2,580 research outputs found
Modeling the two-point correlation of the vector stream function
A new model for the two-point vector stream function correlation has been developed using tensor invariant arguments and evaluated by the comparison of model predictions with DNS data for incompressible homogeneous turbulent shear flow. This two-point vector stream function model correlation can then be used to calculate the two-point velocity correlation function and other quantities useful in turbulence modeling. The model assumes that the two-point vector stream function correlation can be written in terms of the separation vector and a new tensor function that depends only on the magnitude of the separation vector. The model has a single free model coefficient, which has been chosen by comparison with the DNS data. The relative error of the model predictions of the two-point vector stream function correlation is only a few percent for a broad range of the model coefficient. Predictions of the derivatives of this correlation, which are of interest in turbulence modeling, may not be this accurate
Production of 26Al in stellar hydrogen-burning environments: spectroscopic properties of states in 27Si
Model predictions of the amount of the radioisotope 26Al produced in
hydrogen-burning environments require reliable estimates of the thermonuclear
rates for the 26gAl(p,{\gamma})27Si and 26mAl(p,{\gamma})27Si reactions. These
rates depend upon the spectroscopic properties of states in 27Si within about 1
MeV of the 26gAl+p threshold (Sp = 7463 keV). We have studied the
28Si(3He,{\alpha})27Si reaction at 25 MeV using a high-resolution
quadrupole-dipole-dipole-dipole magnetic spectrograph. For the first time with
a transfer reaction, we have constrained J{\pi} values for states in 27Si over
Ex = 7.0 - 8.1 MeV through angular distribution measurements. Aside from a few
important cases, we generally confirm the energies and spin-parity assignments
reported in a recent {\gamma}-ray spectroscopy study. The magnitudes of neutron
spectroscopic factors determined from shell-model calculations are in
reasonable agreement with our experimental values extracted using this
reaction.Comment: accepted for publication in Phys. Rev.
The COMPTEL instrumental line background
The instrumental line background of the Compton telescope COMPTEL onboard the
Compton Gamma-Ray Observatory is due to the activation and/or decay of many
isotopes. The major components of this background can be attributed to eight
individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl.
The identification of instrumental lines with specific isotopes is based on the
line energies as well as on the variation of the event rate with time,
cosmic-ray intensity, and deposited radiation dose during passages through the
South-Atlantic Anomaly. The characteristic variation of the event rate due to a
specific isotope depends on its life-time, orbital parameters such as the
altitude of the satellite above Earth, and the solar cycle. A detailed
understanding of the background contributions from instrumental lines is
crucial at MeV energies for measuring the cosmic diffuse gamma-ray background
and for observing gamma-ray line emission in the interstellar medium or from
supernovae and their remnants. Procedures to determine the event rate from each
background isotope are described, and their average activity in spacecraft
materials over the first seven years of the mission is estimated.Comment: accepted for publication in A&A, 22 pages, 21 figure
The Galactic 26Al Problem and The Close Binary SNIb/c Solution?
The origin of the long-lived radioactive 26Al, which has been observed in the
Galactic interstellar medium from its 1.809 MeV decay gamma-ray line emission,
has been a persistent problem for over twenty years. Wolf-Rayet (WR) winds were
thought to be the most promising source, but their calculated 26Al yields are
not consistent with recent analyses of the 1.809 MeV emission from the nearest
WR star and nearby OB associations. The expected 26Al yield from the WR star
exceeds by as much as a factor of 3, that set by the 2-sigma upper limit on the
1.809 MeV emission, while the WR yields in the OB associations are only about
1/3 of that required by the 1.809 MeV emission. We suggest that a solution to
these problems may lie in 26Al from a previously ignored source: explosive
nucleosynthesis in the core collapse SNIb/c supernovae of WR stars that have
lost most of their mass to close binary companions. Recent nucleosynthetic
calculations of SNIb/c suggest that their 26Al yields depend very strongly on
the final, pre-supernova mass of the WR star, and that those with final masses
around 6 to 8 solar masses are expected to produce as much as 0.01 solar masses
of 26Al per supernova. Such binary SNIb/c make up only a small fraction of the
current SNIb/c and only about 1% of all Galactic core collapse supernovae. They
appear to be such prolific sources that the bulk of the present 26Al in the
Galaxy may come from just a few hundred close binary SNIb/c and the intense
1.809 MeV emission from nearby OB associations may come from just one or two
such supernova.Comment: Accepted for publication in Astrophysical Journal Letters, 611,10
August 200
Positron Annihilation in the Galaxy
The 511 keV line from positron annihilation in the Galaxy was the first γ-ray line detected to originate from outside our solar system. Going into the fifth decade since the discovery, the source of positrons is still unconfirmed and remains one of the enduring mysteries in γ-ray astronomy. With a large flux of ∼10−3 γ/cm2/s, after 15 years in operation INTEGRAL/SPI has detected the 511 keV line at >50σ and has performed high-resolution spectral studies which conclude that Galactic positrons predominantly annihilate at low energies in warm phases of the interstellar medium. The results from imaging are less certain, but show a spatial distribution with a strong concentration in the center of the Galaxy. The observed emission from the Galactic disk has low surface brightness and the scale height is poorly constrained, therefore, the shear number of annihilating positrons in our Galaxy is still not well know. Positrons produced in β+-decay of nucleosynthesis products, such as 26Al, can account for some of the annihilation emission in the disk, but the observed spatial distribution, in particular the excess in the Galactic bulge, remains difficult to explain. Additionally, one of the largest uncertainties in these studies is the unknown distance that positrons propagate before annihilation. In this paper, we will summarize the current knowledge base of Galactic positrons, and discuss how next-generation instruments could finally provide the answers.Non peer reviewedFinal Accepted Versio
A search for light dark matter in XENON10 data
We report results of a search for light (<10 GeV) particle dark matter with
the XENON10 detector. The event trigger was sensitive to a single electron,
with the analysis threshold of 5 electrons corresponding to 1.4 keV nuclear
recoil energy. Considering spin-independent dark matter-nucleon scattering, we
exclude cross sections \sigma_n>3.5x10^{-42} cm^2, for a dark matter particle
mass m_{\chi}=8 GeV. We find that our data strongly constrain recent elastic
dark matter interpretations of excess low-energy events observed by CoGeNT and
CRESST-II, as well as the DAMA annual modulation signal.Comment: Manuscript identical to v2 (published version) but also contains
erratum. Note v3==v2 but without \linenumber
Constraints on inelastic dark matter from XENON10
It has been suggested that dark matter particles which scatter inelastically
from detector target nuclei could explain the apparent incompatibility of the
DAMA modulation signal (interpreted as evidence for particle dark matter) with
the null results from CDMS-II and XENON10. Among the predictions of
inelastically interacting dark matter are a suppression of low-energy events,
and a population of nuclear recoil events at higher nuclear recoil equivalent
energies. This is in stark contrast to the well-known expectation of a falling
exponential spectrum for the case of elastic interactions. We present a new
analysis of XENON10 dark matter search data extending to E keV
nuclear recoil equivalent energy. Our results exclude a significant region of
previously allowed parameter space in the model of inelastically interacting
dark matter. In particular, it is found that dark matter particle masses
GeV are disfavored.Comment: 8 pages, 4 figure
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