2,543 research outputs found
The Fourth Positive System of Carbon Monoxide in the Hubble Space Telescope Spectra of Comets
The rich structure of the Fourth Positive System (A-X) of carbon monoxide
accounts for many of the spectral features seen in long slit HST-STIS
observations of comets 153P/Ikeya-Zhang, C/2001 Q4 (NEAT), and C/2000 WM1
(LINEAR), as well as in the HST-GHRS spectrum of comet C/1996 B2 Hyakutake. A
detailed CO fluorescence model is developed to derive the CO abundances in
these comets by simultaneously fitting all of the observed A-X bands. The model
includes the latest values for the oscillator strengths and state parameters,
and accounts for optical depth effects due to line overlap and self-absorption.
The model fits yield radial profiles of CO column density that are consistent
with a predominantly native source for all the comets observed by STIS. The
derived CO abundances relative to water in these comets span a wide range, from
0.44% for C/2000 WM1 (LINEAR), 7.2% for 153P/Ikeya-Zhang, 8.8% for C/2001 Q4
(NEAT) to 20.9% for C/1996 B2 (Hyakutake). The subtraction of the CO spectral
features using this model leads to the first identification of a molecular
hydrogen line pumped by solar HI Lyman-beta longward of 1200A in the spectrum
of comet 153P/Ikeya-Zhang. (Abridged)Comment: 12 pages, 11 figures, ApJ accepte
Recommended from our members
Toward a theory of coordinating: Creating coordinating mechanisms in practice
This paper uses a practice perspective to study coordinating as dynamic activities that are continuously created and modified in order to enact organizational relationships and activities. It is based on the case of Servico, an organization undergoing a major restructuring of its value chain in response to a change in government regulation. In our case, the actors iterate between the abstract concept of a coordinating mechanism referred to as end-to-end management and its performance in practice. They do this via five performative-ostensive cycles: (1) enacting disruption, (2) orienting to absence, (3) creating elements, (4) forming new patterns, and (5) stabilizing new patterns. These cycles and the relationships between them constitute a process model of coordinating. This model highlights the importance of absence in the coordinating process and demonstrates how experiencing absence shapes subsequent coordinating activity
Critical exponents predicted by grouping of Feynman diagrams in phi^4 model
Different perturbation theory treatments of the Ginzburg-Landau phase
transition model are discussed. This includes a criticism of the perturbative
renormalization group (RG) approach and a proposal of a novel method providing
critical exponents consistent with the known exact solutions in two dimensions.
The usual perturbation theory is reorganized by appropriate grouping of Feynman
diagrams of phi^4 model with O(n) symmetry. As a result, equations for
calculation of the two-point correlation function are obtained which allow to
predict possible exact values of critical exponents in two and three dimensions
by proving relevant scaling properties of the asymptotic solution at (and near)
the criticality. The new values of critical exponents are discussed and
compared to the results of numerical simulations and experiments.Comment: 34 pages, 6 figure
Peptidoglycan editing provides immunity to Acinetobacter baumannii during bacterial warfare
Peptidoglycan (PG) is essential in most bacteria. Thus, it is often targeted by various assaults, including interbacterial attacks via the type VI secretion system (T6SS). Here, we report that the Gram-negative bacteriu
Probing EWSB Naturalness in Unified SUSY Models with Dark Matter
We have studied Electroweak Symmetry Breaking (EWSB) fine-tuning in the
context of two unified Supersymmetry scenarios: the Constrained Minimal
Supersymmetric Model (CMSSM) and models with Non-Universal Higgs Masses (NUHM),
in light of current and upcoming direct detection dark matter experiments. We
consider both those models that satisfy a one-sided bound on the relic density
of neutralinos, , and also the subset that satisfy
the two-sided bound in which the relic density is within the 2 sigma best fit
of WMAP7 + BAO + H0 data. We find that current direct detection searches for
dark matter probe the least fine-tuned regions of parameter-space, or
equivalently those of lowest Higgs mass parameter , and will tend to probe
progressively more and more fine-tuned models, though the trend is more
pronounced in the CMSSM than in the NUHM. Additionally, we examine several
subsets of model points, categorized by common mass hierarchies; M_{\chi_0}
\sim M_{\chi^\pm}, M_{\chi_0} \sim M_{\stau}, M_{\chi_0} \sim M_{\stop_1}, the
light and heavy Higgs poles, and any additional models classified as "other";
the relevance of these mass hierarchies is their connection to the preferred
neutralino annihilation channel that determines the relic abundance. For each
of these subsets of models we investigated the degree of fine-tuning and
discoverability in current and next generation direct detection experiments.Comment: 26 pages, 10 figures. v2: references added. v3: matches published
versio
Functional Renormalization Group and the Field Theory of Disordered Elastic Systems
We study elastic systems such as interfaces or lattices, pinned by quenched
disorder. To escape triviality as a result of ``dimensional reduction'', we use
the functional renormalization group. Difficulties arise in the calculation of
the renormalization group functions beyond 1-loop order. Even worse,
observables such as the 2-point correlation function exhibit the same problem
already at 1-loop order. These difficulties are due to the non-analyticity of
the renormalized disorder correlator at zero temperature, which is inherent to
the physics beyond the Larkin length, characterized by many metastable states.
As a result, 2-loop diagrams, which involve derivatives of the disorder
correlator at the non-analytic point, are naively "ambiguous''. We examine
several routes out of this dilemma, which lead to a unique renormalizable
field-theory at 2-loop order. It is also the only theory consistent with the
potentiality of the problem. The beta-function differs from previous work and
the one at depinning by novel "anomalous terms''. For interfaces and random
bond disorder we find a roughness exponent zeta = 0.20829804 epsilon + 0.006858
epsilon^2, epsilon = 4-d. For random field disorder we find zeta = epsilon/3
and compute universal amplitudes to order epsilon^2. For periodic systems we
evaluate the universal amplitude of the 2-point function. We also clarify the
dependence of universal amplitudes on the boundary conditions at large scale.
All predictions are in good agreement with numerical and exact results, and an
improvement over one loop. Finally we calculate higher correlation functions,
which turn out to be equivalent to those at depinning to leading order in
epsilon.Comment: 42 pages, 41 figure
Forecasting Cosmological Constraints from Redshift Surveys
Observations of redshift-space distortions in spectroscopic galaxy surveys
offer an attractive method for observing the build-up of cosmological
structure, which depends both on the expansion rate of the Universe and our
theory of gravity. In this paper we present a formalism for forecasting the
constraints on the growth of structure which would arise in an idealized
survey. This Fisher matrix based formalism can be used to study the power and
aid in the design of future surveys.Comment: 7 pages, 5 figures, minor revisions to match version accepted by
MNRA
The critical equation of state of the three-dimensional O(N) universality class: N>4
We determine the scaling equation of state of the three-dimensional O(N)
universality class, for N=5, 6, 32, 64. The N=5 model is relevant for the SO(5)
theory of high-T_c superconductivity, while the N=6 model is relevant for the
chiral phase transition in two-color QCD with two flavors. We first obtain the
critical exponents and the small-field, high-temperature, expansion of the
effective potential (Helmholtz free energy) by analyzing the available
perturbative series, in both fixed-dimension and epsilon-expansion schemes.
Then, we determine the critical equation of state by using a systematic
approximation scheme, based on polynomial representations valid in the whole
critical region, which satisfy the known analytical properties of the equation
of state, take into account the Goldstone singularities at the coexistence
curve and match the small-field, high-temperature, expansion of the effective
potential. This allows us also to determine several universal amplitude ratios.
We also compare our approximate solutions with those obtained in the large-N
expansion, up to order 1/N, finding good agreement for N\gtrsim 32.Comment: 27 pages, 8 figures. v2: Improved presentation, updated references.
Nucl. Phys. B in pres
Massive Field-Theory Approach to Surface Critical Behavior in Three-Dimensional Systems
The massive field-theory approach for studying critical behavior in fixed
space dimensions is extended to systems with surfaces.This enables one to
study surface critical behavior directly in dimensions without having to
resort to the expansion. The approach is elaborated for the
representative case of the semi-infinite |\bbox{\phi}|^4 -vector model
with a boundary term {1/2} c_0\int_{\partial V}\bbox{\phi}^2 in the action.
To make the theory uv finite in bulk dimensions , a renormalization
of the surface enhancement is required in addition to the standard mass
renormalization. Adequate normalization conditions for the renormalized theory
are given. This theory involves two mass parameter: the usual bulk `mass'
(inverse correlation length) , and the renormalized surface enhancement .
Thus the surface renormalization factors depend on the renormalized coupling
constant and the ratio . The special and ordinary surface transitions
correspond to the limits with and ,
respectively. It is shown that the surface-enhancement renormalization turns
into an additive renormalization in the limit . The
renormalization factors and exponent functions with and
that are needed to determine the surface critical exponents of the special and
ordinary transitions are calculated to two-loop order. The associated series
expansions are analyzed by Pad\'e-Borel summation techniques. The resulting
numerical estimates for the surface critical exponents are in good agreement
with recent Monte Carlo simulations. This also holds for the surface crossover
exponent .Comment: Revtex, 40 pages, 3 figures, and 8 pictograms (included in equations
- âŚ