1,198 research outputs found
The Spitzer Atlas of Stellar Spectra
We present the Spitzer Atlas of Stellar Spectra (SASS), which includes 159
stellar spectra (5 to 32 mic; R~100) taken with the Infrared Spectrograph on
the Spitzer Space Telescope. This Atlas gathers representative spectra of a
broad section of the Hertzsprung-Russell diagram, intended to serve as a
general stellar spectral reference in the mid-infrared. It includes stars from
all luminosity classes, as well as Wolf-Rayet (WR) objects. Furthermore, it
includes some objects of intrinsic interest, like blue stragglers and certain
pulsating variables. All the spectra have been uniformly reduced, and all are
available online. For dwarfs and giants, the spectra of early-type objects are
relatively featureless, dominated by Hydrogen lines around A spectral types.
Besides these, the most noticeable photospheric features correspond to water
vapor and silicon monoxide in late-type objects and methane and ammonia
features at the latest spectral types. Most supergiant spectra in the Atlas
present evidence of circumstellar gas. The sample includes five M supergiant
spectra, which show strong dust excesses and in some cases PAH features.
Sequences of WR stars present the well-known pattern of lines of HeI and HeII,
as well as forbidden lines of ionized metals. The characteristic flat-top shape
of the [Ne III] line is evident even at these low spectral resolutions. Several
Luminous Blue Variables and other transition stars are present in the Atlas and
show very diverse spectra, dominated by circumstellar gas and dust features. We
show that the [8]-[24] Spitzer colors (IRAC and MIPS) are poor predictors of
spectral type for most luminosity classes.Comment: Accepted by ApJS; Atlas contents available from:
http://web.ipac.caltech.edu/staff/ardila/Atlas/index.html;
http://irsa.ipac.caltech.edu/data/SPITZER/SASS/; 70 PDF pages, including
figure
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.
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
Exchange Current Corrections to Neutrino--Nucleus Scattering
Relativistic exchange current corrections to neutrino--nucleus cross sections
are presented assuming non--vanishing strange quark form factors for the
constituent nucleons. For charged current processes the exchange current
corrections can lower the impulse approximation results by 10\% while these
corrections are found to be sensitive to both the nuclear density and the
strange quark axial form factor of the nucleon for neutral current processes.
Implications on the LSND experiment to determine this form factor are
discussed.Comment: 11 pages, 2 figures, revtex 3.0, full postscript version of the file
and figures available at
http://www.nikhefk.nikhef.nl/projects/Theory/preprints/preprints.html To
appear in Phys. Rev. Lett
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
Complete 0 hbar omega calculations of Gamow-Teller strengths for nuclei in the iron region
Gamow-Teller strengths for selected nuclei in the iron region (A~56) have
been investigated via shell-model Monte Carlo calculations with realistic
interactions in the complete fp basis. Results for all cases show significant
quenching relative to single-particle estimates, in quantitative agreement with
(n,p) data. The J=1,T=0 residual interaction and the f_{7/2}-f_{5/2} spin-orbit
splitting are shown to play major roles in the quenching mechanism. Calculated
B(E2, 2^+_1 -> 0^+_1) values are in fair agreement with experiment using
effective charges of e_p=1.1e and e_n=0.1e.Comment: 13 pages + 1 postscript file, Caltech preprint MAP-16
Quark Description of Hadronic Phases
We extend our proposal that major universality classes of hadronic matter can
be understood, and in favorable cases calculated, directly in the microscopic
quark variables, to allow for splitting between strange and light quark masses.
A surprisingly simple but apparently viable picture emerges, featuring
essentially three phases, distinguished by whether strangeness is conserved
(standard nuclear matter), conserved modulo two (hypernuclear matter), or
locked to color (color flavor locking). These are separated by sharp phase
transitions. There is also, potentially, a quark phase matching hadronic
K-condensation. The smallness of the secondary gap in two-flavor color
superconductivity corresponds to the disparity between the primary dynamical
energy scales of QCD and the much smaller energy scales of nuclear physics.Comment: 21 pages, 2 figure
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
Flavor symmetry breaking effects on SU(3) Skyrmion
We study the massive SU(3) Skyrmion model to investigate the flavor symmetry
breaking (FSB) effects on the static properties of the strange baryons in the
framework of the rigid rotator quantization scheme combined with the improved
Dirac quantization one. Both the chiral symmetry breaking pion mass and FSB
kinetic terms are shown to improve the ratio of the strange-light to
light-light interaction strengths and that of the strange-strange to
light-light.Comment: 12 pages, latex, no figure
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