6,660 research outputs found
Design reliability goal developed from small sample
Sampling distributions, constructed by Monte Carlo simulation are used in hardware development to establish a design reliability goal, to place a confidence coefficient on reliability estimates, and to determine whether sample stress/strength data demonstrate a specified reliability at a specified confidence level
Epitaxial growth and surface reconstruction of CrSb(0001)
Smooth CrSb(0001) films have been grown by molecular beam epitaxy on MnSb(0001) – GaAs(111) substrates. CrSb(0001) shows (2 × 2), triple domain (1 × 4) and (√3×√3)R30° reconstructed surfaces as well as a (1 × 1) phase. The dependence of reconstruction on substrate temperature and incident fluxes is very similar to MnSb(0001)
Theoretical Interpretation of the Measurements of the Secondary Eclipses of TrES-1 and HD209458b
We calculate the planet-star flux-density ratios as a function of wavelength
from 0.5 microns to 25 microns for the transiting extrasolar giant planets
TrES-1 and HD209458b and compare them with the recent Spitzer/IRAC-MIPS
secondary eclipse data in the 4.5, 8.0, and 24 micron bands. With only three
data points and generic calibration issues, detailed conclusions are difficult,
but inferences regarding atmospheric composition, temperature, and global
circulation can be made. Our models reproduce the observations reasonably well,
but not perfectly, and we speculate on the theoretical consequences of
variations around our baseline models. One preliminary conclusion is that we
may be seeing in the data indications that the day side of a close-in
extrasolar giant planet is brighter in the mid-infrared than its night side,
unlike Jupiter and Saturn. This correspondence will be further tested when the
data anticipated in other Spitzer bands are acquired, and we make predictions
for what those data may show.Comment: 15 pages, including 3 color figures, submitted to the Astrophysical
Journa
Studies of high latitude current systems using MAGSAT vector data
The magnetic disturbance fields caused by global external current systems are considered with particular emphasis on improving the understanding of the physical processes which control high latitude current systems. Following processing the MAGSAT data were routinely plotted in the Universal Time (UT) format as well as in a polar plot format. The H'D'U' coordinate system, was adopted as the standard for representing the MAGSAT residual magnetic field vectors. A data file was generated and the TPOLAR computer code was developed to determine from the orbital elements, the time, latitude, and MLT of the extremum latitude of each transpolar segment of orbit. The precision of the vector data set from MAGSAT prompted an extended exploratory phase for data analysis procedures, modeling techniques and phenomenology
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Evolutionarily stable sexual allocation by both stressed and unstressed potentially simultaneous hermaphrodites within the same population.
Factors influencing allocation of resources to male and female offspring continue to be of great interest to evolutionary biologists. A simultaneous hermaphrodite is capable of functioning in both male and female mode at the same time, and such a life-history strategy is adopted by most flowering plants and by many sessile aquatic animals. In this paper, we focus on hermaphrodites that nourish post-zygotic stages, e.g. flowering plants and internally fertilising invertebrates, and consider how their sex allocation should respond to an environmental stress that reduces prospects of survival but does not affect all individuals equally, rather acting only on a subset of the population. Whereas dissemination of pollen and sperm can begin at sexual maturation, release of seeds and larvae is delayed by embryonic development. We find that the evolutionarily stable strategy for allocation between male and female functions will be critically dependent on the effect of stress on the trade-off between the costs of male and female reproduction, (i.e. of sperm and embryos). Thus, we identify evaluation of this factor as an important challenge to empiricists interested in the effects of stress on sex allocation. When only a small fraction of the population is stressed, we predict that stressed individuals will allocate their resources entirely to male function and unstressed individuals will increase their allocation to female function. Conversely, when the fraction of stress-affected individuals is high, stressed individuals should respond to this stressor by increasing investment in sperm and unstressed individuals should invest solely in embryos. A further prediction of the model is that we would not expect to find populations in the natural world where both stressed and unstressed individuals are both hermaphrodite
Studies of high latitude current systems using Magsat vector data
Disturbance fields caused by global external current systems are analyzed in order to gain an improved understanding of the phydical processes which control high latitude current systems and to increase the confidence level in the identification of internal field levels. The basic approach is to: (1) categorize the vector data by those physical parameters important for investigation of external current systems; (2) map the disturbances for appropriate conditions; (3) model the currents which might cause the mapped disturbances; and (4) correlate results with data from other sources. It is concluded that the Magsat data set appears to have remarkably high precision and quality and should permit major advances to be made in modeling current distribution at high latitudes in the ionosphere and magnetosphere
Mu and Tau Neutrino Thermalization and Production in Supernovae: Processes and Timescales
We investigate the rates of production and thermalization of and
neutrinos at temperatures and densities relevant to core-collapse
supernovae and protoneutron stars. Included are contributions from electron
scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and
nucleon scattering. For the scattering processes, in order to incorporate the
full scattering kinematics at arbitrary degeneracy, the structure function
formalism developed by Reddy et al. (1998) and Burrows and Sawyer (1998) is
employed. Furthermore, we derive formulae for the total and differential rates
of nucleon-nucleon bremsstrahlung for arbitrary nucleon degeneracy in
asymmetric matter. We find that electron scattering dominates nucleon
scattering as a thermalization process at low neutrino energies
( MeV), but that nucleon scattering is always faster
than or comparable to electron scattering above MeV. In
addition, for g cm, MeV, and
neutrino energies MeV, nucleon-nucleon bremsstrahlung always
dominates electron-positron annihilation as a production mechanism for
and neutrinos.Comment: 29 pages, LaTeX (RevTeX), 13 figures, submitted to Phys. Rev. C. Also
to be found at anonymous ftp site http://www.astrophysics.arizona.edu; cd to
pub/thompso
Testing the standard fireball model of GRBs using late X-ray afterglows measured by Swift
We show that all X-ray decay curves of GRBs measured by Swift can be fitted
using one or two components both of which have exactly the same functional form
comprised of an early falling exponential phase followed by a power law decay.
The 1st component contains the prompt gamma-ray emission and the initial X-ray
decay. The 2nd component appears later, has a much longer duration and is
present for ~80% of GRBs. It most likely arises from the external shock which
eventually develops into the X-ray afterglow. In the remaining ~20% of GRBs the
initial X-ray decay of the 1st component fades more slowly than the 2nd and
dominates at late times to form an afterglow but it is not clear what the
origin of this emission is.
The temporal decay parameters and gamma/X-ray spectral indices derived for
107 GRBs are compared to the expectations of the standard fireball model
including a search for possible "jet breaks". For ~50% of GRBs the observed
afterglow is in accord with the model but for the rest the temporal and
spectral indices do not conform to the expected closure relations and are
suggestive of continued, late, energy injection. We identify a few possible jet
breaks but there are many examples where such breaks are predicted but are
absent.
The time, T_a, at which the exponential phase of the 2nd component changes to
a final powerlaw decay afterglow is correlated with the peak of the gamma-ray
spectrum, E_peak. This is analogous to the Ghirlanda relation, indicating that
this time is in some way related to optically observed break times measured for
pre-Swift bursts.Comment: submitted to Ap
z'-band Ground-Based Detection of the Secondary Eclipse of WASP-19b
We present the ground-based detection of the secondary eclipse of the
transiting exoplanet WASP-19b. The observations were made in the Sloan z'-band
using the ULTRACAM triple-beam CCD camera mounted on the NTT. The measurement
shows a 0.088\pm0.019% eclipse depth, matching previous predictions based on H-
and K-band measurements. We discuss in detail our approach to the removal of
errors arising due to systematics in the data set, in addition to fitting a
model transit to our data. This fit returns an eclipse centre, T0, of
2455578.7676 HJD, consistent with a circular orbit. Our measurement of the
secondary eclipse depth is also compared to model atmospheres of WASP-19b, and
is found to be consistent with previous measurements at longer wavelengths for
the model atmospheres we investigated.Comment: 20 pages, 10 figures. Published in the ApJ Supplement serie
Experimental study of acoustic displays of flight parameters in a simulated aerospace vehicle
Evaluating acoustic displays of target location in target detection and of flight parameters in simulated aerospace vehicle
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