5,675 research outputs found
On the Excitation and Formation of Circumstellar Fullerenes
We compare and analyze the Spitzer mid-infrared spectrum of three
fullerene-rich planetary nebulae in the Milky Way and the Magellanic Clouds;
Tc1, SMP SMC16, and SMP LMC56. The three planetary nebulae share many
spectroscopic similarities. The strongest circumstellar emission bands
correspond to the infrared active vibrational modes of the fullerene species
C60 and little or no emission is present from Polycyclic Aromatic Hydrocarbons
(PAHs). The strength of the fullerene bands in the three planetary nebulae is
very similar, while the ratio of the [NeIII]15.5um/[NeII]12.8um fine structure
lines, an indicator of the strength of the radiation field, is markedly
different. This raises questions about their excitation mechanism and we
compare the fullerene emission to fluorescent and thermal models. In addition,
the spectra show other interesting and common features, most notably in the
6-9um region, where a broad plateau with substructure dominates the emission.
These features have previously been associated with mixtures of
aromatic/aliphatic hydrocarbon solids. We hypothesize on the origin of this
band, which is likely related to the fullerene formation mechanism, and compare
it with modeled Hydrogenated Amorphous Carbon that present emission in this
region.Comment: 13 pages, 2 tables, 7 figures, Accepted for publication in Ap
Spin alignment of dark matter haloes in filaments and walls
The MMF technique is used to segment the cosmic web as seen in a cosmological
N-body simulation into wall-like and filament-like structures. We find that the
spins and shapes of dark matter haloes are significantly correlated with each
other and with the orientation of their host structures. The shape orientation
is such that the halo minor axes tend to lie perpendicular to the host
structure, be it a wall or filament. The orientation of the halo spin vector is
mass dependent. Low mass haloes in walls and filaments have a tendency to have
their spins oriented within the parent structure, while higher mass haloes in
filaments have spins that tend to lie perpendicular to the parent structure.Comment: 4 pages, 2 figure
Development and experimental validation of a dynamic numerical model for human articular cartilage
The Off-diagonal Goldberger-Treiman Relation and Its Discrepancy
We study the off-diagonal Goldberger-Treiman relation (ODGTR) and its
discrepancy (ODGTD) in the N, Delta, pi sector through O(p^2) using heavy
baryon chiral perturbation theory. To this order, the ODGTD and axial vector N
to Delta transition radius are determined solely by low energy constants. Loop
corrections appear at O(p^4). For low-energy constants of natural size, the
ODGTD would represent a ~ 2% correction to the ODGTR. We discuss the
implications of the ODGTR and ODGTD for lattice and quark model calculations of
the transition form factors and for parity-violating electroexcitation of the
Delta.Comment: 11 pages, 1 eps figur
Matrix degradation in osteoarthritis primes the superficial region of cartilage for mechanical damage
Baryon Decuplet to Octet Electromagnetic Transitions in Quenched and Partially Quenched Chiral Perturbation Theory
We calculate baryon decuplet to octet electromagnetic transition form factors
in quenched and partially quenched chiral perturbation theory. We work in the
isospin limit of SU(3) flavor, up to next-to-leading order in the chiral
expansion, and to leading order in the heavy baryon expansion. Our results are
necessary for proper extrapolation of lattice calculations of these
transitions. We also derive expressions for the case of SU(2) flavor away from
the isospin limit.Comment: 16 pages, 3 figures, revtex
Evidence for dust evolution within the Taurus Complex from Spitzer images
We present Spitzer images of the Taurus Complex (TC) and take advantage of
the sensitivity and spatial resolution of the observations to characterize the
diffuse IR emission across the cloud. This work highlights evidence of dust
evolution within the translucent sections of the archetype reference for
studies of quiescent molecular clouds. We combine Spitzer 160 um and IRAS 100
um observations to produce a dust temperature map and a far-IR dust opacity map
at 5' resolution. The average dust temperature is about 14.5K with a dispersion
of +/-1K across the cloud. The far-IR dust opacity is a factor 2 larger than
the average value for the diffuse ISM. This opacity increase and the
attenuation of the radiation field (RF) both contribute to account for the
lower emission temperature of the large grains. The structure of the TC
significantly changes in the mid-IR images that trace emission from PAHs and
VSGs. We focus our analysis of the mid-IR emission to a range of ecliptic
latitudes where the zodiacal light residuals are small. Within this cloud area,
there are no 8 and 24 um counterparts to the brightest 160 um emission
features. Conversely, the 8 and 24 um images reveal filamentary structure that
is strikingly inconspicuous in the 160 um and extinction maps. The IR colors
vary over sub-parsec distances across this filamentary structure. We compare
the observed colors with model calculations quantifying the impact of the RF
intensity and the abundance of stochastically heated particles on the dust SED.
To match the range of observed colors, we have to invoke variations by a factor
of a few of both the interstellar RF and the abundance of PAHs and VSGs. We
conclude that within this filamentary structure a significant fraction of the
dust mass cycles in and out the small size end of the dust size distribution.Comment: 43 pages, 13 figures, accepted for publication in Ap
Dilepton and Photon Emission Rates from a Hadronic Gas
We analyze the dilepton and photon emission rates from a hadronic gas using
chiral reduction formulas and a virial expansion. The emission rates are
reduced to pertinent vacuum correlation functions, most of which can be
assessed from experiment. Our results indicate that in the low mass region, the
dilepton and photon rates are enhanced compared to most of the calculations
using chiral Lagrangians. The enhancement is further increased through a finite
pion chemical potential. An estimate of the emission rates is also made using
Haag's expansion for the electromagnetic current. The relevance of these
results to dilepton and photon emission rates in heavy-ion collisions is
discussed.Comment: 7 pages, LaTeX using revTeX, 6 figures imbedded in text. Figures
slightly changed, text left unchange
Chiral Behaviour of the Rho Meson in Lattice QCD
In order to guide the extrapolation of the mass of the rho meson calculated
in lattice QCD with dynamical fermions, we study the contributions to its
self-energy which vary most rapidly as the quark mass approaches zero; from the
processes and . It turns out that in
analysing the most recent data from CP-PACS it is crucial to estimate the
self-energy from using the same grid of discrete momenta as
included implicitly in the lattice simulation. The correction associated with
the continuum, infinite volume limit can then be found by calculating the
corresponding integrals exactly. Our error analysis suggests that a factor of
10 improvement in statistics at the lowest quark mass for which data currently
exists would allow one to determine the physical rho mass to within 5%.
Finally, our analysis throws new light on a long-standing problem with the
J-parameter.Comment: 13 pages, 7 figures. Full analytic forms of the self-energies are
included and a correction in the omega-pi self-energ
Effective theory of the Delta(1232) in Compton scattering off the nucleon
We formulate a new power-counting scheme for a chiral effective field theory
of nucleons, pions, and Deltas. This extends chiral perturbation theory into
the Delta-resonance region. We calculate nucleon Compton scattering up to
next-to-leading order in this theory. The resultant description of existing
p cross section data is very good for photon energies up to about 300
MeV. We also find reasonable numbers for the spin-independent polarizabilities
and .Comment: 29 pp, 9 figs. Minor revisions. To be published in PR
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