5,823 research outputs found
Localization and conductance fluctuations in the integer quantum Hall effect: Real--space renormalization group approach
We consider the network model of the integer quantum Hall effect transition.
By generalizing the real--space renormalization group procedure for the
classical percolation to the case of quantum percolation, we derive a closed
renormalization group (RG) equation for the universal distribution of
conductance of the quantum Hall sample at the transition. We find an
approximate solution of the RG equation and use it to calculate the critical
exponent of the localization length and the central moments of the conductance
distribution. The results obtained are compared with the results of recent
numerical simulations.Comment: 17 pages, RevTex, 7 figure
Magnetic Field Structure of the Large Magellanic Cloud from Faraday Rotation Measures of Diffuse Polarized Emission
We present a study of the magnetic field of the Large Magellanic Cloud (LMC),
carried out using diffuse polarized synchrotron emission data at 1.4 GHz
acquired at the Parkes Radio Telescope and the Australia Telescope Compact
Array. The observed diffuse polarized emission is likely to originate above the
LMC disk on the near side of the galaxy. Consistent negative rotation measures
(RMs) derived from the diffuse emission indicate that the line-of-sight
magnetic field in the LMC's near-side halo is directed coherently away from us.
In combination with RMs of extragalactic sources that lie behind the galaxy, we
show that the LMC's large scale magnetic field is likely to be of quadrupolar
geometry, consistent with the prediction of dynamo theory. On smaller scales,
we identify two brightly polarized filaments southeast of the LMC, associated
with neutral hydrogen arms. The filaments' magnetic field potentially aligns
with the direction towards the Small Magellanic Cloud. We suggest that tidal
interactions between the Small and the Large Magellanic Clouds in the past 10^9
years is likely to have shaped the magnetic field in these filaments.Comment: 42 pages, 22 figures, 2 tables. Accepted for publication in ApJ.
Electronic version of Table 2 is available via email from the first autho
Recovery of the Historical SN1957D in X-rays with Chandra
SN1957D, located in one of the spiral arms of M83, is one of the small number
of extragalactic supernovae that has remained detectable at radio and optical
wavelengths during the decades after its explosion. Here we report the first
detection of SN1957D in X-rays, as part of a 729 ks observation of M83 with
\chandra. The X-ray luminosity (0.3 - 8 keV) is 1.7 (+2.4,-0.3) 10**37 ergs/s.
The spectrum is hard and highly self-absorbed compared to most sources in M83
and to other young supernova remnants, suggesting that the system is dominated
at X-ray wavelengths by an energetic pulsar and its pulsar wind nebula. The
high column density may be due to absorption within the SN ejecta. HST WFC3
images resolve the supernova remnant from the surrounding emission and the
local star field. Photometry of stars around SN1957D, using WFC3 images,
indicates an age of less than 10**7 years and a main sequence turnoff mass more
than 17 solar masses. New spectra obtained with Gemini-South show that the
optical spectrum continues to be dominated by broad [O III] emission lines, the
signature of fast-moving SN ejecta. The width of the broad lines has remained
about 2700 km/s (FWHM). The [O III] flux dropped precipitously between 1989 and
1991, but continued monitoring shows the flux has been almost constant since.
In contrast, radio observations over the period 1990-2011 show a decline rate
inf the flux proportional to t**-4, far steeper than the rate observed earlier,
suggesting that the primary shock has overrun the edge of a pre-SN wind.Comment: 28 pages, including 3 tables and 7 figures, accepted for publication
in Ap
Vegetation response to invasive Tamarix control in southwestern U.S. rivers: a collaborative study including 416 sites
Most studies assessing vegetation response following control of invasive Tamarix trees along southwestern U.S. rivers have been small in scale (e.g., river reach), or at a regional scale but with poor spatial-temporal replication, and most have not included testing the effects of a now widely used biological control. We monitored plant composition following Tamarix control along hydrologic, soil, and climatic gradients in 244 treated and 172 reference sites across six U.S. states. This represents the largest comprehensive assessment to date on the vegetation response to the four most common Tamarix control treatments. Biocontrol by a defoliating beetle (treatment 1) reduced the abundance of Tamarix less than active removal by mechanically using hand and chain-saws (2), heavy machinery (3) or burning (4). Tamarix abundance also decreased with lower temperatures, higher precipitation, and follow-up treatments for Tamarix resprouting. Native cover generally increased over time in active Tamarix removal sites, however, the increases observed were small and was not consistently increased by active revegetation. Overall, native cover was correlated to permanent stream flow, lower grazing pressure, lower soil salinity and temperatures, and higher precipitation. Species diversity also increased where Tamarix was removed. However, Tamarix treatments, especially those generating the highest disturbance (burning and heavy machinery), also often promoted secondary invasions of exotic forbs. The abundance of hydrophytic species was much lower in treated than in reference sites, suggesting that management of southwestern U.S. rivers has focused too much on weed control, overlooking restoration of fluvial processes that provide habitat for hydrophytic and floodplain vegetation. These results can help inform future management of Tamarix-infested rivers to restore hydrogeomorphic processes, increase native biodiversity and reduce abundance of noxious species
Exceptionally Slow Rise in Differential Reflectivity Spectra of Excitons in GaN: Effect of Excitation-induced Dephasing
Femtosecond pump-probe (PP) differential reflectivity spectroscopy (DRS) and
four-wave mixing (FWM) experiments were performed simultaneously to study the
initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats
between the A-B excitons were found \textit{only for positive time delay} in
both PP and FWM experiments. The rise time at negative time delay for the
differential reflection spectra was much slower than the FWM signal or PP
differential transmission spectroscopy (DTS) at the exciton resonance. A
numerical solution of a six band semiconductor Bloch equation model including
nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS
results from excitation induced dephasing (EID), that is, the strong density
dependence of the dephasing time which changes with the laser excitation
energy.Comment: 8 figure
Impact of Portion-Size Control for School a la Carte Items: Changes in Kilocalories and Macronutrients Purchased by Middle School Students
We assessed the impact of a pilot middle school a la carte intervention on food and beverage purchases, kilocalories, fat, carbohydrate, and protein sold per student, and nutrient density of the foods sold. A la carte sales were obtained from six middle schools in three states for 1 baseline week and daily during the 6-week intervention. Intervention goals included reducing sizes of sweetened beverages and chips, and increasing the availability of water and reduced-fat/baked chips. Nutrients sold per day were computed and weekly nutrient means per student and per number of items sold were calculated and compared between baseline and week 6. Five schools achieved all goals at 6 weeks. Four schools showed increases in the percentage of kilocalories from protein and decreases in the amount of sweetened beverages sold; five showed substantial increases in water sales. Changes in regular chips varied by school. There were significant changes in energy density of foods sold. School foodservice changes in middle school snack bar/a la carte lines can be implemented and can lead to a reduction in the caloric density of foods purchased
Global Dynamical Evolution of the ISM in Star Forming Galaxies - I. High Resolution 3D HD and MHD Simulations: Effect of the Magnetic Field
In star forming disk galaxies, matter circulation between stars and the
interstellar gas, and, in particular the energy input by random and clustered
supernova explosions, determine the dynamical and chemical evolution of the
ISM, and hence of the galaxy as a whole. Using a 3D MHD code with adaptive mesh
refinement developed for this purpose, we have investigated the r\^ole of
magnetized matter circulation between the gaseous disk and the surrounding
galactic halo. Special emphasis has been put on the effect of the magnetic
field with respect to the volume and mass fractions of the different ISM
``phases'', the relative importance of ram, thermal and magnetic pressures, and
whether the field can prevent matter transport from the disk into the halo. The
simulations were performed on a grid with an area of 1 kpc, centered on
the solar circle, extending kpc perpendicular to the galactic disk
with a resolution as high as 1.25 pc. The simulations were run for a time scale
of 400 Myr, sufficiently long to avoid memory effects of the initial setup, and
to allow for a global dynamical equilibrium to be reached in case of a constant
energy input rate. (...) We find that in general gas transport into the halo in
3D is not prevented by an initial disk parallel magnetic field, but only
delayed initially, for as long as it is needed to punch holes into the thick
magnetized gas disk. The mean volume filling factor of the hot phase in the
disk is similar in HD and MHD (the latter with a total field strength of 4.4
G) runs, amounting to for the Galactic supernova rate.Comment: Paper accepted for publication in A&A (31 Jan); 17 pages with 10
Figures including 6 images and 16 plots (all in low quality jpeg files). For
better resolution images contact the author
The global dust SED: Tracing the nature and evolution of dust with DustEM
The Planck and Herschel missions are currently measuring the farIR-mm
emission of dust, which combined with existing IR data, will for the first time
provide the full SED of the galactic ISM dust emission with an unprecedented
sensitivity and angular resolution. It will allow a systematic study of the
dust evolution processes that affect the SED. Here we present a versatile
numerical tool, DustEM, that predicts the emission and extinction of dust given
their size distribution and their optical and thermal properties. In order to
model dust evolution, DustEM has been designed to deal with a variety of grain
types, structures and size distributions and to be able to easily include new
dust physics. We use DustEM to model the dust SED and extinction in the diffuse
interstellar medium at high-galactic latitude (DHGL), a natural reference SED.
We present a coherent set of observations for the DHGL SED. The dust components
in our DHGL model are (i) PAHs, (ii) amorphous carbon and (iii) amorphous
silicates. We use amorphous carbon dust, rather than graphite, because it
better explains the observed high abundances of gas-phase carbon in shocked
regions of the interstellar medium. Using the DustEM model, we illustrate how,
in the optically thin limit, the IRAS/Planck HFI (and likewise Spitzer/Herschel
for smaller spatial scales) photometric band ratios of the dust SED can
disentangle the influence of the exciting radiation field intensity and
constrain the abundance of small grains relative to the larger grains. We also
discuss the contributions of the different grain populations to the IRAS,
Planck and Herschel channels. Such information is required to enable a study of
the evolution of dust as well as to systematically extract the dust thermal
emission from CMB data and to analyze the emission in the Planck polarized
channels. The DustEM code described in this paper is publically available.Comment: accepted for publication in A&
- …