful for droplet size analysis.

USE OF A MACERATING PECTIC ENZYME IN APPLE NECTAR PROCESSING

ABSTRACTThe use of a commercial enzyme in the production of apple pulp for nectar beverages was investigated. During enzymatic treatment, apple pulp Theologically behaved initially as a pseudoplastic body with a yield value and later gradually shifted towards a pseudoplastic fluid. The pulp was separated by centrifugation into serum and insoluble residue fractions. Pectin analyses of the serum and residue showed that pectic substances were solubilized from the cell wall matrix essentially as highly esterified pectins. Viscosity measurements of the serum indicated some decrease in pectin chain length. The presence of water‐soluble, highly esterified pectins is believed to be responsible for the desired cloud stability of apple nectar.

STABILIZATION OF CLOUD IN PECTINESTERASE ACTIVE ORANGE JUICE BY PECTIC ACID HYDROLYSATES

The cloud of pectinesterase active orange juice was stabilized by inhibiting pectinesterase competitively with pectic acid hydrolysates. Both chemically and enzymatically prepared hydrolysates with an average degree of polymerization of 8 to 15 proved to be suitable, inasmuch as their molecular weight is large enough to inhibit pectinesterase, but too small to cause juice clarification by coagulating themselves. The period of cloud stability of orange juices from various origins could be extended by a factor of 3 to 5 through the additions of 1 mg per ml of hydrolysate with a degree of polymerization of 12. The effect of delayed gelation in orange concentrates was found to be smaller, and higher concentrations of hydrolysates with a somewhat lower degree of plymerization had to be used. Measurement of methanol release proved that pectic acid hydrolysate, added to orange juice, inhibits pectinesterase.

CLOUD STABILITY TEST FOR PASTEURIZED CITRUS JUICES

ABSTRACTA 24‐hr storage test is presented for evaluating. the cloud stability of pasteurized citrus juices. Enzymatic flocculation is accelerated by the addition of a special pectin and a barium salt while controlling pH and temperature. A citrus juice which retains greater than 60% of its initial cloud after 24 hr of this storage will have a satisfactory cloud stability for at least 1 yr under normal conditions.

Stability of gas clouds near quasi-stellar objects

Gas clouds near quasi-stellar objects are coherently accelerated by the ultraviolet ionizing continuum in such a way that the gas density remains nearly uniform. Outward propagating sound waves, considered in the rest frame of the (accelerating) clouds, will grow in amplitude due to radiative forces unless damped by self-radiation. The instability growth rate is large only for gapproximately-greater-than0.326 kc/sub 0//sup 2/, where g is the acceleration, k=2..pi../lambda, and c/sub 0/ is the isothermal sound speed in the cloud. Relatively few optically thin QSO clouds having napproximately-greater-than10/sup 7/ cm/sup -3/ will be unstable to this process, and even these may be stabilized by the pressure in L..cap alpha.. photons.In the Appendix we show that the diffuse radiation pressure in L..cap alpha.. and in other resonance lines is negligible compared with gas pressure for most QSO clouds of interest. (AIP)

Thermal-chemical instabilities in CO clouds

The stability of interstellar clouds containing CO is analyzed taking account of formation processes for CO. Two such processes are examined: O(+) charge exchange and C(+) radiative association. It is found that the C(+) radiative-association chemistry leads to low-temperature instabilities which influence the evolution of clouds. It is also found that instability may set in if CO production increases sufficiently with density, that the O(+) charge-exchange chemistry leads to instability associated with attenuation of the interstellar radiation field by grains, and that thermal instabilities will also result if grain formation, rather than ion-molecule chemistry, dominates CO production. It is suggested that such instabilities play a role in the fragmentation of interstellar clouds and in the formation of protostellar objects.

Effect of concentration process on cloud stability of reconstituted lemon juice

AbstractThe concentration process is a major factor in the low cloud stability of lemon juice reconstituted from a concentrate. A mechanism, based on the intermolecular bonding of pectin molecules, is suggested for this phenomenon, but actual proof of this has not yet been provided. It seems that the mechanism involved consists in reduction of the water activity below the critical level (inversely proportional to the pH of the system) necessary for maintaining the pectin in solution. This reduction favours an irreversible flocculation process through the intermolecular bonds of the pectin. This reduction favours an irreversible flocculation process through the intermolecular bonds of the pectin, thus resulting in the precipitation of the cloud particles trapped within the floes.

Retardation of condensation nuclei growth by surfactant

A one-dimensional cloud model is used to show how the coverage of droplets by surfactant films can suddenly change the growth of certain sizes of drops in a cloud drop spectrum. The retarded or accelerated growth depends upon the nature and thickness of the surfactant film, the size of the nuclei, and the cooling rate of the air parcel. Some conclusions are made concerning the practical application of interfering with the colloidal stability of warm clouds or fogs.