2,020 research outputs found
Forming short-period Wolf-Rayet X-ray binaries and double black holes through stable mass transfer
We show that black-hole High-Mass X-ray Binaries (HMXBs) with O- or B-type
donor stars and relatively short orbital periods, of order one week to several
months may survive spiral in, to then form Wolf-Rayet (WR) X-ray binaries with
orbital periods of order a day to a few days; while in systems where the
compact star is a neutron star, HMXBs with these orbital periods never survive
spiral-in. We therefore predict that WR X-ray binaries can only harbor black
holes. The reason why black-hole HMXBs with these orbital periods may survive
spiral in is: the combination of a radiative envelope of the donor star, and a
high mass of the compact star. In this case, when the donor begins to overflow
its Roche lobe, the systems are able to spiral in slowly with stable Roche-lobe
overflow, as is shown by the system SS433. In this case the transferred mass is
ejected from the vicinity of the compact star (so-called "isotropic
re-emission" mass loss mode, or "SS433-like mass loss"), leading to gradual
spiral-in. If the mass ratio of donor and black hole is , these systems
will go into CE evolution and are less likely to survive. If they survive, they
produce WR X-ray binaries with orbital periods of a few hours to one day.
Several of the well-known WR+O binaries in our Galaxy and the Magellanic
Clouds, with orbital periods in the range between a week and several months,
are expected to evolve into close WR-Black-Hole binaries,which may later
produce close double black holes. The galactic formation rate of double black
holes resulting from such systems is still uncertain, as it depends on several
poorly known factors in this evolutionary picture. It might possibly be as high
as per year.Comment: MNRAS in pres
Expected Coalescence Rate of Double Neutron Stars for Ground Based Interferometers
In this paper we present new estimates of the coalescence rate of neutron
star binaries in the local universe and we discuss its consequences for the
first generations of ground based interferometers. Our approach based on both
evolutionary and statistical methods gives a galactic merging rate of 1.7
10 yr, in the range of previous estimates 10 - 10
yr. The local rate which includes the contribution of elliptical
galaxies is two times higher, in the order of 3.4 10 yr. We
predict one detection every 148 and 125 years with initial VIRGO and LIGO, and
up to 6 events per year with their advanced configuration. Our recent detection
rate estimates from investigations on VIRGO future improvements are quoted.Comment: talk given at the GWDAW9 (Annecy, 2004) to be published in CQ
Mutation supply and the repeatability of selection for antibiotic resistance
Whether evolution can be predicted is a key question in evolutionary biology.
Here we set out to better understand the repeatability of evolution. We
explored experimentally the effect of mutation supply and the strength of
selective pressure on the repeatability of selection from standing genetic
variation. Different sizes of mutant libraries of an antibiotic resistance
gene, TEM-1 -lactamase in Escherichia coli, were subjected to different
antibiotic concentrations. We determined whether populations went extinct or
survived, and sequenced the TEM gene of the surviving populations. The
distribution of mutations per allele in our mutant libraries- generated by
error-prone PCR- followed a Poisson distribution. Extinction patterns could be
explained by a simple stochastic model that assumed the sampling of beneficial
mutations was key for survival. In most surviving populations, alleles
containing at least one known large-effect beneficial mutation were present.
These genotype data also support a model which only invokes sampling effects to
describe the occurrence of alleles containing large-effect driver mutations.
Hence, evolution is largely predictable given cursory knowledge of mutational
fitness effects, the mutation rate and population size. There were no clear
trends in the repeatability of selected mutants when we considered all
mutations present. However, when only known large-effect mutations were
considered, the outcome of selection is less repeatable for large libraries, in
contrast to expectations. Furthermore, we show experimentally that alleles
carrying multiple mutations selected from large libraries confer higher
resistance levels relative to alleles with only a known large-effect mutation,
suggesting that the scarcity of high-resistance alleles carrying multiple
mutations may contribute to the decrease in repeatability at large library
sizes.Comment: 31pages, 9 figure
The Evolution of Globular Clusters in the Galaxy
We investigate the evolution of globular clusters using N-body calculations
and anisotropic Fokker-Planck (FP) calculations. The models include a mass
spectrum, mass loss due to stellar evolution, and the tidal field of the parent
galaxy. Recent N-body calculations have revealed a serious discrepancy between
the results of N-body calculations and isotropic FP calculations. The main
reason for the discrepancy is an oversimplified treatment of the tidal field
employed in the isotropic FP models. In this paper we perform a series of
calculations with anisotropic FP models with a better treatment of the tidal
boundary and compare these with N-body calculations. The new tidal boundary
condition in our FP model includes one free parameter. We find that a single
value of this parameter gives satisfactory agreement between the N-body and FP
models over a wide range of initial conditions.
Using the improved FP model, we carry out an extensive survey of the
evolution of globular clusters over a wide range of initial conditions varying
the slope of the mass function, the central concentration, and the relaxation
time. The evolution of clusters is followed up to the moment of core collapse
or the disruption of the clusters in the tidal field of the parent galaxy. In
general, our model clusters, calculated with the anisotropic FP model with the
improved treatment for the tidal boundary, live longer than isotropic models.
The difference in the lifetime between the isotropic and anisotropic models is
particularly large when the effect of mass loss via stellar evolution is rather
significant. On the other hand the difference is small for relaxation-
dominated clusters which initially have steep mass functions and high central
concentrations.Comment: 36 pages, 11 figures, LaTeX; added figures and tables; accepted by
Ap
Simple Stellar Population Models as probed by the Large Magellanic Cloud Star Cluster ESO 121-SC03
The presence of blue straggler stars (BSs) in star clusters has proven a
challenge to conventional simple stellar population (SSP) models. Conventional
SSP models are based on the evolution theory of single stars. Meanwhile, the
typical locations of BSs in the colour-magnitude diagram of a cluster are
brighter and bluer than the main sequence turn-off point. Such loci cannot be
predicted by single-star evolution theory. However, stars with such properties
contribute significantly to the integrated light of the cluster. In this paper,
we reconstruct the integrated properties of the Large Magellanic Cloud cluster
ESO 121-SC03, based on a detailed exploration of the individual cluster stars,
and with particular emphasis on the cluster's BSs. We find that the integrated
light properties of ESO 121-SC03 are dramatically modified by its BS component.
The integrated spectral energy distribution (ISED) flux level is significantly
enhanced toward shorter wavelengths, and all broad-band colours become bluer.
When fitting the fully integrated ISED of this cluster based on conventional
SSP models, the best-fitting values of age and metallicity are significantly
underestimated compared to the true cluster parameters. The age underestimate
is per cent if we only include the BSs within the cluster's half-light
radius and per cent if all BSs are included. The corresponding
underestimates of the cluster's metallicity are and per cent,
respectively. The populous star clusters in the Magellanic Clouds are ideal
objects to explore the potential importance of BSs for the integrated light
properties of more distant unresolved star clusters in a statistically robust
manner, since they cover a large range in age and metallicity.Comment: 11 pages, 7 figures, 2 tables, accepted for publication in MNRA
Een Nieuwe Koekelt : kloppend groen hart van Ede
De tuinders van VAT-Ede (Vereniging Amateurtuinders) wilden een verkenning uitvoeren naar de mogelijkheden om het bestaande volkstuincomplex De Koekelt om te vormen tot een multifunctioneel tuinenpark. Het volkstuinencomplex De Koekelt biedt door haar ligging en grootte ongekende mogelijkheden om een multifunctioneel tuinenpark te realiseren. Het volkstuinenterrein kan door herstructurering veranderen in een groene zone waar ecologie en milieu de ruimte krijgen, waar meerdere vormen van recreatie mogelijk zijn en waar meer aansluiting ontstaat met de omgevingEen multifunctioneel tuinenpark zou wel eens de groene motor kunnen zijn voor de herstructurering van het hele Peppelensteeggebied
Budget Impact Analysis of Metformin Sustained Release for the Treatment of Type 2 Diabetes in The Netherlands
BACKGROUND: Adverse drug reactions and medication nonadherence are well-known causes of sub-optimal disease control and worsened disease outcomes in patients who are treated for type 2 diabetes. Metformin sustained release (SR) might reduce these adverse events and improve medication adherence via a simplified treatment regimen for metformin immediate release (IR)-intolerant patients. OBJECTIVES: The aim of this study is to estimate the budget impact of metformin SR for the treatment of type 2 diabetes in the Netherlands, compared to the current standard of care (SoC) with metformin IR. METHODS: A budget impact model was built to represent the course of the disease and treatment pathway of type 2 diabetes patients eligible for metformin SR from a healthcare payer's perspective. Patients were considered eligible if they used less than 2000 mg metformin IR per day, but suffered from adverse events that might lead to therapy discontinuation, and if they were newly diagnosed with type 2 diabetes. The costs of type 2 diabetes treatment and related complications over a time horizon of 3 years were calculated. Univariate sensitivity analyses were conducted to show which parameters have the biggest influence on the budget impact. RESULTS: The budget impact analysis showed cost-savings of - €1,962,335 over a period of 3 years through implementation of metformin SR as an alternative to SoC with metformin IR. Savings were mostly driven by the delay of other, more expensive type 2 diabetes treatments, such as insulin. In sensitivity analyses, medication adherence and persistence appeared to have the biggest influence on the budget impact. CONCLUSION: Metformin SR could potentially be a cost-saving alternative to metformin IR for the treatment of type 2 diabetes in the Netherlands, especially in patients experiencing adverse events with metformin IR. However, more research is needed to better predict the effect of using once-daily metformin, compared to multiple dosages, on medication adherence and persistence and to evaluate whether metformin SR really decreases the amount of adverse events
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