12,024 research outputs found
Development of acoustically lined ejector technology for multitube jet noise suppressor nozzles by model and engine tests over a wide range of jet pressure ratios and temperatures
An experimental program comprising model nozzle and full-scale engine tests was undertaken to acquire parametric data for acoustically lined ejectors applied to primary jet noise suppression. Ejector lining design technology and acoustical scaling of lined ejector configurations were the major objectives. Ground static tests were run with a J-75 turbojet engine fitted with a 37-tube, area ratio 3.3 suppressor nozzle and two lengths of ejector shroud (L/D = 1 and 2). Seven ejector lining configurations were tested over the engine pressure ratio range of 1.40 to 2.40 with corresponding jet velocities between 305 and 610 M/sec. One-fourth scale model nozzles were tested over a pressure ratio range of 1.40 to 4.0 with jet total temperatures between ambient and 1088 K. Scaling of multielement nozzle ejector configurations was also studied using a single element of the nozzle array with identical ejector lengths and lining materials. Acoustic far field and near field data together with nozzle thrust performance and jet aerodynamic flow profiles are presented
Coherent versus Incoherent Light Scattering from a Quantum Dot
We analyze the light scattered by a single InAs quantum dot interacting with
a resonant continuous-wave laser. High resolution spectra reveal clear
distinctions between coherent and incoherent scattering, with the laser
intensity spanning over four orders of magnitude. We find that the fraction of
coherently scattered photons can approach unity under sufficiently weak or
detuned excitation, ruling out pure dephasing as a relevant decoherence
mechanism. We show how spectral diffusion shapes spectra, correlation
functions, and phase-coherence, concealing the ideal radiatively-broadened
two-level system described by Mollow.Comment: to appear in PRB 85, 23531
Revealing natural relationships among arbuscular mycorrhizal fungi: culture line BEG47 represents Diversispora epigaea, not Glomus versiforme
Background: Understanding the mechanisms underlying biological phenomena, such as evolutionarily conservative trait inheritance, is predicated on knowledge of the natural relationships among organisms. However, despite their enormous ecological significance, many of the ubiquitous soil inhabiting and plant symbiotic arbuscular mycorrhizal fungi (AMF, phylum Glomeromycota) are incorrectly classified.
Methodology/Principal Findings:
Here, we focused on a frequently used model AMF registered as culture BEG47. This fungus is a descendent of the ex-type culture-lineage of Glomus epigaeum, which in 1983 was synonymised with Glomus versiforme. It has since then been used as ‘G. versiforme BEG47’. We show by morphological comparisons, based on type material, collected 1860–61, of G. versiforme and on type material and living ex-type cultures of G. epigaeum, that these two AMF species cannot be conspecific, and by molecular phylogenetics that BEG47 is a member of the genus Diversispora.
Conclusions: This study highlights that experimental works published during the last >25 years on an AMF named ‘G. versiforme’ or ‘BEG47’ refer to D. epigaea, a species that is actually evolutionarily separated by hundreds of millions of years from all members of the genera in the Glomerales and thus from most other commonly used AMF ‘laboratory strains’. Detailed redescriptions substantiate the renaming of G. epigaeum (BEG47) as D. epigaea, positioning it systematically in the order Diversisporales, thus enabling an evolutionary understanding of genetical, physiological, and ecological traits, relative to those of other AMF. Diversispora epigaea is widely cultured as a laboratory strain of AMF, whereas G. versiforme appears not to have been cultured nor found in the field since its original description
Asymmetric magnetic interference patterns in 0-pi Josephson junctions
We examine the magnetic interference patterns of Josephson junctions with a
region of 0- and of pi-phase shift. Such junctions have recently been realized
as c-axis YBCO-Pb junctions with a single twin boundary in YBCO. We show that
in general the junction generates self-fields which introduces an asymmetry in
the critical current under reversal of the magnetic field. Numerical
calculations of these asymmetries indicate they account well for the
unexplained features observed in single twin boundary junctions.Comment: 4 pages, 3 figure
REAM intensity modulator-enabled 10Gb/s colorless upstream transmission of real-time optical OFDM signals in a single-fiber-based bidirectional PON architecture
Reflective electro-absorption modulation-intensity modulators (REAM-IMs) are utilized, for the first time, to experimentally demonstrate colorless ONUs in single-fiber-based, bidirectional, intensity-modulation and direct-detection (IMDD), optical OFDM PONs (OOFDM-PONs) incorporating 25km SSMFs and OLT-side-seeded CW optical signals. The colorlessness of the REAM-IMs is characterized, based on which optimum REAM-IM operating conditions are identified. In the aforementioned PON architecture, 10Gb/s colorless upstream transmissions of end-to-end realtime OOFDM signals are successfully achieved for various wavelengths within the entire C-band. Over such a wavelength window, corresponding minimum received optical powers at the FEC limit vary in a range as small as <0.5dB. In addition, experimental measurements also indicate that Rayleigh backscattering imposes a 2.8dB optical power penalty on the 10Gb/s over 25km upstream OOFDM signal transmission. Furthermore, making use of on-line adaptive bit and power loading, a linear trade-off between aggregated signal line rate and optical power budget is observed, which shows that, for the present PON system, a 10% reduction in signal line rate can improve the optical power budget by 2.6dB. © 2012 Optical Society of America
The Spectral Energy Distribution of Powerful Starburst Galaxies I : Modelling the Radio Continuum
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We have acquired radio-continuum data between 70MHz and 48 GHz for a sample of 19 southern starburst galaxies at moderate redshifts (0.067 < z < 0.227) with the aim of separating synchrotron and free-free emission components. Using a Bayesian framework, we find the radio continuum is rarely characterized well by a single power law, instead often exhibiting lowfrequency turnovers below 500 MHz, steepening at mid to high frequencies, and a flattening at high frequencies where free-free emission begins to dominate over the synchrotron emission. These higher order curvature components may be attributed to free-free absorption across multiple regions of star formation with varying optical depths. The decomposed synchrotron and free-free emission components in our sample of galaxies form strong correlations with the total-infrared bolometric luminosities. Finally, we find that without accounting for free-free absorption with turnovers between 90 and 500MHz the radio continuum at low frequency (v < 200 MHz) could be overestimated by upwards of a factor of 12 if a simple power-law extrapolation is used from higher frequencies. The mean synchrotron spectral index of our sample is constrained to be α = -1.06, which is steeper than the canonical value of -0.8 for normal galaxies. We suggest this may be caused by an intrinsically steeper cosmic ray distribution.Peer reviewe
'The Brick' is not a brick : A comprehensive study of the structure and dynamics of the Central Molecular Zone cloud G0.253+0.016
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.In this paper we provide a comprehensive description of the internal dynamics of G0.253+0.016 (a.k.a. 'the Brick'); one of the most massive and dense molecular clouds in the Galaxy to lack signatures of widespread star formation. As a potential host to a future generation of high-mass stars, understanding largely quiescent molecular clouds like G0.253+0.016 is of critical importance. In this paper, we reanalyse Atacama Large Millimeter Array cycle 0 HNCO data at 3 mm, using two new pieces of software which we make available to the community. First, scousepy, a Python implementation of the spectral line fitting algorithm scouse. Secondly, acorns (Agglomerative Clustering for ORganising Nested Structures), a hierarchical n-dimensional clustering algorithm designed for use with discrete spectroscopic data. Together, these tools provide an unbiased measurement of the line of sight velocity dispersion in this cloud, kms, which is somewhat larger than predicted by velocity dispersion-size relations for the Central Molecular Zone (CMZ). The dispersion of centroid velocities in the plane of the sky are comparable, yielding . This isotropy may indicate that the line-of-sight extent of the cloud is approximately equivalent to that in the plane of the sky. Combining our kinematic decomposition with radiative transfer modelling we conclude that G0.253+0.016 is not a single, coherent, and centrally-condensed molecular cloud; 'the Brick' is not a \emph{brick}. Instead, G0.253+0.016 is a dynamically complex and hierarchically-structured molecular cloud whose morphology is consistent with the influence of the orbital dynamics and shear in the CMZ.Peer reviewedFinal Accepted Versio
Murchison Widefield Array and XMM-Newton observations of the Galactic supernova remnant G5.9+3.1
In this paper we discuss the radio continuum and X-ray properties of the
so-far poorly studied Galactic supernova remnant (SNR) G5.9+3.1. We present the
radio spectral energy distribution (SED) of the Galactic SNR G5.9+3.1 obtained
with the Murchison Widefield Array (MWA). Combining these new observations with
the surveys at other radio continuum frequencies, we discuss the integrated
radio continuum spectrum of this particular remnant. We have also analyzed an
archival XMM-Newton observation, which represents the first detection of X-ray
emission from this remnant. The SNR SED is very well explained by a simple
power-law relation. The synchrotron radio spectral index of G5.9+3.1, is
estimated to be 0.420.03 and the integrated flux density at 1GHz to be
around 2.7Jy. Furthermore, we propose that the identified point radio source,
located centrally inside the SNR shell, is most probably a compact remnant of
the supernova explosion. The shell-like X-ray morphology of G5.9+3.1 as
revealed by XMM-Newton broadly matches the spatial distribution of the radio
emission, where the radio-bright eastern and western rims are also readily
detected in the X-ray while the radio-weak northern and southern rims are weak
or absent in the X-ray. Extracted MOS1+MOS2+PN spectra from the whole SNR as
well as the north, east, and west rims of the SNR are fit successfully with an
optically thin thermal plasma model in collisional ionization equilibrium with
a column density N_H~0.80x cm and fitted temperatures spanning
the range kT~0.14-0.23keV for all of the regions. The derived electron number
densities n_e for the whole SNR and the rims are also roughly comparable
(ranging from ~ cm to ~ cm, where f
is the volume filling factor). We also estimate the swept-up mass of the X-ray
emitting plasma associated with G5.9+3.1 to be ~.Comment: Accepted for publication in A&
Sterile Neutrinos as Dark Matter
The simplest model that can accomodate a viable nonbaryonic dark matter
candidate is the standard electroweak theory with the addition of right-handed
or sterile neutrinos. We reexamine this model and find that the sterile
neutrinos can be either hot, warm, or cold dark matter. Since their only direct
coupling is to left-handed or active neutrinos, the most efficient production
mechanism is via neutrino oscillations. If the production rate is always less
than the expansion rate, then these neutrinos will never be in thermal
equilibrium. However, enough of them may be produced so that they provide the
missing mass necessary for closure. We consider a single generation of neutrino
fields with a Dirac mass, , and a Majorana
mass for the right-handed components only, . For we show that the
number density of sterile neutrinos is proportional to so that the
energy density today is {\it independent of} . However is crucial in
determining the large scale structure of the Universe. In particular, leads to warm dark matter and a structure formation
scenario that may have some advantages over both the standard hot and cold dark
matter scenarios.Comment: 10 pages (1 figure available upon request) phyzzx,
FERMILAB-Pub-93/057-
Rotation profiles of solar-like stars with magnetic fields
The aim of this work is to investigate rotation profile of solar-like stars
with magnetic fields. A diffusion coefficient of magnetic angular momentum
transport is deduced. Rotating stellar models with different mass are computed
under the effect of the coefficient. Then rotation profiles are obtained from
the theoretical stellar models. The total angular momentum of solar model with
only hydrodynamic instabilities is about 13 times larger than that of the Sun
at the age of the Sun, and this model can not reproduce quasi-solid rotation in
the radiative region. However, not only can the solar model with magnetic
fields reproduce an almost uniform rotation in the radiative region, but its
total angular momentum is consistent with helioseismic result at the level of 3
at the age of the Sun. The rotation of solar-like stars with magnetic
fields is almost uniform in the radiative region. But there is an obvious
transition region of angular velocity between the convective core and the
radiative region of models with 1.2 - 1.5 , where angular velocity
has a sharp radial change, which is different from the rotation profile of the
Sun and massive stars with magnetic fields. Moreover the changes of the angular
velocity in the transition region increase with the increasing in the age and
mass.Comment: Accepted for publication in ChjA
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