789 research outputs found
Collisional excitation of HC3N by para- and ortho-H2
New calculations for rotational excitation of cyanoacetylene by collisions
with hydrogen molecules are performed to include the lowest 38 rotational
levels of HC3N and kinetic temperatures to 300 K. Calculations are based on the
interaction potential of Wernli et al. A&A, 464, 1147 (2007) whose accuracy is
checked against spectroscopic measurements of the HC3N-H2 complex. The quantum
coupled-channel approach is employed and complemented by quasi-classical
trajectory calculations. Rate coefficients for ortho-H2 are provided for the
first time. Hyperfine resolved rate coefficients are also deduced. Collisional
propensity rules are discussed and comparisons between quantum and classical
rate coefficients are presented. This collisional data should prove useful in
interpreting HC3N observations in the cold and warm ISM, as well as in
protoplanetary disks.Comment: 8 pages, 2 tables, 4 figures, accepted for publication in MNRA
Effect of Gain-Dependent Phase Shift on Fiber Laser Synchronization
Recent experiments have demonstrated synchronization of fiber laser arrays at low and moderate pump levels. It has been suggested that a key dynamical process leading to synchronized behavior is the differential phase shift induced by the gain media. We explore theoretically the role of this effect in generating inphase dynamics. We find that its presence can substantially enhance the degree of inphase stability to an extent that could be practically important. At the same time, our analysis shows that a gain-dependent phase shift is not a necessary ingredient in the dynamical selection of the inphase state, thus, leading us to reconsider the essential mechanism behind inphase selection in fiber laser arrays
Collisional excitation of singly deuterated ammonia NHD by H
The availability of collisional rate coefficients with H is a
pre-requisite for interpretation of observations of molecules whose energy
levels are populated under non local thermodynamical equilibrium conditions. In
the current study, we present collisional rate coefficients for the NHD /
para--H() collisional system, for energy levels up to (735 K) and for gas temperatures in the range K. The
cross sections are obtained using the essentially exact close--coupling (CC)
formalism at low energy and at the highest energies, we used the
coupled--states (CS) approximation. For the energy levels up to
(215 K), the cross sections obtained through the CS formalism are
scaled according to a few CC reference points. These reference points are
subsequently used to estimate the accuracy of the rate coefficients for higher
levels, which is mainly limited by the use of the CS formalism. Considering the
current potential energy surface, the rate coefficients are thus expected to be
accurate to within 5\% for the levels below , while we estimate
an accuracy of 30\% for higher levels
Slow Switching in Globally Coupled Oscillators: Robustness and Occurrence through Delayed Coupling
The phenomenon of slow switching in populations of globally coupled
oscillators is discussed. This characteristic collective dynamics, which was
first discovered in a particular class of the phase oscillator model, is a
result of the formation of a heteroclinic loop connecting a pair of clustered
states of the population. We argue that the same behavior can arise in a wider
class of oscillator models with the amplitude degree of freedom. We also argue
how such heteroclinic loops arise inevitably and persist robustly in a
homogeneous population of globally coupled oscillators. Although the
heteroclinic loop might seem to arise only exceptionally, we find that it
appears rather easily by introducing the time-delay in the population which
would otherwise exhibit perfect phase synchrony. We argue that the appearance
of the heteroclinic loop induced by the delayed coupling is then characterized
by transcritical and saddle-node bifurcations. Slow switching arises when the
system with a heteroclinic loop is weakly perturbed. This will be demonstrated
with a vector model by applying weak noises. Other types of weak
symmetry-breaking perturbations can also cause slow switching.Comment: 10 pages, 14 figures, RevTex, twocolumn, to appear in Phys. Rev.
Collisional excitation of doubly and triply deuterated ammonia NDH and ND by H
The availability of collisional rate coefficients is a prerequisite for an
accurate interpretation of astrophysical observations, since the observed media
often harbour densities where molecules are populated under non--LTE
conditions. In the current study, we present calculations of rate coefficients
suitable to describe the various spin isomers of multiply deuterated ammonia,
namely the NDH and ND isotopologues. These calculations are based on
the most accurate NH--H potential energy surface available, which has
been modified to describe the geometrical changes induced by the nuclear
substitutions. The dynamical calculations are performed within the
close--coupling formalism and are carried out in order to provide rate
coefficients up to a temperature of = 50K. For the various
isotopologues/symmetries, we provide rate coefficients for the energy levels
below 100 cm. Subsequently, these new rate coefficients are used
in astrophysical models aimed at reproducing the NHD, NDH and ND
observations previously reported towards the prestellar cores B1b and 16293E.
We thus update the estimates of the corresponding column densities and find a
reasonable agreement with the previous models. In particular, the
ortho--to--para ratios of NHD and NHD are found to be consistent with
the statistical ratios
Rapid Polymerase Chain Reaction–Based Test for the Detection of Female Urogenital Chlamydial Infections
Objective: The purpose of this study was to evaluate the Amplicor
Chlamydia trachomatis Test (Roche Molecular Systems, Branchburg, NJ),
a polymerase chain reaction (PCR)-based technique, as a screening test for the detection
of female urogenital C. trachomatis infections, comparing it to an enzyme
immunoassay method
Dynamics of a Josephson Array in a Resonant Cavity
We derive dynamical equations for a Josephson array coupled to a resonant
cavity by applying the Heisenberg equations of motion to a model Hamiltonian
described by us earlier [Phys. Rev. B {\bf 63}, 144522 (2001); Phys. Rev. B
{\bf 64}, 179902 (E)]. By means of a canonical transformation, we also show
that, in the absence of an applied current and dissipation, our model reduces
to one described by Shnirman {\it et al} [Phys. Rev. Lett. {\bf 79}, 2371
(1997)] for coupled qubits, and that it corresponds to a capacitive coupling
between the array and the cavity mode. From extensive numerical solutions of
the model in one dimension, we find that the array locks into a coherent,
periodic state above a critical number of active junctions, that the
current-voltage characteristics of the array have self-induced resonant steps
(SIRS's), that when active junctions are synchronized on a SIRS, the
energy emitted into the resonant cavity is quadratic in , and that when a
fixed number of junctions is biased on a SIRS, the energy is linear in the
input power. All these results are in agreement with recent experiments. By
choosing the initial conditions carefully, we can drive the array into any of a
variety of different integer SIRS's. We tentatively identify terms in the
equations of motion which give rise to both the SIRS's and the coherence
threshold. We also find higher-order integer SIRS's and fractional SIRS's in
some simulations. We conclude that a resonant cavity can produce threshold
behavior and SIRS's even in a one-dimensional array with appropriate
experimental parameters, and that the experimental data, including the coherent
emission, can be understood from classical equations of motion.Comment: 15 pages, 10 eps figures, submitted to Phys. Rev.
Evaluation of “Dream Herb,” Calea zacatechichi
A recent surge in the use of dietary supplements, including herbal remedies, necessitates investigations into their safety profiles. “Dream herb,” Calea zacatechichi, has long been used in traditional folk medicine for a variety of purposes and is currently being marketed in the US for medicinal purposes, including diabetes treatment. Despite the inherent vulnerability of the renal system to xenobiotic toxicity, there is a lack of safety studies on the nephrotoxic potential of this herb. Additionally, the high frequency of diabetes-associated kidney disease makes safety screening of C. zacatechichi for safety especially important. We exposed human proximal tubule HK-2 cells to increasing doses of this herb alongside known toxicant and protectant control compounds to examine potential toxicity effects of C. zacatechichi relative to control compounds. We evaluated both cellular and mitochondrial functional changes related to toxicity of this dietary supplement and found that even at low doses evidence of cellular toxicity was significant. Moreover, these findings correlated with significantly elevated levels of nephrotoxicity biomarkers, lending further support for the need to further scrutinize the safety of this herbal dietary supplement
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