5,567 research outputs found
Nonlinear dielectric effect of dipolar fluids
The nonlinear dielectric effect for dipolar fluids is studied within the
framework of the mean spherical approximation (MSA) of hard core dipolar Yukawa
fluids. Based on earlier results for the electric field dependence of the
polarization our analytical results show so-called normal saturation effects
which are in good agreement with corresponding NVT ensemble Monte Carlo
simulation data. The linear and the nonlinear dielectric permittivities
obtained from MC simulations are determined from the fluctuations of the total
dipole moment of the system in the absence of an applied electric field. We
compare the MSA based theoretical results with the corresponding Langevin and
Debye-Weiss behaviors.Comment: 10 pages including 4 figure
Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS
We derive the physical properties at the base of the jet from DG Tau both
along and across the flow and as a function of velocity. We analysed seven
optical spectra of the DG Tau jet, taken with the Hubble Space Telescope
Imaging Spectrograph. The spectra were obtained by placing a long-slit parallel
to the jet axis and stepping it across the jet width. The resulting
position-velocity diagrams in optical forbidden emission lines allowed access
to plasma conditions via calculation of emission line ratios.
We find at the base of the jet high electron density, 10, and
very low ionisation, , which combine to give a total
density up to 3 10. This analysis confirms previous reports of
variations in plasma parameters along the jet, (i.e. decrease in density by
several orders of magnitude, increase of from 0.05 to a plateau at 0.7
downstream at 2 from the star). Furthermore, a spatial coincidence is
revealed between sharp gradients in the total density and supersonic velocity
jumps. This strongly suggests that the emission is caused by shock excitation.
The position-velocity diagrams indicate the presence of both fast accelerating
gas and slower, less collimated material. We derive the mass outflow rate,
, in the blue-shifted lobe in different velocity channels, that
contribute to a total of 8 4 10 M
yr. We estimate that a symmetric bipolar jet would transport at the low
and intermediate velocities probed by rotation measurements, an angular
momentum flux of 2.9 1.5 10 M yr
AU km s.
The derived properties of the DG Tau jet are demonstrated to be consistent
with magneto-centrifugal theory. However, non-stationary modelling is required
in order to explain all of the features revealed at high resolution.Comment: 16 pages, 18 figure
On hypergeometric series reductions from integral representations, the Kampe de Feriet function, and elsewhere
Single variable hypergeometric functions pFq arise in connection with the
power series solution of the Schrodinger equation or in the summation of
perturbation expansions in quantum mechanics. For these applications, it is of
interest to obtain analytic expressions, and we present the reduction of a
number of cases of pFp and p+1F_p, mainly for p=2 and p=3. These and related
series have additional applications in quantum and statistical physics and
chemistry.Comment: 17 pages, no figure
The Quantum Mechanics of Hyperion
This paper is motivated by the suggestion [W. Zurek, Physica Scripta, T76,
186 (1998)] that the chaotic tumbling of the satellite Hyperion would become
non-classical within 20 years, but for the effects of environmental
decoherence. The dynamics of quantum and classical probability distributions
are compared for a satellite rotating perpendicular to its orbital plane,
driven by the gravitational gradient. The model is studied with and without
environmental decoherence. Without decoherence, the maximum quantum-classical
(QC) differences in its average angular momentum scale as hbar^{2/3} for
chaotic states, and as hbar^2 for non-chaotic states, leading to negligible QC
differences for a macroscopic object like Hyperion. The quantum probability
distributions do not approach their classical limit smoothly, having an
extremely fine oscillatory structure superimposed on the smooth classical
background. For a macroscopic object, this oscillatory structure is too fine to
be resolved by any realistic measurement. Either a small amount of smoothing
(due to the finite resolution of the apparatus) or a very small amount of
environmental decoherence is sufficient ensure the classical limit. Under
decoherence, the QC differences in the probability distributions scale as
(hbar^2/D)^{1/6}, where D is the momentum diffusion parameter. We conclude that
decoherence is not essential to explain the classical behavior of macroscopic
bodies.Comment: 17 pages, 24 figure
Anomalous transport in biased ac-driven Josephson junctions: Negative conductances
We investigate classical anomalous electrical transport in a driven,
resistively and capacitively shunted Josephson junction device. Novel transport
phenomena are identified in chaotic regimes when the junction is subjected to
both, a time periodic (ac) and a constant, biasing (dc) current. The dependence
of the voltage across the junction on the dc-current exhibits a rich diversity
of anomalous transport characteristics: In particular, depending on the chosen
parameter regime we can identify so termed absolute negative conductance around
zero dc-bias, the occurrence of negative differential conductance and, after
crossing a zero conductance, the emergence of a negative nonlinear conductance
in the non-equilibrium response regime remote from zero dc-bias.Comment: 7 pages, 5 figure
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Effects of a Magazineâs Move to Online-only: Post-print Audience Attention and Readership Retention Revisited
For financial reasons, newspapers and magazines are increasingly going online-only. By doing so, some have returned to profitability, but with what consequences for their audiences? To expand the scant evidence base, we conducted a case study of the UKâs New Musical Express (NME) magazine. By analyzing quantitative audience data from official industry sources, we estimate total time spent with the NME by its British audience fell dramatically post-printâby 72%. This fall mirrors that suffered by The Independent newspaper, which went online-only two years earlier. We also report that the NMEâs official net weekly and monthly readership increased post-print, although these results are difficult to compare with The Independentâs because the two titles differed in their print publication frequencies. We conclude that the attention periodicals attract via their print editions is unlikely to immediately transfer to their online editions should they go online-only. Building a fuller theory of print platform cessation, howeverâone that also encompasses changes in readership/reachârequires more comparable data. This case study provides further evidence to suggest that though, for newspapers and magazines, a post-print existence may be less costly, it is also more constrained, with much of the attention they formerly enjoyed simply stripped away
Accurate Results from Perturbation Theory for Strongly Frustrated Heisenberg Spin Clusters
We investigate the use of perturbation theory in finite sized frustrated spin
systems by calculating the effect of quantum fluctuations on coherent states
derived from the classical ground state. We first calculate the ground and
first excited state wavefunctions as a function of applied field for a 12-site
system and compare with the results of exact diagonalization. We then apply the
technique to a 20-site system with the same three fold site coordination as the
12-site system. Frustration results in asymptotically convergent series for
both systems which are summed with Pad\'e approximants.
We find that at zero magnetic field the different connectivity of the two
systems leads to a triplet first excited state in the 12-site system and a
singlet first excited state in the 20-site system, while the ground state is a
singlet for both. We also show how the analytic structure of the Pad\'e
approximants at evolves in the complex plane at
the values of the applied field where the ground state switches between spin
sectors and how this is connected with the non-trivial dependence of the
number on the strength of quantum fluctuations. We discuss the origin
of this difference in the energy spectra and in the analytic structures. We
also characterize the ground and first excited states according to the values
of the various spin correlation functions.Comment: Final version, accepted for publication in Physical review
Trends in stratospheric minor constituents
Photochemical models predict that increasing source gas concentrations are also expected to lead to changes in the concentrations of both catalytically active radical species (such as NO2, ClO, and OH) and inactive reservoir species (such as HNO3, HCl, and H2O). For simplicity, we will refer to all these as trace species. Those species that are expected to have increasing concentration levels are investigated. Additionally, the trace species concentration levels are monitored for unexpected changes on the basis of the measure increase in source gases. Carrying out these investigations is difficult due to the limited data base of measurements of stratospheric trace species. In situ measurements are made only infrequently, and there are few satelliteborne measurements, most over a time space insufficient for trend determination. Instead, ground-based measurements of column content must be used for many species, and interpretation is complicated by contributions from the troposphere or mesosphere or both. In this chapter, we examine existing measurements as published or tabulated
The circumstellar environment of HD50138 revealed by VLTI/AMBER at high angular resolution
HD50138 is a Herbig B[e] star with a circumstellar disc detected at IR and mm
wavelength. Its brightness makes it a good candidate for NIR interferometry
observations. We aim to resolve, spatially and spectrally, the continuum and
hydrogen emission lines in the 2.12-2.47 micron region, to shed light on the
immediate circumstellar environment of the star. VLTI/AMBER K-band observations
provide spectra, visibilities, differential phases, and closure phases along
three long baselines for the continuum, and HI emission in Br and five
high-n Pfund lines. By computing the pure-line visibilities, we derive the
angular size of the different line-emitting regions. A simple LTE model was
created to constrain the physical conditions of HI emitting region. The
continuum region cannot be reproduced by a geometrical 2D elongated Gaussian
fitting model. We estimate the size of the region to be 1 au. We find the
Br and Pfund lines come from a more compact region of size 0.4 au. The
Br line exhibits an S-shaped differential phase, indicative of
rotation. The continuum and Br line closure phase show offsets of
-255 and 2010, respectively. This is evidence of an
asymmetry in their origin, but with opposing directions. We find that we cannot
converge on constraints for the HI physical parameters without a more detailed
model. Our analysis reveals that HD50138 hosts a complex circumstellar
environment. Its continuum emission cannot be reproduced by a simple disc
brightness distribution. Similarly, several components must be evoked to
reproduce the interferometric observables within the Br, line.
Combining the spectroscopic and interferometric data of the Br and
Pfund lines favours an origin in a wind region with a large opening angle.
Finally, our results point to an evolved source.Comment: accepted for publication in A&
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