1,269 research outputs found
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
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
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
VLA observations of candidate high-mass protostellar objects at 7 mm
We present radio continuum observations at 7 mm made using the Very Large
Array towards three massive star forming regions thought to be in very early
stages of evolution selected from the sample of Sridharan et al. (2002).
Emission was detected towards all three sources (IRAS 18470-0044, IRAS
19217+1651 and IRAS 23151+5912). We find that in all cases the 7 mm emission
corresponds to thermal emission from ionized gas. The regions of ionized gas
associated with IRAS 19217+1651 and IRAS 23151+5912 are hypercompact with
diameters of 0.009 and 0.0006 pc, and emission measures of 7.0 x 10^8 and 2.3 x
10^9 pc cm^(-6), respectively.Comment: 17 pages, 5 figures, accepted by The Astronomical Journa
On the Theory of Gamma Ray Bursts and Hypernovae: The Black Hole Soft X-ray Transient Sources
We show that a common evolutionary history can produce the black hole
binaries in the Galaxy in which the black holes have masses of ~ 5-10 M_sun. In
with low-mass, <~ 2.5 M_sun, ZAMS (zero age main sequence) companions, the
latter remain in main sequence during the active stage of soft X-ray transients
(SXTs), most of them being of K or M classification. In two intermediate cases,
IL Lupi and Nova Scorpii with ZAMS ~ 2.5 M_sun companions the orbits are
greatly widened because of large mass loss in the explosion forming the black
hole, and whereas these companions are in late main sequence evolution, they
are close to evolving. Binaries with companion ZAMS masses >~ 3 M_sun are
initially "silent" until the companion begins evolving across the Herzsprung
gap. We provide evidence that the narrower, shorter period binaries, with
companions now in main sequence, are fossil remnants of gamma ray bursters
(GRBs). We also show that the GRB is generally accompanied by a hypernova
explosion (a very energetic supernova explosion). We further show that the
binaries with evolved companions are good models for some of the ultraluminous
X-ray sources (ULXs) recently seen by Chandra in other galaxies. The great
regularity in our evolutionary history, especially the fact that most of the
companions of ZAMS mass <~ 2.5 M_sun remain in main sequences as K or M stars
can be explained by the mass loss in common envelope evolution to be Case C;
i.g., to occur only after core He burning has finished. Since our argument for
Case C mass transfer is not generally understood in the community, we add an
appendix, showing that with certain assumptions which we outline we can
reproduce the regularities in the evolution of black hole binaries by Case C
mass transfer.Comment: 59 pages, 12 figures, review articl
Spectroscopic study of the O-type runaway supergiant HD 195592
The scope of this paper is to perform a detailed spectroscopic study of the
northern O-type supergiant HD 195592. We use a large sample of high quality
spectra in order to investigate its multiplicity, and to probe the line profile
variability. Our analysis reveals a clear spectroscopic binary signature in the
profile of the He {\sc i} 6678 line, pointing to a probable O + B
system. We report on low amplitude radial velocity variations in every strong
absorption line in the blue spectrum of HD 195592. These variations are ruled
by two time-scales respectively of 5.063 and about 20 days. The former is
firmly established, whilst the latter is poorly constrained. We report also on
a very significant line profile variability of the H line, with time
scales strongly related to those of the radial velocities. Our results provide
significant evidence that HD 195592 is a binary system, with a period that
might be the variability time-scale of about 5 days. The second time scale may
be the signature of an additional star moving along a wider orbit provided its
mass is low enough, even though direct evidence for the presence of a third
star is still lacking. Alternatively, the second time-scale may be the
signature of a variability intrinsic to the stellar wind of the primary,
potentially related to the stellar rotation.Comment: 9 pages, 5 postscript figures, accepted for publication in New
Astronom
Isradipine Twice Daily Lowers Blood Pressure Over 24 H
The objective of this study was to compare the effects of isradipine and placebo on blood pressure (BP) at the end of the dosing interval (‘trough'). Following a three-week placebo period, 187 patients who had previously shown a response to treatment with isradipine (based on office BP measurements) were randomized to double-blind treatment with 2.5 mg isradipine twice daily or placebo for six weeks. Four of these patients withdrew from the study during the double-blind phase because of adverse events (one taking isradipine and three taking placebo). Blood pressure during the double-blind study was always measured 12 h after drug administration (trough values). The rate of normalization [defined as diastolic BP (DBP) ≤ 90 mm Hg] was 52/96 (54%) in the isradipine-treated group compared with 30/87 (33%) in the placebo group. A further 12/96 (12%) patients taking isradipine showed a fall in DBP of ≥ 10 mm Hg, although their DBP was still not < 90 mm Hg, compared with 5/87 (6%) patients receiving placebo. This difference was statistically significant (P = .003). Thus, isradipine in a dose of 2.5 mg twice daily lowers blood pressure over 24 h. Am J Hypertens 1991;4:131S-134
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
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