6,200 research outputs found
Hubble space telescope: Pointing error effects on nonlinear ball joints
The Hubble Space Telescope pointing error produced by optical benches mounted on free ball joints is examined. Spacecraft cable connections are assumed to produce translational and rotational damping and restoring forces which act through the optical bench center of mass. The nonlinear dynamics are modeled and then implemented using an existing computer program for simulating the vehicle dynamics and pointing control system algorithm. Results are presented for the test case which indicate acceptable performance
On Carbon Burning in Super Asymptotic Giant Branch Stars
We explore the detailed and broad properties of carbon burning in Super
Asymptotic Giant Branch (SAGB) stars with 2755 MESA stellar evolution models.
The location of first carbon ignition, quenching location of the carbon burning
flames and flashes, angular frequency of the carbon core, and carbon core mass
are studied as a function of the ZAMS mass, initial rotation rate, and mixing
parameters such as convective overshoot, semiconvection, thermohaline and
angular momentum transport. In general terms, we find these properties of
carbon burning in SAGB models are not a strong function of the initial rotation
profile, but are a sensitive function of the overshoot parameter. We
quasi-analytically derive an approximate ignition density, g cm, to predict the location of first carbon ignition
in models that ignite carbon off-center. We also find that overshoot moves the
ZAMS mass boundaries where off-center carbon ignition occurs at a nearly
uniform rate of / 1.6
. For zero overshoot, =0.0, our models in the ZAMS mass
range 8.9 to 11 show off-center carbon ignition. For
canonical amounts of overshooting, =0.016, the off-center carbon
ignition range shifts to 7.2 to 8.8 . Only systems with
and ZAMS mass 7.2-8.0 show
carbon burning is quenched a significant distance from the center. These
results suggest a careful assessment of overshoot modeling approximations on
claims that carbon burning quenches an appreciable distance from the center of
the carbon core.Comment: Accepted ApJ; 23 pages, 21 figures, 5 table
Development of a three-dimensional time-dependent flow field model
A three-dimensional, time-dependent mathematical model to represent Mobile Bay was developed. Computer programs were developed which numerically solve the appropriate conservation equations for predicting bay and estuary flow fields. The model is useful for analyzing the dispersion of sea water into fresh water and the transport of sediment, and for relating field and physical model data
Properties of Carbon-Oxygen White Dwarfs From Monte Carlo Stellar Models
We investigate properties of carbon-oxygen white dwarfs with respect to the
composite uncertainties in the reaction rates using the stellar evolution
toolkit, Modules for Experiments in Stellar Astrophysics (MESA) and the
probability density functions in the reaction rate library STARLIB. These are
the first Monte Carlo stellar evolution studies that use complete stellar
models. Focusing on 3 M models evolved from the pre main-sequence to
the first thermal pulse, we survey the remnant core mass, composition, and
structure properties as a function of 26 STARLIB reaction rates covering
hydrogen and helium burning using a Principal Component Analysis and Spearman
Rank-Order Correlation. Relative to the arithmetic mean value, we find the
width of the 95\% confidence interval to be
0.019 M for the core mass at the first thermal pulse,
12.50 Myr for the age, 0.013 for the central temperature, 0.060 for the central density,
2.610 for the central electron
fraction, 5.810,
0.392, and 0.392. Uncertainties in the experimental
C(, triple-, and
N( reaction rates dominate these variations. We
also consider a grid of 1 to 6 M models evolved from the pre
main-sequence to the final white dwarf to probe the sensitivity of the
initial-final mass relation to experimental uncertainties in the hydrogen and
helium reaction rates.Comment: Accepted for publication in The Astrophysical Journal; 19 Pages, 23
Figures, 5 Table
Ultrasound enhancement of microfiltration performance for natural organic matter removal
Sonication of water at 1500 W power prior to microfiltration showed that short sonication times (60 s) gave a reduced flux decline. It is suggested that a less potent, smaller molecular form of the natural organic matter (NOM) was produced by sonication. Longer sonication times diminished this beneficial effect. This may be due to the formation of aggregates or compounds that are more readily adsorbed on the membrane. Where the sonication was preceded by an alum treatment, the flux loss showed a regular decrease with longer sonication times. It is suggested that the effects of sonication on the alum flocs and on the flocs; NOM interactions may play a critical role in regulating the flux. Where sand was present on sonication at 800 and 1400 W, the cavitational energy was focussed on adsorbed organic material, resulting in more efficient destruction and the formation of compounds that counteracted the flux enhancement
Laminar Flame Speeds in Degenerate Oxygen-Neon Mixtures
The collapse of degenerate oxygen-neon cores (i.e., electron-capture
supernovae or accretion-induced collapse) proceeds through a phase in which a
deflagration wave ("flame") forms at or near the center and propagates through
the star. In models, the assumed speed of this flame influences whether this
process leads to an explosion or to the formation of a neutron star. We
calculate the laminar flame speeds in degenerate oxygen-neon mixtures with
compositions motivated by detailed stellar evolution models. These mixtures
include trace amounts of carbon and have a lower electron fraction than those
considered in previous work. We find that trace carbon has little effect on the
flame speeds, but that material with electron fraction
has laminar flame speeds that are times faster than those at . We provide tabulated flame speeds and a corresponding fitting function so
that the impact of this difference can be assessed via full star hydrodynamical
simulations of the collapse process.Comment: 12 pages, 12 figures; Accepted to Ap
A three-dimensional, time-dependent model of Mobile Bay
A three-dimensional, time-variant mathematical model for momentum and mass transport in estuaries was developed and its solution implemented on a digital computer. The mathematical model is based on state and conservation equations applied to turbulent flow of a two-component, incompressible fluid having a free surface. Thus, bouyancy effects caused by density differences between the fresh and salt water, inertia from thare river and tidal currents, and differences in hydrostatic head are taken into account. The conservation equations, which are partial differential equations, are solved numerically by an explicit, one-step finite difference scheme and the solutions displayed numerically and graphically. To test the validity of the model, a specific estuary for which scaled model and experimental field data are available, Mobile Bay, was simulated. Comparisons of velocity, salinity and water level data show that the model is valid and a viable means of simulating the hydrodynamics and mass transport in non-idealized estuaries
Power-laws in recurrence networks from dynamical systems
Recurrence networks are a novel tool of nonlinear time series analysis
allowing the characterisation of higher-order geometric properties of complex
dynamical systems based on recurrences in phase space, which are a fundamental
concept in classical mechanics. In this Letter, we demonstrate that recurrence
networks obtained from various deterministic model systems as well as
experimental data naturally display power-law degree distributions with scaling
exponents that can be derived exclusively from the systems' invariant
densities. For one-dimensional maps, we show analytically that is not
related to the fractal dimension. For continuous systems, we find two distinct
types of behaviour: power-laws with an exponent depending on a
suitable notion of local dimension, and such with fixed .Comment: 6 pages, 7 figure
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