573 research outputs found
Solar System: Sifting through the debris
A quadrillion previously unnoticed small bodies beyond Neptune have been
spotted as they dimmed X-rays from a distant source. Models of the dynamics of
debris in the Solar System's suburbs must now be reworked.Comment: 3 pages, 1 figure; Nature News and Views on Chang et al. 2006,
Nature, 442, 660-66
Intensive care unit (ICU)-acquired bacteraemia and ICU mortality and discharge:Addressing time-varying confounding using appropriate methodology
Background: Studies often ignore time-varying confounding or may use inappropriate methodology to adjust for time-varying confounding.
Aim: To estimate the effect of intensive care unit (ICU)-acquired bacteraemia on ICU mortality and discharge using appropriate methodology.
Methods: Marginal structural models with inverse probability weighting were used to estimate the ICU mortality and discharge associated with ICU-acquired bacteraemia among patients who stayed more than two days at the general ICU of a London teaching hospital and remained bacteraemia-free during those first two days. For comparison, the same associations were evaluated with (i) a conventional Cox model, adjusting only for baseline confounders and (ii) a Cox model adjusting for baseline and time-varying confounders.
Findings: Using the marginal structural model with inverse probability weighting, bacteraemia was associated with an increase in ICU mortality (cause-specific hazard ratio (CSHR): 1.29; 95% confidence interval (CI): 1.02-1.63)and a decrease in discharge (CSHR: 0.52; 95% CI: 0.45-0.60). By 60 days, among patients still in the ICU after two days and without prior bacteraemia, 8.0% of ICU deaths could be prevented by preventing all ICU-acquired bacteraemia cases. The conventional Cox model adjusting for time-varying confounders gave substantially different results [for ICU mortality, CSHR: 1.08 (95% CI: 0.88-1.32); for discharge, CSHR: 0.68 (95% CI: 0.60-0.77)].
Conclusion: In this study, even after adjusting for the timing of acquiring bacteraemia and time-varying confounding using inverse probability weighting for marginal structura
A Minimum-Mass Extrasolar Nebula
By analogy with the minimum-mass solar nebula, we construct a surface-density
profile using the orbits of the 26 precise-Doppler planets found in multiple
planet systems: Sigma = 2200 grams per square centimeter (a/1 AU)^- beta, where
a is the circumstellar radius, and beta = 2.0 plus or minus 0.5. The
minimum-mass solar nebula is consistent with this model, but the uniform-alpha
accretion disk model is not. In a nebula with beta > 2, the center of the disk
is the likely cradle of planet formation.Comment: 15 pages, including 2 figures. To appear in ApJ, 9/04 new version
with prettier page layou
Planet Formation in the Outer Solar System
This paper reviews coagulation models for planet formation in the Kuiper
Belt, emphasizing links to recent observations of our and other solar systems.
At heliocentric distances of 35-50 AU, single annulus and multiannulus
planetesimal accretion calculations produce several 1000 km or larger planets
and many 50-500 km objects on timescales of 10-30 Myr in a Minimum Mass Solar
Nebula. Planets form more rapidly in more massive nebulae. All models yield two
power law cumulative size distributions, N_C propto r^{-q} with q = 3.0-3.5 for
radii larger than 10 km and N_C propto r^{-2.5} for radii less than 1 km. These
size distributions are consistent with observations of Kuiper Belt objects
acquired during the past decade. Once large objects form at 35-50 AU,
gravitational stirring leads to a collisional cascade where 0.1-10 km objects
are ground to dust. The collisional cascade removes 80% to 90% of the initial
mass in the nebula in roughly 1 Gyr. This dust production rate is comparable to
rates inferred for alpha Lyr, beta Pic, and other extrasolar debris disk
systems.Comment: invited review for PASP, March 2002. 33 pages of text and 12 figure
The TAOS Project Stellar Variability II. Detection of 15 Variable Stars
The Taiwanese-American Occultation Survey (TAOS) project has collected more
than a billion photometric measurements since 2005 January. These sky survey
data-covering timescales from a fraction of a second to a few hundred days-are
a useful source to study stellar variability. A total of 167 star fields,
mostly along the ecliptic plane, have been selected for photometric monitoring
with the TAOS telescopes. This paper presents our initial analysis of a search
for periodic variable stars from the time-series TAOS data on one particular
TAOS field, No. 151 (RA = 17^{\rm h}30^{\rm m}6\fs67, Dec = 27\degr17\arcmin
30\arcsec, J2000), which had been observed over 47 epochs in 2005. A total of
81 candidate variables are identified in the 3 square degree field, with
magnitudes in the range 8 < R < 16. On the basis of the periodicity and shape
of the lightcurves, 29 variables, 15 of which were previously unknown, are
classified as RR Lyrae, Cepheid, delta Scuti, SX Phonencis, semi-regular and
eclipsing binaries.Comment: 20 pages, 6 figures, accepted in The Astronomical Journa
Accretion in the Early Kuiper Belt I. Coagulation and Velocity Evolution
We describe planetesimal accretion calculations in the Kuiper Belt.
Our evolution code simulates planetesimal growth in a single annulus and
includes velocity evolution but not fragmentation. Test results match analytic
solutions and duplicate previous simulations at 1 AU.
In the Kuiper Belt, simulations without velocity evolution produce a single
runaway body with a radius of 1000 km on a time scale inversely proportional to
the initial mass in the annulus. Runaway growth occurs in 100 Myr for 10 earth
masses and an initial eccentricity of 0.001 in a 6 AU annulus centered at 35
AU. This mass is close to the amount of dusty material expected in a minimum
mass solar nebula extrapolated into the Kuiper Belt.
Simulations with velocity evolution produce runaway growth on a wide range of
time scales. Dynamical friction and viscous stirring increase particle
velocities in models with large (8 km radius) initial bodies. This velocity
increase delays runaway growth by a factor of two compared to models without
velocity evolution. In contrast, collisional damping dominates over dynamical
friction and viscous stirring in models with small (80--800 m radius) initial
bodies. Collisional damping decreases the time scale to runaway growth by
factors of 4--10 relative to constant velocity calculations. Simulations with
minimum mass solar nebulae, 10 earth masses, reach runaway growth on time
scales of 20-40 Myr with 80 m initial bodies, 50-100 Myr with 800 m bodies, and
75-250 Myr for 8 km initial bodies. These growth times vary linearly with the
mass of the annulus but are less sensitive to the initial eccentricity than
constant velocity models.Comment: 45 pages of text (including 5 tables), 31 pages of figur
Fully reduced HMGB1 accelerates the regeneration of multiple tissues by transitioning stem cells to G(ALERT)
A major discovery of recent decades has been the existence of stem cells and their potential to repair many, if not most, tissues. With the aging population, many attempts have been made to use exogenous stem cells to promote tissue repair, so far with limited success. An alternative approach, which may be more effective and far less costly, is to promote tissue regeneration by targeting endogenous stem cells. However, ways of enhancing endogenous stem cell function remain poorly defined. Injury leads to the release of danger signals which are known to modulate the immune response, but their role in stem cell-mediated repair in vivo remains to be clarified. Here we show that high mobility group box 1 (HMGB1) is released following fracture in both humans and mice, forms a heterocomplex with CXCL12, and acts via CXCR4 to accelerate skeletal, hematopoietic, and muscle regeneration in vivo. Pretreatment with HMGB1 2 wk before injury also accelerated tissue regeneration, indicating an acquired proregenerative signature. HMGB1 led to sustained increase in cell cycling in vivo, and using Hmgb1 -/- mice we identified the underlying mechanism as the transition of multiple quiescent stem cells from G0 to GAlert HMGB1 also transitions human stem and progenitor cells to GAlert Therefore, exogenous HMGB1 may benefit patients in many clinical scenarios, including trauma, chemotherapy, and elective surgery
A genomic portrait of the emergence, evolution, and global spread of a methicillin-resistant staphylococcus aureus pandemic
The widespread use of antibiotics in association with high-density clinical care has driven the emergence of drug-resistant bacteria that are adapted to thrive in hospitalized patients. Of particular concern are globally disseminated methicillin-resistant Staphylococcus aureus (MRSA) clones that cause outbreaks and epidemics associated with health care. The most rapidly spreading and tenacious health-care-associated clone in Europe currently is EMRSA-15, which was first detected in the UK in the early 1990s and subsequently spread throughout Europe and beyond. Using phylogenomic methods to analyze the genome sequences for 193 S. aureus isolates, we were able to show that the current pandemic population of EMRSA-15 descends from a health-care-associated MRSA epidemic that spread throughout England in the 1980s, which had itself previously emerged from a primarily community-associated methicillin-sensitive population. The emergence of fluoroquinolone resistance in this EMRSA-15 subclone in the English Midlands during the mid-1980s appears to have played a key role in triggering pandemic spread, and occurred shortly after the first clinical trials of this drug. Genome-based coalescence analysis estimated that the population of this subclone over the last 20 yr has grown four times faster than its progenitor. Using comparative genomic analysis we identified the molecular genetic basis of 99.8% of the antimicrobial resistance phenotypes of the isolates, highlighting the potential of pathogen genome sequencing as a diagnostic tool. We document the genetic changes associated with adaptation to the hospital environment and with increasing drug resistance over time, and how MRSA evolution likely has been influenced by country-specific drug use regimens
Oxidation resistance of graphene-coated Cu and Cu/Ni alloy
The ability to protect refined metals from reactive environments is vital to
many industrial and academic applications. Current solutions, however,
typically introduce several negative effects, including increased thickness and
changes in the metal physical properties. In this paper, we demonstrate for the
first time the ability of graphene films grown by chemical vapor deposition to
protect the surface of the metallic growth substrates of Cu and Cu/Ni alloy
from air oxidation. SEM, Raman spectroscopy, and XPS studies show that the
metal surface is well protected from oxidation even after heating at 200
\degree C in air for up to 4 hours. Our work further shows that graphene
provides effective resistance against hydrogen peroxide. This protection method
offers significant advantages and can be used on any metal that catalyzes
graphene growth
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