74 research outputs found
The Progeny of Stellar Dynamics and Stellar Evolution within an N-body model of NGC 188
We present a direct N-body simulation modeling the evolution of the old (7
Gyr) open cluster NGC 188. This is the first N-body open cluster simulation
whose initial binary population is directly defined by observations of a
specific open cluster: M35 (150 Myr). We compare the simulated color-magnitude
diagram at 7 Gyr to that of NGC 188, and discuss the blue stragglers produced
in the simulation. We compare the solar-type main sequence binary period and
eccentricity distributions of the simulation to detailed observations of
similar binaries in NGC 188. These results demonstrate the importance of
detailed observations in guiding N-body open cluster simulations. Finally, we
discuss the implications of a few discrepancies between the NGC 188 model and
observations and suggest a few methods for bringing N-body open cluster
simulations into better agreement with observations.Comment: Contributed talk at IAUS 266 'Star clusters: basic galactic building
blocks', R. de Grijs and J.R.D. Lepine, eds. 6 pages, 4 figure
Modelling the evolution and nucleosynthesis of carbon-enhanced metal-poor stars
We present the results of binary population simulations of carbon-enhanced
metal-poor (CEMP) stars. We show that nitrogen and fluorine are useful tracers
of the origin of CEMP stars, and conclude that the observed paucity of very
nitrogen-rich stars puts strong constraints on possible modifications of the
initial mass function at low metallicity. The large number fraction of CEMP
stars may instead require much more efficient dredge-up from low-metallicity
asymptotic giant branch stars.Comment: 6 pages, 1 figure, to appear in the proceedings of IAU Symposium 252
"The Art of Modelling Stars in the 21st Century", April 6-11, 2008, Sanya,
Chin
Stellar population synthesis of post-AGB stars: the s-process in MACHO47.2496.8
The low-metallicity RV Tauri star MACHO47.2496.8, recently discovered in the
Large Magellanic Cloud, is highly enriched in carbon and heavy elements
produced by the slow neutron capture process (s-process), and is most probably
a genuine post-C(N-type) asymptotic giant branch (AGB) star. We use the
analysis of the abundances of MACHO47.2496.8 to constrain free parameters in
AGB models. We test which values of the free parameters describing uncertain
physical mechanisms in AGB stars, namely the third dredge-up and the features
of the 13C neutron source, produce models that better match the abundances
observed in MACHO47.2496.8. We carry out stellar population synthesis coupled
with s-process nucleosynthesis using a synthetic stellar evolution code. The
s-process ratios observed in MACHO47.2496.8 can be matched by the same models
that explain the s-process ratios of Galactic AGB and post-AGB stars of
metallicity > Z_sun/10, except for the choice of the effectiveness of 13C as a
neutron source, which has to be lower by roughly a factor of 3 to 6. The less
effective neutron source for lower metallicities is also required when
comparing population synthesis results to observations of Galactic halo
-enhanced stars, such as Pb stars. The 12C/13C ratio in MACHO47.2496.8
cannot be matched simultaneously and requires the occurrence of extra-mixing
processes. The confirmed trend of the decreased efficiency of the 13C neutron
source with metallicity requires an explanation from AGB s-process models. The
present work is to date the first comparison between theoretical models and the
detailed abundances of an extragalactic post-AGB star.Comment: accepted for publication on Astronomy & Astrophysics Letter
Investigation of the single neutron exposure model for the s-process: the primary nature of the neutron source
The primary nature of the C neutron source is very significant for the
studies of the s-process nucleosynthesis. In this paper we present an attempt
to fit the element abundances observed in 16 s-rich stars using parametric
model of the single neutron exposure. The calculated results indicate that
almost all s-elements were made in a single neutron exposure for 9 sample
stars. Although a large spread of neutron exposure is obtained, the maximum
value of the neutron exposure will reach about 7.0 mbarn, which is close
to the theoretical predictions by the AGB model. The calculated result is a
significant evidence for the primary nature of the neutron source. Combining
the result obtained in this work and the neutron exposure-initial mass
relations, a large spread of neutron exposure can be explained by the different
initial stellar mass and their time evolution. The possibility that the
rotationally induced mixing process can lead to a spread of the neutron
exposure in AGB stars is also existent.Comment: 6 pages, 2 figure
Orbital eccentricities of binary systems with a former AGB star
Many binary stellar systems in which the primary star is beyond the
asymptotic giant branch (AGB) evolutionary phase show significant orbital
eccentricities whereas current binary interaction models predict their orbits
to be circularised. We analyse how the orbital parameters in a system are
modified under mass loss and mass exchange among its binary components and
propose a model for enhanced mass-loss from the AGB star due to tidal
interaction with its companion, which allows a smooth transition between the
wind and Roche-lobe overflow mass-loss regimes. We explicitly follow its effect
along the orbit on the change of eccentricity and orbital semi-major axis, as
well as the effect of accretion by the companion. We calculate timescales for
the variation of these orbital parameters and compare them to the tidal
circularisation timescale. We find that in many cases, due to the enhanced mass
loss of the AGB component at orbital phases closer to the periastron, the net
eccentricity growth rate in one orbit is comparable to the rate of tidal
circularisation. We show that with this eccentricity enhancing mechanism it is
possible to reproduce the orbital period and eccentricity of the Sirius system,
which under the standard assumptions of binary interaction is expected to be
circularised. We also show that this mechanism may provide an explanation for
the eccentricities of most barium star systems, which are expected to be
circularised due to tidal dissipation. By proposing a tidally enhanced model of
mass loss from AGB stars we find a mechanism which efficiently works against
the tidal circularisation of the orbit, which explains the significant
eccentricities observed in binary systems containing a white dwarf and a less
evolved companion, such as Sirius and systems with barium stars.Comment: 9 pages, 5 figures, accepted for publication in Astronomy and
Astrophysics on 24th of October of 200
A statistical modelling approach for source attribution meta-analysis of sporadic infection with foodborne pathogens
Numerous source attribution studies for foodborne pathogens based on epidemiological and microbiological methods are available. These studies provide empirical data for modelling frameworks that synthetize the quantitative evidence at our disposal and reduce reliance on expert elicitations. Here, we develop a statistical model within a Bayesian estimation framework to integrate attribution estimates from expert elicitations with estimates from microbial subtyping and case-control studies for sporadic infections with four major bacterial zoonotic pathogens in the Netherlands (Campylobacter, Salmonella, Shiga toxin-producing E. coli [STEC] O157 and Listeria). For each pathogen, we pooled the published fractions of human cases attributable to each animal reservoir from the microbial subtyping studies, accounting for the uncertainty arising from the different typing methods, attribution models, and year(s) of data collection. We then combined the population attributable fractions (PAFs) from the case-control studies according to five transmission pathways (domestic food, environment, direct animal contact, human-human transmission and travel) and 11 groups within the foodborne pathway (beef/lamb, pork, poultry meat, eggs, dairy, fish/shellfish, fruit/vegetables, beverages, grains, composite foods and food handlers/vermin). The attribution estimates were biologically plausible, allowing the human cases to be attributed in several ways according to reservoirs, transmission pathways and food groups. All pathogens were predominantly foodborne, with Campylobacter being mostly attributable to the chicken reservoir, Salmonella to pigs (albeit closely followed by layers), and Listeria and STEC O157 to cattle. Food-wise, the attributions reflected those at the reservoir level in terms of ranking. We provided a modelling solution to reach consensus attribution estimates reflecting the empirical evidence in the literature that is particularly useful for policy-making and is extensible to other pathogens and domains
A social cost-benefit analysis of two One Health interventions to prevent toxoplasmosis
In the Netherlands, toxoplasmosis ranks second in disease burden among foodborne pathogens with an estimated health loss of 1,900 Disability Adjusted Life Years and a cost-of-illness estimated at €45 million annually. Therefore, effective and preferably cost-effective preventive interventions are warranted. Freezing meat intended for raw or undercooked consumption and improving biosecurity in pig farms are promising interventions to prevent Toxoplasma gondii infections in humans. Putting these interventions into practice would expectedly reduce the number of infections; however, the net benefits for society are unknown. Stakeholders bearing the costs for these interventions will not necessary coincide with the ones having the benefits. We performed a Social Cost-Benefit Analysis to evaluate the net value of two potential interventions for the Dutch society. We assessed the costs and benefits of the two interventions and compared them with the current practice of education, especially during pregnancy. A ‘minimum scenario’ and a ‘maximum scenario’ was assumed, using input parameters with least benefits to society and input parameters with most benefits to society, respectively. For both interventions, we performed different scenario analyses. The freezing meat intervention was far more effective than the biosecurity intervention. Despite high freezing costs, freezing two meat products: steak tartare and mutton leg yielded net social benefits in both the minimum and maximum scenario, ranging from €10.6 million to €31 million for steak tartare and €0.6 million to €1.5 million for mutton leg. The biosecurity intervention would result in net costs in all scenarios ranging from €1 million to €2.5 million, due to high intervention costs and limited benefits. From a public health perspective (i.e. reducing the burden of toxoplasmosis) and the societal perspective (i.e. a net benefit for the Dutch society) freezing steak tartare and leg of mutton is to be considered
Detection of opsonizing antibodies directed against a recently circulating Bordetella pertussis strain in paired plasma samples from symptomatic and recovered pertussis patients.
Correlates of protection (CoPs) against the highly contagious respiratory disease whooping cough, caused by Bordetella pertussis, remain elusive. Characterizing the antibody response to this pathogen is essential towards identifying potential CoPs. Here, we evaluate levels, avidity and functionality of B. pertussis-specific-antibodies from paired plasma samples derived from symptomatic and recovered pertussis patients, as well as controls. Natural infection is expected to induce protective immunity. IgG levels and avidity to nine B. pertussis antigens were determined using a novel multiplex panel. Furthermore, opsonophagocytosis of a B. pertussis clinical isolate by neutrophils was measured. Findings indicate that following infection, B. pertussis-specific antibody levels of (ex-) pertussis patients waned, while the avidity of antibodies directed against the majority of studied antigens increased. Opsonophagocytosis indices decreased upon recovery, but remained higher than controls. Random forest analysis of all the data revealed that 28% of the opsonophagocytosis index variances could be explained by filamentous hemagglutinin- followed by pertussis toxin-specific antibodies. We propose to further explore which other B. pertussis-specific antibodies can better predict opsonophagocytosis. Moreover, other B. pertussis-specific antibody functions as well as the possible integration of these functions in combination with other immune cell properties should be evaluated towards the identification of CoPs against pertussis
A statistical modelling approach for source attribution meta-analysis of sporadic infection with foodborne pathogens
Numerous source attribution studies for foodborne pathogens based on epidemiological and microbiological methods are available. These studies provide empirical data for modelling frameworks that synthetize the quantitative evidence at our disposal and reduce reliance on expert elicitations. Here, we develop a statistical model within a Bayesian estimation framework to integrate attribution estimates from expert elicitations with estimates from microbial subtyping and case-control studies for sporadic infections with four major bacterial zoonotic pathogens in the Netherlands (Campylobacter, Salmonella, Shiga toxin-producing E. coli [STEC] O157 and Listeria). For each pathogen, we pooled the published fractions of human cases attributable to each animal reservoir from the microbial subtyping studies, accounting for the uncertainty arising from the different typing methods, attribution models, and year(s) of data collection. We then combined the population attributable fractions (PAFs) from the case-control studies according to five transmission pathways (domestic food, environment, direct animal contact, human-human transmission and travel) and 11 groups within the foodborne pathway (beef/lamb, pork, poultry meat, eggs, dairy, fish/shellfish, fruit/vegetables, beverages, grains, composite foods and food handlers/vermin). The attribution estimates were biologically plausible, allowing the human cases to be attributed in several ways according to reservoirs, transmission pathways and food groups. All pathogens were predominantly foodborne, with Campylobacter being mostly attributable to the chicken reservoir, Salmonella to pigs (albeit closely followed by layers), and Listeria and STEC O157 to cattle. Food-wise, the attributions reflected those at the reservoir level in terms of ranking. We provided a modelling solution to reach consensus attribution estimates reflecting the empirical evidence in the literature that is particularly useful for policy-making and is extensible to other pathogens and domains
Test, trace, isolate:Evidence for declining SARS-CoV-2 PCR sensitivity in a clinical cohort
Real-time reverse transcription-polymerase chain reaction (RT-PCR) on upper respiratory tract (URT) samples is the primary method to diagnose SARS-CoV-2 infections and guide public health measures, with a supportive role for serology. We reinforce previous findings on limited sensitivity of PCR testing, and solidify this fact by statistically utilizing a firm basis of multiple tests per individual. We integrate stratifications with respect to several patient characteristics such as severity of disease and time since onset of symptoms. Bayesian statistical modelling was used to retrospectively determine the sensitivity of RT-PCR using SARS-CoV-2 serology in 644 COVID-19-suspected patients with varying degrees of disease severity and duration. The sensitivity of RT-PCR ranged between 80% − 95%; increasing with disease severity, it decreased rapidly over time in mild COVID-19 cases. Negative URT RT-PCR results should be interpreted in the context of clinical characteristics, especially with regard to containment of viral transmission based on ‘test, trace and isolate’
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