9,046 research outputs found
Test of nuclear level density inputs for Hauser-Feshbach model calculations
The energy spectra of neutrons, protons, and alpha-particles have been
measured from the d+59Co and 3He+58Fe reactions leading to the same compound
nucleus, 61$Ni. The experimental cross sections have been compared to
Hauser-Feshbach model calculations using different input level density models.
None of them have been found to agree with experiment. It manifests the serious
problem with available level density parameterizations especially those based
on neutron resonance spacings and density of discrete levels. New level
densities and corresponding Fermi-gas parameters have been obtained for
reaction product nuclei such as 60Ni,60Co, and 57Fe
Digital receiver study and implementation
Computer software was developed which makes it possible to use any general purpose computer with A/D conversion capability as a PSK receiver for low data rate telemetry processing. Carrier tracking, bit synchronization, and matched filter detection are all performed digitally. To aid in the implementation of optimum computer processors, a study of general digital processing techniques was performed which emphasized various techniques for digitizing general analog systems. In particular, the phase-locked loop was extensively analyzed as a typical non-linear communication element. Bayesian estimation techniques for PSK demodulation were studied. A hardware implementation of the digital Costas loop was developed
Red Supergiants in the Andromeda Galaxy (M31)
Red supergiants are a short-lived stage in the evolution of moderately
massive stars (10-25Mo), and as such their location in the H-R diagram provides
an exacting test of stellar evolutionary models. Since massive star evolution
is strongly affected by the amount of mass-loss a star suffers, and since the
mass-loss rates depend upon metallicity, it is highly desirable to study the
physical properties of these stars in galaxies of various metallicities. Here
we identify a sample of red supergiants in M31 (the most metal-rich of the
Local Group galaxies) and derive their physical properties by fitting MARCS
atmosphere models to moderate resolution optical spectroscopy, and from V-K
photometry.Comment: Accepted for publication in the Astrophysical Journa
Sersiclets - A Matched Filter Extension of Shapelets for Weak Lensing Studies
The precision study of dark matter using weak lensing by large scale
structure is strongly constrained by the accuracy with which one can measure
galaxy shapes. Several methods have been devised but none have demonstrated the
ability to reach the level of precision required by future weak lensing
surveys. In this Letter we explore new avenues to the existing Shapelets
approach, combining a priori knowledge of the galaxy profile with the power of
orthogonal basis function decomposition. This Letter discusses the new issues
raised by this matched filter approach and proposes promising alternatives to
shape measurement techniques. In particular it appears that the use of a
matched filter (e.g. Sersic profile) restricted to elliptical radial fitting
functions resolves several well known Shapelet issues.Comment: 6 pages, 6 figures. MNRAS Accepte
Hemiparasitic plant impacts animal and plant communities across four trophic levels
1.Understanding the impact of species on community structure is a fundamental question in ecology. There is a growing body of evidence that suggests that both sub-dominant species and parasites can have a disproportionately large impact.
2.Here we report the impacts of an organism that is both subdominant and parasitic, the hemiparasite Rhinanthus minor. Whilst the impact of parasitic angiosperms on their hosts and, to a lesser degree, co-existing plant species, have been well characterized, much less is known about their impacts on higher trophic levels.
3.We experimentally manipulated field densities of the hemiparasite Rhinanthus minor in a species rich grassland, comparing the plant and invertebrate communities in plots where it was removed, at natural densities or at enhanced densities.
4.Plots with natural and enhanced densities of R. minor had lower plant biomass than plots without the hemiparasite, but enhanced densities almost doubled the abundance of invertebrates within the plots across all trophic levels, with effects evident in herbivores, predators and detritivores.
5.The hemiparasite R. minor, despite being a sub-dominant and transient component within plant communities that it inhabits, has profound effects on four different trophic levels. These effects persist beyond the life of the hemiparasite,
emphasizing its role as a keystone species in grassland communitie
Acoustic interactions between an altitude test facility and jet engine plumes: Theory and experiments
The overall objective of the described effort was to develop an understanding of the physical mechanisms involved in the flow/acoustic interactions experienced in full-scale altitude engine test facilities. This is done by conducting subscale experiments and through development of a theoretical model. Model cold jet experiments with an axisymmetric convergent nozzle are performed in a test setup that stimulates a supersonic jet exhausting into a cylindrical diffuser. The measured data consist of detailed flow visualization data and acoustic spectra for a free and a ducted plume. It is shown that duct resonance is most likely responsible by theoretical calculations. Theoretical calculations also indicate that the higher discrete tones observed in the measurements are related to the screech phenomena. Limited experiments on the sensitivity of a free 2-D, C-D nozzle to externally imposed sound are also presented. It is shown that a 2-D, C-D nozzle with a cutback is less excitable than a 2-D C-D nozzle with no cutback. At a pressure ratio of 1.5 unsteady separation from the diverging walls of the nozzle is noticed. This separation switches from one wall to the opposite wall thus providing an unsteady deflection of the plume. It is shown that this phenomenon is related to the venting provided by the cutback section
The impact of mass-loss on the evolution and pre-supernova properties of red supergiants
The post main-sequence evolution of massive stars is very sensitive to many
parameters of the stellar models. Key parameters are the mixing processes, the
metallicity, the mass-loss rate and the effect of a close companion. We study
how the red supergiant lifetimes, the tracks in the Hertzsprung-Russel diagram
(HRD), the positions in this diagram of the pre-supernova progenitor as well as
the structure of the stars at that time change for various mass-loss rates
during the red supergiant phase (RSG), and for two different initial rotation
velocities. The surface abundances of RSGs are much more sensitive to rotation
than to the mass-loss rates during that phase. A change of the RSG mass-loss
rate has a strong impact on the RSG lifetimes and therefore on the luminosity
function of RSGs. At solar metallicity, the enhanced mass-loss rate models do
produce significant changes on the populations of blue, yellow and red
supergiants. When extended blue loops or blue ward excursions are produced by
enhanced mass-loss, the models predict that a majority of blue (yellow)
supergiants are post RSG objects. These post RSG stars are predicted to show
much smaller surface rotational velocities than similar blue supergiants on
their first crossing of the HR gap. The position in the HRD of the end point of
the evolution depends on the mass of the hydrogen envelope. More precisely,
whenever, at the pre-supernova stage, the H-rich envelope contains more than
about 5\% of the initial mass, the star is a red supergiant, and whenever the
H-rich envelope contains less than 1\% of the total mass the star is a blue
supergiant. For intermediate situations, intermediate colors/effective
temperatures are obtained. Yellow progenitors for core collapse supernovae can
be explained by the enhanced mass-loss rate models, while the red progenitors
are better fitted by the standard mass-loss rate models.Comment: 19 pages, 11 figures, 6 tables, accepted for publication in Astronomy
and Astrophysic
Oxygen-rich dust production in IC 10
We report the detection of oxygen-rich circumstellar envelopes in stars of
the nearby (700 kpc) starburst galaxy IC 10. The star formation history and the
chemical environment of this galaxy makes it an ideal target to observe dust
production by high-mass stars in a low-metallicity environment. The goal of
this study is to identify oxygen-rich stars in IC 10 and to constrain their
nature between asymptotic giant branch stars (AGBs), red supergiants (RSGs),
and other infrared bright sources. We examine the mass-loss rate of the stars
and compare to results obtained for the Magellanic Clouds. Our objectives are
to (1) assess whether RSGs can be significant dust producers in IC 10, and (2),
solve the discrepancy between the star formation history of IC 10 and the
relatively low number of RSGs detected in the optical. We search for silicate
dust in emission by using the spectral map observed with the Infrared
Spectrograph on board the Spitzer Space Telescope. The optical (UBVRI) and
infrared (JHK, Spitzer/IRAC and Spitzer/MIPS) photometry is used to assert the
membership of the stars to IC 10 and disentangle between AGBs and RSGs.
Radiative models are used to infer mass-loss rates and stellar luminosities.
The luminosity and colors of at least 9 silicate emission sources are
consistent with stars within IC 10. Furthermore, the photometry of 2 of these
sources is consistent with RSGs. We derive dust mass-loss rates similar to the
values found in the Magellanic Clouds. Accounting for the sample completeness,
RSGs are not important contributors to the dust mass budget in IC 10.Comment: Accepted for publication in A&
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