4,335 research outputs found
Radiation Hydrodynamical Instabilities in Cosmological and Galactic Ionization Fronts
Ionization fronts, the sharp radiation fronts behind which H/He ionizing
photons from massive stars and galaxies propagate through space, were
ubiquitous in the universe from its earliest times. The cosmic dark ages ended
with the formation of the first primeval stars and galaxies a few hundred Myr
after the Big Bang. Numerical simulations suggest that stars in this era were
very massive, 25 - 500 solar masses, with H II regions of up to 30,000
light-years in diameter. We present three-dimensional radiation hydrodynamical
calculations that reveal that the I-fronts of the first stars and galaxies were
prone to violent instabilities, enhancing the escape of UV photons into the
early intergalactic medium (IGM) and forming clumpy media in which supernovae
later exploded. The enrichment of such clumps with metals by the first
supernovae may have led to the prompt formation of a second generation of
low-mass stars, profoundly transforming the nature of the first protogalaxies.
Cosmological radiation hydrodynamics is unique because ionizing photons coupled
strongly to both gas flows and primordial chemistry at early epochs,
introducing a hierarchy of disparate characteristic timescales whose relative
magnitudes can vary greatly throughout a given calculation. We describe the
adaptive multistep integration scheme we have developed for the self-consistent
transport of both cosmological and galactic ionization fronts.Comment: 6 pages, 4 figures, accepted for proceedings of HEDLA2010, Caltech,
March 15 - 18, 201
Toward the assessment of the susceptibility of a digital system to lightning upset
Accomplishments and directions for further research aimed at developing methods for assessing a candidate design of an avionic computer with respect to susceptability to lightning upset are reported. Emphasis is on fault tolerant computers. Both lightning stress and shielding are covered in a review of the electromagnetic environment. Stress characterization, system characterization, upset detection, and positive and negative design features are considered. A first cut theory of comparing candidate designs is presented including tests of comparative susceptability as well as its analysis and simulation. An approach to lightning induced transient fault effects is included
Theory of excitation of Rydberg polarons in an atomic quantum gas
We present a quantum many-body description of the excitation spectrum of
Rydberg polarons in a Bose gas. The many-body Hamiltonian is solved with
functional determinant theory, and we extend this technique to describe Rydberg
polarons of finite mass. Mean-field and classical descriptions of the spectrum
are derived as approximations of the many-body theory. The various approaches
are applied to experimental observations of polarons created by excitation of
Rydberg atoms in a strontium Bose-Einstein condensate.Comment: 14 pages, 9 figures. arXiv admin note: substantial text overlap with
arXiv:1706.0371
Creation of Rydberg Polarons in a Bose Gas
We report spectroscopic observation of Rydberg polarons in an atomic Bose
gas. Polarons are created by excitation of Rydberg atoms as impurities in a
strontium Bose-Einstein condensate. They are distinguished from previously
studied polarons by macroscopic occupation of bound molecular states that arise
from scattering of the weakly bound Rydberg electron from ground-state atoms.
The absence of a -wave resonance in the low-energy electron-atom scattering
in Sr introduces a universal behavior in the Rydberg spectral lineshape and in
scaling of the spectral width (narrowing) with the Rydberg principal quantum
number, . Spectral features are described with a functional determinant
approach (FDA) that solves an extended Fr\"{o}hlich Hamiltonian for a mobile
impurity in a Bose gas. Excited states of polyatomic Rydberg molecules
(trimers, tetrameters, and pentamers) are experimentally resolved and
accurately reproduced with FDA.Comment: 5 pages, 3 figure
The Red Sea, Coastal Landscapes, and Hominin Dispersals
This chapter provides a critical assessment of environment, landscape and resources in the Red Sea region over the past five million years in relation to archaeological evidence of hominin settlement, and of current hypotheses about the role of the region as a pathway or obstacle to population dispersals between Africa and Asia and the possible significance of coastal colonization. The discussion assesses the impact of factors such as topography and the distribution of resources on land and on the seacoast, taking account of geographical variation and changes in geology, sea levels and palaeoclimate. The merits of northern and southern routes of movement at either end of the Red Sea are compared. All the evidence indicates that there has been no land connection at the southern end since the beginning of the Pliocene period, but that short sea crossings would have been possible at lowest sea-level stands with little or no technical aids. More important than the possibilities of crossing the southern channel is the nature of the resources available in the adjacent coastal zones. There were many climatic episodes wetter than today, and during these periods water draining from the Arabian escarpment provided productive conditions for large mammals and human populations in coastal regions and eastwards into the desert. During drier episodes the coastal region would have provided important refugia both in upland areas and on the emerged shelves exposed by lowered sea level, especially in the southern sector and on both sides of the Red Sea. Marine resources may have offered an added advantage in coastal areas, but evidence for their exploitation is very limited, and their role has been over-exaggerated in hypotheses of coastal colonization
Blazar nuclei in radio-loud narrow-line Seyfert 1?
It has been suggested that some radio-loud narrow-line Seyfert 1 contain
relativistic jets, on the basis of their flat-spectrum radio nuclei and studies
on variability. We present preliminary results of an ongoing investigation of
the X-ray and multiwavelength properties of 5 radio-loud NLS1 based on archival
data from Swift and XMM-Newton. Some sources present interesting
characteristics, very uncharacteristic for a radio-quiet narrow-line Seyfert 1,
such as very hard X-ray spectra, and correlated optical and ultraviolet
variability. However, none of the studied sources show conclusive evidence for
relativistic jets. gamma-ray observations with Fermi are strongly recommended
to definitely decide on the presence or not of relativistic jets.Comment: 9 pages, 4 figures. Talk presented at the 37th COSPAR Assembly
(Montreal, Canada, July 13-20, 2008), Session E17. Accepted for publication
on Advances in Space Researc
Using biomarkers to predict TB treatment duration (Predict TB): a prospective, randomized, noninferiority, treatment shortening clinical trial
Background : By the early 1980s, tuberculosis treatment was shortened from 24 to 6 months, maintaining relapse rates of 1-2%. Subsequent trials attempting shorter durations have failed, with 4-month arms consistently having relapse rates of 15-20%. One trial shortened treatment only among those without baseline cavity on chest x-ray and whose month 2 sputum culture converted to negative. The 4-month arm relapse rate decreased to 7% but was still significantly worse than the 6-month arm (1.6%, P<0.01). Â We hypothesize that PET/CT characteristics at baseline, PET/CT changes at one month, and markers of residual bacterial load will identify patients with tuberculosis who can be cured with 4 months (16 weeks) of standard treatment.Methods: This is a prospective, multicenter, randomized, phase 2b, noninferiority clinical trial of pulmonary tuberculosis participants. Those eligible start standard of care treatment. PET/CT scans are done at weeks 0, 4, and 16 or 24. Participants who do not meet early treatment completion criteria (baseline radiologic severity, radiologic response at one month, and GeneXpert-detectable bacilli at four months) are placed in Arm A (24 weeks of standard therapy). Those who meet the early treatment completion criteria are randomized at week 16 to continue treatment to week 24 (Arm B) or complete treatment at week 16 (Arm C). The primary endpoint compares the treatment success rate at 18 months between Arms B and C.Discussion: Multiple biomarkers have been assessed to predict TB treatment outcomes. This study uses PET/CT scans and GeneXpert (Xpert) cycle threshold to risk stratify participants. PET/CT scans are not applicable to global public health but could be used in clinical trials to stratify participants and possibly become a surrogate endpoint. If the Predict TB trial is successful, other immunological biomarkers or transcriptional signatures that correlate with treatment outcome may be identified. TRIAL REGISTRATION: NCT02821832
Co-expression of multiple myosin heavy chain genes, in addition to a tissue-specific one, in extraocular musculature.
Soil bacteria and archaea found in long-term corn (Zea mays L.) agroecosystems in Quebec, Canada
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