200 research outputs found
Super-AGB Stars and their role as Electron Capture Supernova progenitors
We review the lives, deaths and nucleosynthetic signatures of intermediate
mass stars in the range approximately 6.5-12 Msun, which form super-AGB stars
near the end of their lives. We examine the critical mass boundaries both
between different types of massive white dwarfs (CO, CO-Ne, ONe) and between
white dwarfs and supernovae and discuss the relative fraction of super-AGB
stars that end life as either an ONe white dwarf or as a neutron star (or an
ONeFe white dwarf), after undergoing an electron capture supernova. We also
discuss the contribution of the other potential single-star channels to
electron-capture supernovae, that of the failed massive stars. We describe the
factors that influence these different final fates and mass limits, such as
composition, the efficiency of convection, rotation, nuclear reaction rates,
mass loss rates, and third dredge-up efficiency. We stress the importance of
the binary evolution channels for producing electron-capture supernovae. We
discuss recent nucleosynthesis calculations and elemental yield results and
present a new set of s-process heavy element yield predictions. We assess the
contribution from super-AGB star nucleosynthesis in a Galactic perspective, and
consider the (super-)AGB scenario in the context of the multiple stellar
populations seen in globular clusters. A brief summary of recent works on dust
production is included. Lastly we conclude with a discussion of the
observational constraints and potential future advances for study into these
stars on the low mass/high mass star boundary.Comment: 28 pages, 11 figures. Invited review for Publications of the
Astronomical Society of Australia, to be published in special issue on
"Electron Capture Supernovae". Submitte
A biophysical model explains the spontaneous bursting behavior in the developing retina
During early development, waves of activity propagate across the retina and
play a key role in the proper wiring of the early visual system. During the
stage II these waves are triggered by a transient network of neurons, called
Starburst Amacrine Cells (SACs), showing a bursting activity which disappears
upon further maturation. While several models have attempted to reproduce
retinal waves, none of them is able to mimic the rhythmic autonomous bursting
of individual SACs and reveal how these cells change their intrinsic properties
during development. Here, we introduce a mathematical model, grounded on
biophysics, which enables us to reproduce the bursting activity of SACs and to
propose a plausible, generic and robust, mechanism that generates it. The core
parameters controlling repetitive firing are fast depolarizing -gated
calcium channels and hyperpolarizing -gated potassium channels. The
quiescent phase of bursting is controlled by a slow after hyperpolarization
(sAHP), mediated by calcium-dependent potassium channels. Based on a
bifurcation analysis we show how biophysical parameters, regulating calcium and
potassium activity, control the spontaneously occurring fast oscillatory
activity followed by long refractory periods in individual SACs. We make a
testable experimental prediction on the role of voltage-dependent potassium
channels on the excitability properties of SACs and on the evolution of this
excitability along development. We also propose an explanation on how SACs can
exhibit a large variability in their bursting periods, as observed
experimentally within a SACs network as well as across different species, yet
based on a simple, unique, mechanism. As we discuss, these observations at the
cellular level have a deep impact on the retinal waves description.Comment: 25 pages, 13 figures, submitte
Hiding in plain sight - red supergiant imposters? Super-AGB stars
Super Asymptotic Giant Branch (Super-AGB) stars reside in the mass range ˜ 6.5-10 M¿ and bridge the divide between low/intermediate-mass and massive stars. They are characterised by off-centre carbon ignition prior to a thermally pulsing phase which can consist of many tens to even thousands of thermal pulses. With their high luminosities and very large, cool, red stellar envelopes, these stars appear seemingly identical to their slightly more massive red supergiant counterparts. Due to their similarities, super-AGB stars may therefore act as stellar imposters and contaminate red supergiant surveys. The final fate of super-AGB stars is also quite uncertain and depends primarily on the competition between the core growth and mass-loss rates. If the stellar envelope is removed prior to the core reaching ˜ 1.375 M¿, an O-Ne white dwarf will remain, otherwise the star will undergo an electron-capture supernova (EC-SN) leaving behind a neutron star. We determine the relative fraction of super-AGB stars that end life as either an O-Ne white dwarf or as a neutron star, and provide a mass limit for the lowest mass supernova over a broad range of metallicities from the Z=0.02 to 0.0001.Peer ReviewedPostprint (published version
Arsenic Biotransformation as a Cancer Promoting Factor by Inducing DNA Damage and Disruption of Repair Mechanisms
Chronic exposure to arsenic in drinking water poses a major global health concern. Populations exposed to high concentrations of arsenic-contaminated drinking water suffer serious health consequences, including alarming cancer incidence and death rates. Arsenic is biotransformed through sequential addition of methyl groups, acquired from s-adenosylmethionine (SAM). Metabolism of arsenic generates a variety of genotoxic and cytotoxic species, damaging DNA directly and indirectly, through the generation of reactive oxidative species and induction of DNA adducts, strand breaks and cross links, and inhibition of the DNA repair process itself. Since SAM is the methyl group donor used by DNA methyltransferases to maintain normal epigenetic patterns in all human cells, arsenic is also postulated to affect maintenance of normal DNA methylation patterns, chromatin structure, and genomic stability. The biological processes underlying the cancer promoting factors of arsenic metabolism, related to DNA damage and repair, will be discussed here
Biophysical reaction-diffusion model for stage II retinal waves and bifurcations analysis
International audienceRetinal waves are spontaneous waves of spiking activity observed in the retina, during development only,playing a central role in shaping the visual system and retinal circuitry. Understanding how these waves areinitiated and propagate in the retina could enable one to control, guide and predict them in the in vivo adultretina as inducing them is expected to reintroduce some plasticity in the retinal tissue and in the projectionsto the LGN. In this context, we propose a physiologically realistic reaction-diffusion model for the mechanismsof the emergence of stage II cholinergic retinal waves during development. We perform the bifurcation analysiswhen varying two biophysically relevant parameters, the conductances of calcium and potassium g_Ca,g_K respectively. The two main goals of our work are: firstly, reproduce the experimental recordings ofdevelopmental retinal waves by simulating our model and secondly, explore the different dynamical behavioursobserved when varying these two parameters
Strategic approaches to informing the public about biotechnology in Latin America
The benefits of today's biotechnology products are not evident to
consumers. The public will accept biotechnology only when individuals
decide for themselves that biotec products will contribute to their
personal well-being. To make such a decision, people will need greater
awareness and understanding of how biotechnology will affect the
environment, human health, local and national economies, and the
well-being of society. A low level of awareness and understanding about
biotechnology is characteristic of Latin America and the Caribbean
countries, as elsewhere, efforts to remedy poor public perception often
seem inadequate and do not reflect a well-designed strategy. In order
to improve the understanding of the biotechnology and their human
applications, a strategic plan for public communications is required.
Specific objectives for this initiative may include: (1) to make
evident to decision makers that modern biotechnology can be an
effective tool for increasing agricultural productivity, and thereby
economic growth, without imposing unacceptable risk to the environment
or human and animal health; (2) to enable members of the public to make
informed decisions about appropriate uses of biotechnology by providing
accurate information about benefits, risks and impacts; or (3) to
incorporate modern biotechnology into science curricula for secondary
schools, university and college students, and agriculture extension
officers. A variety of specialized expertise, including communication
specialists, technical writers, graphic artists and illustrators to
design information materials and conduct training is needed to
implement this. Ideally, members bring expertise in biotechnology and
biosafety, public communications and project management. The plan will
need to identify scientists and technical experts who can provide
expertise in science writing for general audiences, advertising,
graphic arts, public opinion polling and media communications. These
people can provide basic information about the techniques of modern
biotechnology; the products now available and those being developed;
what is known about the nature, probability and consequences of
potential risks. Governments, industry, universities and media must
play an important role to improve public perception about
biotechnology, this is a requirement to develop biotechnology in the
Region
Super and massive AGB stars - IV. Final fates - Initial to final mass relation
We explore the final fates of massive intermediate-mass stars by computing
detailed stellar models from the zero age main sequence until near the end of
the thermally pulsing phase. These super-AGB and massive AGB star models are in
the mass range between 5.0 and 10.0 Msun for metallicities spanning the range
Z=0.02-0.0001. We probe the mass limits M_up, M_n and M_mass, the minimum
masses for the onset of carbon burning, the formation of a neutron star, and
the iron core-collapse supernovae respectively, to constrain the white
dwarf/electron-capture supernova boundary. We provide a theoretical initial to
final mass relation for the massive and ultra-massive white dwarfs and specify
the mass range for the occurrence of hybrid CO(Ne) white dwarfs. We predict
electron-capture supernova (EC-SN) rates for lower metallicities which are
significantly lower than existing values from parametric studies in the
literature. We conclude the EC-SN channel (for single stars and with the
critical assumption being the choice of mass-loss rate) is very narrow in
initial mass, at most approximately 0.2 Msun. This implies that between ~ 2-5
per cent of all gravitational collapse supernova are EC-SNe in the metallicity
range Z=0.02 to 0.0001. With our choice for mass-loss prescription and computed
core growth rates we find, within our metallicity range, that CO cores cannot
grow sufficiently massive to undergo a Type 1.5 SN explosion.Comment: 15 pages, 7 figures, accepted for publication in MNRA
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