361 research outputs found
The Primordial Helium Abundance: Towards Understanding and Removing the Cosmic Scatter in the dY/dZ Relation
We present results from photoionization models of low-metallicity HII
regions. These nebulae form the basis for measuring the primordial helium
abundance. Our models show that the helium ionization correction factor (ICF)
can be non-negligible for nebulae excited by stars with effective temperatures
larger than 40,000 K. Furthermore, we find that when the effective temperature
rises to above 45,000 K, the ICF can be significantly negative. This result is
independent of the choice of stellar atmosphere. However, if an HII region has
an [O III] 5007/[O I] 6300 ratio greater than 300, then our models show that,
regardless of its metallicity, it will have a negligibly small ICF. A similar,
but metallicity dependent, result was found using the [O III] 5007/H
ratio. These two results can be used as selection criteria to remove nebulae
with potentially non-negligible ICFs. Using our metallicity independent
criterion on the data of Izotov & Thuan (1998) results in a 20% reduction of
the rms scatter about the best fit line. A fit to the selected data
results in a slight increase of the value of the primordial helium abundance.Comment: 10 pages, 5 figures, accepted by the Ap
Tooth Discoloration in Patients With Neonatal Diabetes After Transfer Onto Glibenclamide: A previously unreported side effect
PublishedJournal ArticleMulticenter StudyResearch Support, N.I.H., ExtramuralResearch Support, Non-U.S. Gov'tOBJECTIVE To assess if tooth discoloration is a novel side effect of sulfonylurea therapy in patients with permanent neonatal diabetes due to mutations in KCNJ11. RESEARCH DESIGN AND METHODS A total of 67 patients with a known KCNJ11 mutation who had been successfully transferred from insulin injections onto oral sulfonylureas were contacted and asked about the development of tooth discoloration after transfer. RESULTS Altered tooth appearance was identified in 5 of the 67 patients. This was variable in severity, ranging from mild discoloration/staining (n = 4) to loss of enamel (n = 1) and was only seen in patients taking glibenclamide (glyburide). CONCLUSIONS These previously unreported side effects may relate to the developing tooth and/or to the high local concentrations in the children who frequently chewed glibenclamide tablets or took it as a concentrated solution. Given the multiple benefits of sulfonylurea treatment for patients with activating KCNJ11 mutations, this association warrants further investigation but should not preclude such treatment.This work was funded by the Welcome Trust (grant 067463/Z/2/Z), National Institutes of Health Grants DK-44752 and DK-20595, and a gift from the Kovler Family Foundation. S.E.F. is the Sir
Graham Wilkins, Peninsula Medical School Research Fellow. A.T.H. is a Welcome Trust
Research Leave Fellow. O.R.-C. was supported by an “Ayuda para contratos post-Formacio´n
Sanitaria Especializada” from the “Instituto de Salud Carlos III” (FIS CM06/00013
An Approach to the Bio-Inspired Control of Self-reconfigurable Robots
Self-reconfigurable robots are robots built by modules which
can move in relationship to each other. This ability of changing its physical
form provides the robots a high level of adaptability and robustness.
Given an initial configuration and a goal configuration of the robot, the
problem of self-regulation consists on finding a sequence of module moves
that will reconfigure the robot from the initial configuration to the goal
configuration. In this paper, we use a bio-inspired method for studying
this problem which combines a cluster-flow locomotion based on cellular
automata together with a decentralized local representation of the
spatial geometry based on membrane computing ideas. A promising 3D
software simulation and a 2D hardware experiment are also presented.National Natural Science Foundation of China No. 6167313
The Low- and Intermediate-Mass Stellar Population in the Small Magellanic Cloud: The Central Stars of Planetary Nebulae
We present a study on the central stars (CSs) of Planetary Nebulae (PNe)
observed in the Small Magellanic Cloud (SMC) with the Space Telescope Imaging
Spectrograph instrument on-board the HST. The stellar magnitudes have been
measured using broad-band photometry, and Zanstra analysis of the nebulae
provided the stellar temperatures. From the location of the CSs on the HR
diagram, and by comparing the observed CSs with current models of stellar
evolution, we infer the CSs masses. We examine closely the possibility of light
contamination in the bandpass from an unrecognized stellar companion, and we
establish strong constraints on the existence and nature of any binary
companion. We find an average mass of 0.63 Msun, which is similar to the mass
obtained for a sample of CSs in the LMC (0.65 Msun). However, the SMC and LMC
CS mass distributions differ slightly, the SMC sample lacking an
intermediate-mass stellar population (0.65 to 0.75 Msun). We discuss the
significance and possible reasons for the difference between the two mass
distributions. In particular, we consider the differences in the star formation
history between the clouds and the mass-loss rate dependence on metallicity.Comment: 30 pages, 6 figures, 5 tables. To be published in ApJ (October 20
Three Dimensional Electrical Impedance Tomography
The electrical resistivity of mammalian tissues varies widely and is correlated with physiological
function. Electrical impedance tomography (EIT) can be used to probe such variations in vivo, and offers a
non-invasive means of imaging the internal conductivity distribution of the human body. But the
computational complexity of EIT has severe practical limitations, and previous work has been restricted to
considering image reconstruction as an essentially two-dimensional problem. This simplification can limit
significantly the imaging capabilities of EIT, as the electric currents used to determine the conductivity variations will not in general be confined to a two-dimensional plane. A few studies have attempted three-dimensional EIT image reconstruction, but have not yet succeeded in generating images of a quality suitable for clinical applications. Here we report the development of a three-dimensional EIT system with greatly improved imaging capabilities, which combines our 64-electrode data-collection apparatus with customized matrix inversion techniques. Our results demonstrate the practical potential of EIT for clinical applications, such as lung or brain imaging and diagnostic screening
Excessive reactive oxygen species induce transcription-dependent replication stress.
Elevated levels of reactive oxygen species (ROS) reduce replication fork velocity by causing dissociation of the TIMELESS-TIPIN complex from the replisome. Here, we show that ROS generated by exposure of human cells to the ribonucleotide reductase inhibitor hydroxyurea (HU) promote replication fork reversal in a manner dependent on active transcription and formation of co-transcriptional RNA:DNA hybrids (R-loops). The frequency of R-loop-dependent fork stalling events is also increased after TIMELESS depletion or a partial inhibition of replicative DNA polymerases by aphidicolin, suggesting that this phenomenon is due to a global replication slowdown. In contrast, replication arrest caused by HU-induced depletion of deoxynucleotides does not induce fork reversal but, if allowed to persist, leads to extensive R-loop-independent DNA breakage during S-phase. Our work reveals a link between oxidative stress and transcription-replication interference that causes genomic alterations recurrently found in human cancer. [Abstract copyright: © 2023. The Author(s).
Farms, pipes, streams and reforestation : reasoning about structured parallel processes using types and hylomorphisms
The increasing importance of parallelism has motivated the creation of better abstractions for writing parallel software, including structured parallelism using nested algorithmic skeletons. Such approaches provide high-level abstractions that avoid common problems, such as race conditions, and often allow strong cost models to be defined. However, choosing a combination of algorithmic skeletons that yields good parallel speedups for a program on some specific parallel architecture remains a difficult task. In order to achieve this, it is necessary to simultaneously reason both about the costs of different parallel structures and about the semantic equivalences between them. This paper presents a new type-based mechanism that enables strong static reasoning about these properties. We exploit well-known properties of a very general recursion pattern, hylomorphisms, and give a denotational semantics for structured parallel processes in terms of these hylomorphisms. Using our approach, it is possible to determine formally whether it is possible to introduce a desired parallel structure into a program without altering its functional behaviour, and also to choose a version of that parallel structure that minimises some given cost model.Postprin
Modelling the Pan-Spectral Energy Distribution of Starburst Galaxies: III. Emission Line Diagnostics of Ensembles of Evolving HII Regions
We build, as far as theory will permit, self consistent model HII regions
around central clusters of aging stars. These produce strong emission line
diagnostics applicable to either individual HII regions in galaxies, or to the
integrated emission line spectra of disk or starburst galaxies. The models
assume that the expansion and internal pressure of individual HII regions is
driven by the net input of mechanical energy from the central cluster, be it
through winds or supernova events. This eliminates the ionization parameter as
a free variable, replacing it with a parameter which depends on the ratio of
the cluster mass to the pressure in the surrounding interstellar medium. These
models explain why HII regions with low abundances have high excitation, and
demonstrate that at least part of the warm ionized medium is the result of
overlapping faint, old, large, and low pressure HII regions. We present line
ratios (at both optical and IR wavelengths) which provide reliable abundance
diagnostics for both single HII regions or for integrated galaxy spectra, and
we find a number that can be used to estimate the mean age of the cluster stars
exciting individual HII regions.Comment: 22 pages. 18 figures. Accepted for publication in Astrophysical
journal Supplements. Electronic tabular material is available on request to
[email protected]
Three-dimensional chemically homogeneous and bi-abundance photoionization models of the "super-metal-rich" planetary nebula NGC 6153
Deep spectroscopy of the planetary nebula (PN) NGC\,6153 shows that its heavy
element abundances derived from optical recombination lines (ORLs) are ten
times higher than those derived from collisionally excited lines (CELs), and
points to the existence of H-deficient inclusions embedded in the diffuse
nebula. In this study, we have constructed chemically homogeneous and
bi-abundance three-dimensional photoionization models, using the Monte Carlo
photoionization code {\sc mocassin}. We attempt to reproduce the multi-waveband
spectroscopic and imaging observations of NGC\,6153, and investigate the nature
and origin of the postulated H-deficient inclusions, as well as their impacts
on the empirical nebular analyses assuming a uniform chemical composition. Our
results show that chemically homogeneous models yield small electron
temperature fluctuations and fail to reproduce the strengths of ORLs from C, N,
O and Ne ions. In contrast, bi-abundance models incorporating a small amount of
metal-rich inclusions ( per cent of the total nebular mass) are able
to match all the observations within the measurement uncertainties. The
metal-rich clumps, cooled down to a very low temperature (~K) by
ionic infrared fine-structure lines, dominate the emission of heavy element
ORLs, but contribute almost nil to the emission of most CELs. We find that the
abundances of C, N, O and Ne derived empirically from CELs, assuming a uniform
chemical composition, are about 30 per cent lower than the corresponding
average values of the whole nebula, including the contribution from the
H-deficient inclusions. Ironically, in the presence of H-deficient inclusions,
the traditional standard analysis of the optical helium recombination lines,
assuming a chemically homogeneous nebula, overestimates the helium abundance by
40 per cent.Comment: 19 pages, 18 figures, accepted for publication in MNRA
DNA damage during S-phase mediates the proliferation-quiescence decision in the subsequent G1 via p21 expression
Following DNA damage caused by exogenous sources, such as ionizing radiation, the tumour suppressor p53 mediates cell cycle arrest via expression of the CDK inhibitor, p21. However, the role of p21 in maintaining genomic stability in the absence of exogenous DNA-damaging agents is unclear. Here, using live single-cell measurements of p21 protein in proliferating cultures, we show that naturally occurring DNA damage incurred over S-phase causes p53-dependent accumulation of p21 during mother G2- and daughter G1-phases. High p21 levels mediate G1 arrest via CDK inhibition, yet lower levels have no impact on G1 progression, and the ubiquitin ligases CRL4Cdt2 and SCFSkp2 couple to degrade p21 prior to the G1/S transition. Mathematical modelling reveals that a bistable switch, created by CRL4Cdt2, promotes irreversible S-phase entry by keeping p21 levels low, preventing premature S-phase exit upon DNA damage. Thus, we characterize how p21 regulates the proliferation-quiescence decision to maintain genomic stability
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