142 research outputs found
On the mass of supernova progenitors: the role of the CC reaction
A precise knowledge of the masses of supernova progenitors is essential to
answer various questions of modern astrophysics, such as those related to the
dynamical and chemical evolution of Galaxies. In this paper we revise the upper
bound for the mass of the progenitors of CO white dwarfs (\mup) and the lower
bound for the mass of the progenitors of normal type II supernovae (\mups). In
particular, we present new stellar models with mass between 7 and 10 \msun,
discussing their final destiny and the impact of recent improvements in our
understanding of the low energy rate of the \c12c12 reaction.Comment: To be published on the proceedings of NIC 201
Teaching English Language Arts Methods in the United States: A Review of the Research
This is the author's accepted manuscript. Copyright 2014 WileyWhat is the state of the English education methods course in the 21st century? Summarizing the research in English teacher education since the last major study (Smagorinsky & Whiting, 1995) of how English teachers are prepared, the authors review the state of the profession to examine trends in the field since the recent revision of the NCTE guidelines for teacher preparation, the redefinition of what constitutes methods coursework in and across programs, the rising numbers of culturally and linguistically diverse learners, the demands of assessment and accountability, and the integration of the field experience with content. The authors review research related to teaching reading strategies, integrating fieldwork with English education coursework, addressing standards in planning and teaching, meeting the needs of English language learners, and teaching with and about technology to determine how English teacher education is adapting to the demands of educating English teachers in the 21st century
Multidimensional hydrodynamic simulations of the hydrogen injection flash
The injection of hydrogen into the convection shell powered by helium burning
during the core helium flash is commonly encountered during the evolution of
metal-free and extremely metal-poor low-mass stars. With specifically designed
multidimensional hydrodynamic simulations, we aim to prove that an entropy
barrier is no obstacle for the growth of the helium-burning shell convection
zone in the helium core of a metal-rich Pop I star, i.e. convection can
penetrate into the hydrogen-rich layers for these stars, too. We further study
whether this is also possible in one-dimensional stellar evolutionary
calculations. Our hydrodynamical simulations show that the helium-burning shell
convection zone in the helium core moves across the entropy barrier and reaches
the hydrogen-rich layers. This leads to mixing of protons into the hotter
layers of the core and to a rapid increase of the nuclear energy production at
the upper edge of the helium-burning convection shell - the hydrogen injection
flash. As a result a second convection zone appears in the hydrogen-rich
layers. Contrary to 1D models, the entropy barrier separating the two
convective shells from each other is largely permeable to chemical transport
when allowing for multidimensional flow, and consequently, hydrogen is
continuously mixed deep into the helium core. We find it difficult to achieve
such a behavior in one-dimensional stellar evolutionary calculations.Comment: 8 pages, 8 figures - accepted for publication in Astronomy and
Astrophysics. Animations related to the manuscript can be downloaded from
http://www-astro.ulb.ac.be/~mocak/index.php/Main/AnimationsHeFlas
Non-resonant direct p- and d-wave neutron capture by 12C
Discrete gamma-rays from the neutron capture state of 13C to its low-lying
bound states have been measured using pulsed neutrons at En = 550 keV. The
partial capture cross sections have been determined to be 1.7+/-0.5,
24.2+/-1.0, 2.0+/-0.4 and 1.0+/-0.4 microb for the ground (1/2-), first (1/2+),
second (3/2-) and third (5/2+) excited states, respectively. From a comparison
with theoretical predictions based on the non-resonant direct radiative capture
mechanism, we could determine the spectroscopic factor for the 1/2+ state to be
0.80 +/- 0.04, free from neutron-nucleus interaction ambiguities in the
continuum. In addition we have detected the contribution of the non-resonant
d-wave capture component in the partial cross sections for transitions leading
to the 1/2- and 3/2- states. While the s-wave capture dominates at En < 100
keV, the d-wave component turns out to be very important at higher energies.
From the present investigation the 12C(n,gamma)13C reaction rate is obtained
for temperatures in the range 10E+7 - 10E+10 K.Comment: Accepted for publication in Phys. Rev. C. - 16 pages + 8 figure
Pulsations of massive ZZ Ceti stars with carbon/oxygen and oxygen/neon cores
We explore the adiabatic pulsational properties of massive white dwarf stars
with hydrogen-rich envelopes and oxygen/neon and carbon/oxygen cores. To this
end, we compute the cooling of massive white dwarf models for both core
compositions taking into account the evolutionary history of the progenitor
stars and the chemical evolution caused by time-dependent element diffusion. In
particular, for the oxygen/neon models, we adopt the chemical profile resulting
from repeated carbon-burning shell flashes expected in very massive white dwarf
progenitors. For carbon/oxygen white dwarfs we consider the chemical profiles
resulting from phase separation upon crystallization. For both compositions we
also take into account the effects of crystallization on the oscillation
eigenmodes. We find that the pulsational properties of oxygen/neon white dwarfs
are notably different from those made of carbon/oxygen, thus making
asteroseismological techniques a promising way to distinguish between both
types of stars and, hence, to obtain valuable information about their
progenitors.Comment: 11 pages, including 11 postscript figures. Accepted for publication
in Astronomy and Astrophysic
The s-process weak component: uncertainties due to convective overshooting
Using a new s-nucleosynthesis code, coupled with the stellar evolution code
Star2003, we performed simulations to study the impact of the convection
treatment on the s-process during core He-burning of a 25 Msun star (ZAMS mass)
with an initial metallicity of Z=0.02. Particular attention was devoted to the
impact of the extent of overshooting on the s-process efficiency. The results
show enhancements of about a factor 2-3 in s-process efficiency (measured as
the average overproduction factor of the 6 s-only nuclear species with
) with overshooting parameter values in the range
0.01-0.035, compared to results obtained with the same model but without
overshooting. The impact of these results on the p-process model based on type
II supernovae is discussed.Comment: 7 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Asteroseismology of the Kepler V777 Her variable white dwarf with fully evolutionary models
DBV stars are pulsating white dwarfs with atmospheres rich in He.
Asteroseismology of DBV stars can provide valuable clues about the origin,
structure and evolution of hydrogen-deficient white dwarfs, and may allow to
study neutrino and axion physics. Recently, a new DBV star, KIC 8626021, has
been discovered in the field of the \emph{Kepler} spacecraft. It is expected
that further monitoring of this star in the next years will enable astronomers
to determine its detailed asteroseismic profile. We perform an
asteroseismological analysis of KIC 8626021 on the basis of fully evolutionary
DB white-dwarf models. We employ a complete set of evolutionary DB white-dwarf
structures covering a wide range of effective temperatures and stellar masses.
They have been obtained on the basis of a complete treatment of the
evolutionary history of progenitors stars. We compute g-mode adiabatic
pulsation periods for this set of models and compare them with the pulsation
properties exhibited by KIC 8626021. On the basis of the mean period spacing of
the star, we found that the stellar mass should be substantially larger than
spectroscopy indicates. From period-to-period fits we found an
asteroseismological model characterized by an effective temperature much higher
than the spectroscopic estimate. In agreement with a recent asteroseismological
analysis of this star by other authors, we conclude that KIC 8626021 is located
near the blue edge of the DBV instability strip, contrarily to spectroscopic
predictions. We also conclude that the mass of KIC 8626021 should be
substantially larger than thought.Comment: 7 pages, 5 figures, 3 tables. To be published in Astronomy and
Astrophysic
Type Ia supernovae and the ^{12}C+^{12}C reaction rate
The experimental determination of the cross-section of the ^{12}C+^{12}C
reaction has never been made at astrophysically relevant energies (E<2 MeV).
The profusion of resonances throughout the measured energy range has led to
speculation that there is an unknown resonance at E\sim1.5 MeV possibly as
strong as the one measured for the resonance at 2.14 MeV. We study the
implications that such a resonance would have for the physics of SNIa, paying
special attention to the phases that go from the crossing of the ignition curve
to the dynamical event. We use one-dimensional hydrostatic and hydrodynamic
codes to follow the evolution of accreting white dwarfs until they grow close
to the Chandrasekhar mass and explode as SNIa. In our simulations, we account
for a low-energy resonance by exploring the parameter space allowed by
experimental data. A change in the ^{12}C+^{12}C rate similar to the one
explored here would have profound consequences for the physical conditions in
the SNIa explosion, namely the central density, neutronization, thermal
profile, mass of the convective core, location of the runaway hot spot, or time
elapsed since crossing the ignition curve. For instance, with the largest
resonance strength we use, the time elapsed since crossing the ignition curve
to the supernova event is shorter by a factor ten than for models using the
standard rate of ^{12}C+^{12}C, and the runaway temperature is reduced from
\sim8.14\times10^{8} K to \sim4.26\times10^{8} K. On the other hand, a
resonance at 1.5 MeV, with a strength ten thousand times smaller than the one
measured at 2.14 MeV, but with an {\alpha}/p yield ratio substantially
different from 1 would have a sizeable impact on the degree of neutronization
of matter during carbon simmering. We conclude that a robust understanding of
the links between SNIa properties and their progenitors will not be attained
until the ^{12}C+^{12}C reaction rate is measured at energies \sim1.5 MeV.Comment: 15 pages, 6 tables, 10 figures, accepted for Astronomy and
Astrophysic
Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster
Rotational evolution in the pre-main sequence (PMS) is described with new
sets of PMS evolutionary tracks including rotation, non-gray boundary
conditions (BCs) and either low (LCE) or high convection efficiency (HCE).
Using observational data and our theoretical predictions, we aim at
constraining 1) the differences obtained for the rotational evolution of stars
within the ONC by means of these different sets of models; 2) the initial
angular momentum of low mass stars, by means of their templates in the ONC. We
discuss the reliability of current stellar models for the PMS. While the 2D
radiation hydrodynamic simulations predict HCE in PMS, semi-empirical
calibrations either seem to require that convection is less efficient in PMS
than in the following MS phase or are still contradictory. We derive stellar
masses and ages for the ONC by using both LCE and HCE. The resulting mass
distribution for the bulk of the ONC population is in the range 0.20.3
{\msun} for our non-gray models and in the range 0.10.3{\msun} for models
having gray BCs. In agreement with Herbst et al. (2002) we find that a large
percentage (70%) of low-mass stars (M\simlt 0.5{\msun} for LCE;
M\simlt0.35{\msun} for HCE) in the ONC appears to be fast rotators (P4days).
Three possibilities are open: 1) 70% of the ONC low mass stars lose their
disk at early evolutionary phases; 2)their locking period is shorter; 3) the
period evolution is linked to a different morphology of the magnetic fields of
the two groups of stars. We also estimate the range of initial angular momentum
consistent with the observed periods. The comparisons made indicate that a
second parameter is needed to describe convection in the PMS, possibly related
to the structural effect of a dynamo magnetic field.Comment: 17 pages, 11 figure
Evolution of a 3 \msun star from the main sequence to the ZZ Ceti stage: the role played by element diffusion
The purpose of this paper is to present new full evolutionary calculations
for DA white dwarf stars with the major aim of providing a physically sound
reference frame for exploring the pulsation properties of the resulting models
in future communications. Here, white dwarf evolution is followed in a
self-consistent way with the predictions of time dependent element diffusion
and nuclear burning. In addition, full account is taken of the evolutionary
stages prior to the white dwarf formation. In particular, we follow the
evolution of a 3 \msun model from the zero-age main sequence (the adopted
metallicity is Z=0.02) all the way from the stages of hydrogen and helium
burning in the core up to the thermally pulsing phase. After experiencing 11
thermal pulses, the model is forced to evolve towards its white dwarf
configuration by invoking strong mass loss episodes. Further evolution is
followed down to the domain of the ZZ Ceti stars on the white dwarf cooling
branch. Emphasis is placed on the evolution of the chemical abundance
distribution due to diffusion processes and the role played by hydrogen burning
during the white dwarf evolution. Furthermore, the implications of our
evolutionary models for the main quantities relevant for adiabatic pulsation
analysis are discussed. Interestingly, the shape of the Ledoux term is markedly
smoother as compared with previous detailed studies of white dwarfs. This is
translated into a different behaviour of the Brunt-Vaisala frequency.Comment: 11 pages, 11 figures, accepted for publication in MNRA
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