10,474 research outputs found
Heterogeneity in CO/CO Ratios Toward Solar-Type Young Stellar Objects
This study reports an unusual heterogeneity in
[CO]/[CO] abundance ratios of carbon monoxide
observed in the gas phase toward seven ~ solar-mass YSOs and three dense
foreground clouds in the nearby star-forming regions, Ophiuchus, Corona
Australis, Orion, Vela and an isolated core, L43. Robust isotope ratios were
derived using infrared absorption spectroscopy of the 4.7 m fundamental
and 2.3 m overtone rovibrational bands of CO at very high resolution
(/), observed with the CRIRES
spectrograph on the Very Large Telescope. We find
[CO]/[CO] values ranging from ~ 85 to 165,
significantly higher than those of the local interstellar medium (~ 65 to 69).
These observations are evidence for isotopic heterogeneity in carbon reservoirs
in solar-type YSO environments, and encourage the need for refined Galactic
chemical evolution models to explain the C/C discrepancy between
the solar system and local ISM. The oxygen isotope ratios are consistent with
isotopologue-specific photodissociation by CO self-shielding toward the disks,
VV CrA N and HL Tau, further substantiating models predicting CO self-shielding
on disk surfaces. However, we find that CO self-shielding is an unlikely
general explanation for the high [CO]/[CO] ratios
observed in this study. Comparison of the solid CO against gas-phase
[CO]/[CO] suggests that interactions between CO ice
and gas reservoirs need to be further investigated as at least a partial
explanation for the unusually high [CO]/[CO]
observed.Comment: 16 pages, 14 figures, 7 tables. Accepted for publication in The
Astrophysical Journa
Casimir effect for curved geometries: PFA validity limits
We compute Casimir interaction energies for the sphere-plate and
cylinder-plate configuration induced by scalar-field fluctuations with
Dirichlet boundary conditions. Based on a high-precision calculation using
worldline numerics, we quantitatively determine the validity bounds of the
proximity force approximation (PFA) on which the comparison between all
corresponding experiments and theory are based. We observe the quantitative
failure of the PFA on the 1% level for a curvature parameter a/R > 0.00755.
Even qualitatively, the PFA fails to predict reliably the correct sign of
genuine Casimir curvature effects. We conclude that data analysis of future
experiments aiming at a precision of 0.1% must no longer be based on the PFA.Comment: 4 pages, 4 figure
Geometric quantum gate for trapped ions based on optical dipole forces induced by Gaussian laser beams
We present an implementation of quantum logic gates via internal state
dependent displacements of ions in a linear Paul trap caused by optical dipole
forces. Based on a general quantum analysis of the system dynamics we consider
specific implementations with alkaline earth ions. For experimentally realistic
parameters gate infidelities as low as can be obtained.Comment: 10 pages, 4 figure
Micro-canonical pentaquark production in \ee annihilations
The existence of pentaquarks, namely baryonic states made up of four quarks
and one antiquark, became questionable, because the candidates, i.e. the
peak, are seen in certain reactions, i.e. p+p collisions, but not in
others, i.e. \ee annihilations. In this paper, we estimate the production of
and in \ee annihilations at different
energies using Fermi statistical model as originally proposed in its
microcanonical form. The results is compared with that from pp collisions at
SPS and RHIC energies. We find that, if pentaquark states exist, the production
is highly possible in \ee annihilations. For example, at LEP energy
=91.2 GeV, both and yield more
than in pp collisions at SPS and RHIC energy.Comment: 7 pages 2 figure
Mode Bifurcation and Fold Points of Complex Dispersion Curves for the Metamaterial Goubau Line
In this paper the complex dispersion curves of the four lowest-order
transverse magnetic modes of a dielectric Goubau line () are
compared with those of a dispersive metamaterial Goubau line. The vastly
different dispersion curve structure for the metamaterial Goubau line is
characterized by unusual features such as mode bifurcation, complex fold
points, both proper and improper complex modes, and merging of complex and real
modes
Cosmological Radiation Hydrodynamics with ENZO
We describe an extension of the cosmological hydrodynamics code ENZO to
include the self-consistent transport of ionizing radiation modeled in the
flux-limited diffusion approximation. A novel feature of our algorithm is a
coupled implicit solution of radiation transport, ionization kinetics, and gas
photoheating, making the timestepping for this portion of the calculation
resolution independent. The implicit system is coupled to the explicit
cosmological hydrodynamics through operator splitting and solved with scalable
multigrid methods. We summarize the numerical method, present a verification
test on cosmological Stromgren spheres, and then apply it to the problem of
cosmological hydrogen reionization.Comment: 14 pages, 3 figures, to appear in Recent Directions in Astrophysical
Quantitative Spectroscopy and Radiation Hydrodynamics, Ed. I. Hubeny,
American Institute of Physics (2009
A lattice evaluation of four-quark operators in the nucleon
Nucleon matrix elements of various four-quark operators are evaluated in
quenched lattice QCD using Wilson fermions. Some of these operators give rise
to twist-four contributions to nucleon structure functions. Furthermore, they
bear valuable information about the diquark structure of the nucleon. Mixing
with lower-dimensional operators is avoided by considering appropriate
representations of the flavour group. We find that for a certain flavour
combination of baryon structure functions, twist-four contributions are very
small. This suggests that twist-four effects for the nucleon might be much
smaller than m_p^2/Q^2.Comment: 17 pages, 3 figure
Applied lattice gauge calculations: diquark content of the nucleon
As an example of an application of lattice QCD we describe a computation of
four-quark operators in the nucleon. The results are interpreted in a diquark
language.Comment: 6 pages, 1 figure, Invited talk given by M. G\"ockeler at the
European Workshop on the QCD Structure of the Nucleon (QCD - N'02), Ferrara,
Italy, 3-6 Apr 200
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