1,127 research outputs found
Radiative neutron capture on a proton at BBN energies
The total cross section for radiative neutron capture on a proton, , is evaluated at big bang nucleosynthesis (BBN) energies. The
electromagnetic transition amplitudes are calculated up to next-to leading
order within the framework of pionless effective field theory with dibaryon
fields. We also calculate the cross section and the photon
analyzing power for the process from the amplitudes. The
values of low energy constants that appear in the amplitudes are estimated by a
Markov Chain Monte Carlo analysis using the relevant low energy experimental
data. Our result agrees well with those of other theoretical calculations
except for the cross section at some energies estimated by an
R-matrix analysis. We also study the uncertainties in our estimation of the
cross section at relevant BBN energies and find that the
estimated cross section is reliable to within 1% error.Comment: 21 pages and 12 eps figures; 6 eps figures and 2 references added,
and accepted for publication in Phys. Rev.
Chandra Observations of SN 2004et and the X-ray Emission of Type IIp Supernovae
We report the X-ray detection of the Type II-plateau supernova SN 2004et in
the spiral galaxy NGC 6946, using the Chandra X-Ray Observatory. The position
of the X-ray source was found to agree with the optical position within ~0.4
arcsec. Chandra also surveyed the region before the 2004 event, finding no
X-ray emission at the location of the progenitor. For the post-explosion
observations, a total of 202, 151, and 158 photons were detected in three
pointings, each ~29 ks in length, on 2004 October 22, November 6, and December
3, respectively. The spectrum of the first observation is best fit by a thermal
model with a temperature of kT=1.3 keV and a line-of-sight absorption of
N_H=1.0 x 10^{22} cm^{-2}. The inferred unabsorbed luminosity (0.4-8 keV) is
~4x10^{38} erg/s, adopting a distance of 5.5 Mpc. A comparison between hard and
soft counts on the first and third epochs indicates a softening over this time,
although there is an insufficient number of photons to constrain the variation
of temperature and absorption by spectral fitting. We model the emission as
arising from the reverse shock region in the interaction between the supernova
ejecta and the progenitor wind. For a Type IIP supernova with an extended
progenitor, the cool shell formed at the time of shock wave breakout from the
star can affect the initial evolution of the interaction shell and the
absorption of radiation from the reverse shock. The observed spectral softening
might be due to decreasing shell absorption. We find a pre-supernova mass loss
rate of (2-2.5)x 10^{-6} M_{\odot} /yr for a wind velocity of 10 kms, which is
in line with expectations for a Type IIP supernova.Comment: total 19 pages including 7 figures. ApJ, in press. See
http://spider.ipac.caltech.edu/staff/rho/preprint/SN2004etms.ps for the paper
including full resolution image
Lattice dynamics and the electron-phonon interaction in CaRuO
We present a Raman scattering study of CaRuO, in which we investigate
the temperature-dependence of the lattice dynamics and the electron-phonon
interaction below the metal-insulator transition temperature ({\it T}). Raman spectra obtained in a backscattering geometry with light polarized
in the ab-plane reveal 9 B phonon modes (140, 215, 265, 269, 292, 388,
459, 534, and 683 cm) and 9 A phonon modes (126, 192, 204, 251, 304,
322, 356, 395, and 607 cm) for the orthorhombic crystal structure
(PbcaD). With increasing temperature toward {\it T},
the observed phonon modes shift to lower energies and exhibit reduced spectral
weights, reflecting structural changes associated with the elongation of the
RuO octahedra. Interestingly, the phonons exhibit significant increases in
linewidths and asymmetries for {\it T} {\it T}. These results
indicate that there is an increase in the effective number of electrons and the
electron-phonon interaction strengths as the temperature is raised through {\it
T}, suggesting the presence of orbital fluctuations in the
temperature regime {\it T} {\it T} {\it T}.Comment: 6 pages, 4 figure
Magnetic properties of pure and Gd doped EuO probed by NMR
An Eu NMR study in the ferromagnetic phase of pure and Gd doped EuO was
performed. A complete description of the NMR lineshape of pure EuO allowed for
the influence of doping EuO with Gd impurities to be highlighted. The presence
of a temperature dependent static magnetic inhomogeneity in Gd doped EuO was
demonstrated by studying the temperature dependence of the lineshapes. The
results suggest that the inhomogeneity in 0.6% Gd doped EuO is linked to
colossal magnetoresistance. The measurement of the spin-lattice relaxation
times as a function of temperature led to the determination of the value of the
exchange integral J as a function of Gd doping. It was found that J is
temperature independent and spatially homogeneous for all the samples and that
its value increases abruptly with increasing Gd doping.Comment: 14 pages, 10 figures, to be published in Physical Review
Carbon Monoxide in the Cassiopeia A Supernova Remnant
We report the likely detection of near-infrared 2.29 m first overtone
Carbon Monoxide (CO) emission from the young supernova remnant Cassiopeia A
(Cas A). The continuum-subtracted CO filter map reveals CO knots within the
ejecta-rich reverse shock. We compare the first overtone CO emission with that
found in the well-studied supernova, SN 1987A and find 30 times less CO
in Cas A. The presence of CO suggests that molecule mixing is small in the SN
ejecta and that astrochemical processes and molecule formation may continue at
least ~300 years after the initial explosion.Comment: Accepted for the publication in ApJ Lette
Dynamics of Baryons from String Theory and Vector Dominance
We consider a holographic model of QCD from string theory, a la Sakai and
Sugimoto, and study baryons. In this model, mesons are collectively realized as
a five-dimensional \ Yang-Mills field and baryons
are classically identified as solitons with a unit Pontryagin number
and electric charges. The soliton is shown to be very small in the large
't Hooft coupling limit, allowing us to introduce an effective field . Its coupling to the mesons are dictated by the soliton structure, and
consists of a direct magnetic coupling to the field strength as well
as a minimal coupling to the gauge field. Upon the dimensional
reduction, this effective action reproduces all interaction terms between
nucleons and an infinite tower of mesons in a manner consistent with the large
expansion. We further find that all electromagnetic interactions, as
inferred from the same effective action via a holographic prescription, are
mediated by an infinite tower of vector mesons, rendering the baryon
electromagnetic form factors completely vector-dominated as well. We estimate
nucleon-meson couplings and also the anomalous magnetic moments, which compare
well with nature.Comment: 65pages, 3 figures, vector mesons and axial-vector mesons are now
canonically normalized (comparisons with data and conclusions unaffected
Finite density and temperature in hybrid bag models
We introduce the chemical potential in a system of two-flavored massless
fermions in a chiral bag by imposing boundary conditions in the Euclidean time
direction. We express the fermionic mean number in terms of a functional trace
involving the Green function of the boundary value problem, which is studied
analytically. Numerical evaluations for the fermionic number are presented.Comment: 19 pages, 4 figure
Molecular and Ionic shocks in the Supernova Remnant 3C391
New observations of the supernova remnant 3C391 are in the H2 2.12 micron and
[Fe II] 1.64 micron narrow-band filters at the Palomar 200-inch telescope, and
in the 5-15 micron CVF on ISOCAM. Shocked H2 emission was detected from the
region 3C391:BML, where broad millimeter CO and CS lines had previously been
detected. A new H2 clump was confirmed to have broad CO emission, demonstrating
that the near-infrared H2 images can trace previously undetected molecular
shocks. The [Fe II] emission has a significantly different distribution, being
brightest in the bright radio bar, at the interface between the supernova
remnant and the giant molecular cloud, and following filaments in the radio
shell. The near-infrared [Fe II] and the mid-infrared 12-18 micron filter
images are the first images to reveal the radiative shell of 3C391. The
mid-infrared spectrum is dominated by bright ionic lines and H2 S(2) through
S(7). There are no aromatic hydrocarbons associated with the shocks, nor is
their any mid-infrared continuum, suggesting that macromolecules and very small
grains are destroyed. Comparing 3C391 to the better-studied IC443, both
remnants have molecular- and ionic-dominated regions; for 3C391, the
ionic-dominated region is the interface into the giant molecular cloud, showing
that the main bodies of giant molecular clouds contain significant regions with
densities 100 to 1000/cm^3 and a small filling factor with higher-density. The
molecular shocked region resolves into 16 clumps of H2 emission, with some
fainter diffuse emission but with no associated near-infrared continuum
sources. One of the clumps is coincident with a previously-detected OH 1720 MHz
maser. These clumps are interpreted as a cluster of pre-stellar, dense
molecular cores that are presently being shocked by the supernova blast wave
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