269 research outputs found
Degenerative mitral valve regurgitation: best practice revolution
Degenerative mitral valve disease often leads to leaflet prolapse due to chordal elongation or rupture, and resulting in mitral valve regurgitation. Guideline referral for surgical intervention centres primarily on symptoms and ventricular dysfunction. The recommended treatment for degenerative mitral valve disease is mitral valve reconstruction, as opposed to valve replacement with a bioprosthetic or mechanical valve, because valve repair is associated with improved event free survival. Recent studies have documented a significant number of patients are not referred in a timely fashion according to established guidelines, and when they are subjected to surgery, an alarming number of patients continue to undergo mitral valve replacement. The debate around appropriate timing of intervention for asymptomatic severe mitral valve regurgitation has put additional emphasis on targeted surgeon referral and the need to ensure a very high rate of mitral valve repair, particularly in the non-elderly population. Current clinical practice remains suboptimal for many patients, and this review explores the need for a ‘best practice revolution' in the field of degenerative mitral valve regurgitatio
Nucleosynthetic Yields from "Collapsars"
The "collapsar" engine for gamma-ray bursts invokes as its energy source the
failure of a normal supernova and the formation of a black hole. Here we
present the results of the first three-dimensional simulation of the collapse
of a massive star down to a black hole, including the subsequent accretion and
explosion. The explosion differs significantly from the axisymmetric scenario
obtained in two-dimensional simulations; this has important consequences for
the nucleosynthetic yields. We compare the nucleosynthetic yields to those of
hypernovae. Calculating yields from three-dimensional explosions requires new
strategies in post-process nucleosynthesis; we discuss NuGrid's plan for
three-dimensional yields.Comment: To appear in the Conference Proceedings for the "10th Symposium on
Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island,
Michigan, US
Difficulties in Probing Nuclear Physics: A Study of Ti and Ni
The nucleosynthetic yield from a supernova explosion depends upon a variety
of effects: progenitor evolution, explosion process, details of the nuclear
network, and nuclear rates. Especially in studies of integrated stellar yields,
simplifications reduce these uncertainties. But nature is much more complex,
and to actually study nuclear rates, we will have to understand the full,
complex set of processes involved in nucleosynthesis. Here we discuss a few of
these complexities and detail how the NuGrid collaboration will address them.Comment: To appear in the Conference Proceedings for the "10th Symposium on
Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island,
Michigan, US
Complete nucleosynthesis calculations for low-mass stars from NuGrid
Many nucleosynthesis and mixing processes of low-mass stars as they evolve
from the Main Sequence to the thermal-pulse Asymptotic Giant Branch phase
(TP-AGB) are well understood (although of course important physics components,
e.g. rotation, magnetic fields, gravity wave mixing, remain poorly known).
Nevertheless, in the last years presolar grain measurements with high
resolution have presented new puzzling problems and strong constraints on
nucleosynthesis processes in stars. The goal of the NuGrid collaboration is to
present uniform yields for a large range of masses and metallicities, including
lowmass stars and massive stars and their explosions. Here we present the
first calculations of stellar evolution and high-resolution, post-processing
simulations of an AGB star with an initial mass of 2 M_sun and solar-like
metallicity (Z=0.01), based on the post-processing code PPN. In particular, we
analyze the formation and evolution of the radiative 13C-pocket between the
17th TP and the 18th TP. The s-process nucleosynthesis profile of a sample of
heavy isotopes is also discussed, before the next convective TP occurrence.Comment: To appear in the Conference Proceedings for the "10th Symposium on
Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island,
Michigan, US
Nucleosynthesis Calculations from Core-Collapse Supernovae
We review some of the uncertainties in calculating nucleosynthetic yields,
focusing on the explosion mechanism. Current yield calculations tend to either
use a piston, energy injection, or enhancement of neutrino opacities to drive
an explosion. We show that the energy injection, or more accurately, an entropy
injection mechanism is best-suited to mimic our current understanding of the
convection-enhanced supernova engine. The enhanced neutrino-opacity technique
is in qualitative disagreement with simulations of core-collapse supernovae and
will likely produce errors in the yields. But piston-driven explosions are the
most discrepant. Piston-driven explosion severely underestimate the amount of
fallback, leading to order-of-magnitude errors in the yields of heavy elements.
To obtain yields accurate to the factor of a few level, we must use entropy or
energy injection and this has become the NuGrid collaboration approach.Comment: To appear in the Conference Proceedings for the "10th Symposium on
Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island,
Michigan, US
44Ti and 56Ni in core-collapse supernovae
We investigate the physical conditions where 44Ti and 56Ni are created in
core-collapse supernovae. In this preliminary work we use a series of
post-processing network calculations with parametrized expansion profiles that
are representative of the wide range of temperatures, densities and
electron-to-baryon ratios found in 3D supernova simulations. Critical flows
that affect the final yields of 44Ti and 56Ni are assessed.Comment: To appear in the Conference Proceedings for the "10th Symposium on
Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island,
Michigan, US
The neutron capture process in the He shell in core-collapse supernovae: Presolar silicon carbide grains as a diagnostic tool for nuclear astrophysics
Carbon-rich presolar grains are found in primitive meteorites, with isotopic measurements to date suggesting a core-collapse supernovae origin site for some of them. This holds for about 1-2 % of presolar silicon carbide (SiC) grains, so-called Type X and C grains, and about 30 % of presolar graphite grains. Presolar SiC grains of Type X show anomalous isotopic signatures for several elements heavier than iron compared to the solar abundances: most notably for strontium, zirconium, molybdenum, ruthenium and barium. We study the nucleosynthesis of zirconium and molybdenum isotopes in the He-shell of three core-collapse supernovae models of 15, 20 and 25 M☉ with solar metallicity, and compare the results to measurements of presolar grains. We find the stellar models show a large scatter of isotopic abundances for zirconium and molybdenum, but the mass averaged abundances are qualitatively similar to the measurements. We find all models show an excess of 96Zr relative to the measurements, but the model abundances are affected by the fractionation between Sr and Zr since a large contribution to 90Zr is due to the radiogenic decay of 90Sr. Some supernova models show excesses of 95,97Mo and depletion of 96Mo relative to solar. The mass averaged distribution from these models shows an excess of 100Mo, but this may be alleviated by very recent neutron-capture cross section measurements. We encourage future explorations to assess the impact of the uncertainties in key neutron-capture reaction rates that lie along the n-process path
Advanced burning stages and fate of 8-10 Mo stars
The stellar mass range 8<M/Mo<12 corresponds to the most massive AGB stars
and the most numerous massive stars. It is host to a variety of supernova
progenitors and is therefore very important for galactic chemical evolution and
stellar population studies. In this paper, we study the transition from
super-AGB star to massive star and find that a propagating neon-oxygen burning
shell is common to both the most massive electron capture supernova (EC-SN)
progenitors and the lowest mass iron-core collapse supernova (FeCCSN)
progenitors. Of the models that ignite neon burning off-center, the 9.5Mo model
would evolve to an FeCCSN after the neon-burning shell propagates to the
center, as in previous studies. The neon-burning shell in the 8.8Mo model,
however, fails to reach the center as the URCA process and an extended (0.6 Mo)
region of low Ye (0.48) in the outer part of the core begin to dominate the
late evolution; the model evolves to an EC-SN. This is the first study to
follow the most massive EC-SN progenitors to collapse, representing an
evolutionary path to EC-SN in addition to that from SAGB stars undergoing
thermal pulses. We also present models of an 8.75Mo super-AGB star through its
entire thermal pulse phase until electron captures on 20Ne begin at its center
and of a 12Mo star up to the iron core collapse. We discuss key uncertainties
and how the different pathways to collapse affect the pre-supernova structure.
Finally, we compare our results to the observed neutron star mass distribution.Comment: 20 pages, 14 figures, 1 table. Submitted to ApJ 2013 February 19;
accepted 2013 June
Spatial Distribution of Nucleosynthesis Products in Cassiopeia A: Comparison Between Observations and 3D Explosion Models
We examine observed heavy element abundances in the Cassiopeia A supernova
remnant as a constraint on the nature of the Cas A supernova. We compare bulk
abundances from 1D and 3D explosion models and spatial distribution of elements
in 3D models with those derived from X-ray observations. We also examine the
cospatial production of 26Al with other species. We find that the most reliable
indicator of the presence of 26Al in unmixed ejecta is a very low S/Si ratio
(~0.05). Production of N in O/S/Si-rich regions is also indicative. The
biologically important element P is produced at its highest abundance in the
same regions. Proxies should be detectable in supernova ejecta with high
spatial resolution multiwavelength observations.Comment: To appear in the Conference Proceedings for the "10th Symposium on
Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island,
Michigan, US
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