155 research outputs found
The surface carbon and nitrogen abundances in models of ultra metal-poor stars
We investigate whether the observed high number of carbon- and
nitrogen-enhanced extremely metal-poor stars could be explained by peculiar
evolutionary properties during the core He flash at the tip of the red giant
branch. For this purpose we compute a series of detailed stellar models
expanding upon our previous work; in particular, we investigate if during the
major He flash the penetration of the helium convective zone into the overlying
hydrogen-rich layers can produce carbon- and nitrogen-rich abundances in
agreement with current spectroscopic observations. The dependence of this
phenomenon on selected model input parameters, such as initial metallicity and
treatment of convection is examined in detail.Comment: 8 pages, 4 figures, submitted to A&
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
Astrophysical reaction rate for Be by photodisintegration
We study the astrophysical reaction rate for the formation of Be
through the three body reaction . This reaction is one
of the key reactions which could bridge the mass gap at A = 8 nuclear systems
to produce intermediate-to-heavy mass elements in alpha- and neutron-rich
environments such as r-process nucleosynthesis in supernova explosions,
s-process nucleosynthesis in asymptotic giant branch (AGB) stars, and
primordial nucleosynthesis in baryon inhomogeneous cosmological models. To
calculate the thermonuclear reaction rate in a wide range of temperatures, we
numerically integrate the thermal average of cross sections assuming a
two-steps formation through a metastable Be. Off-resonant and on-resonant
contributions from the ground state in Be are taken into account. As
input cross section, we adopt the latest experimental data by
photodisintegration of Be with laser-electron photon beams, which covers
all relevant resonances in Be. We provide the reaction rate for
Be in the temperature range from T=10
to T=10 both in the tabular form and in the analytical form. The
calculated reaction rate is compared with the reaction rates of the CF88 and
the NACRE compilations. The CF88 rate is valid at due to lack
of the off-resonant contribution. The CF88 rate differs from the present rate
by a factor of two in a temperature range . The NACRE rate,
which adopted different sources of experimental information on resonance states
in Be, is 4--12 times larger than the present rate at ,
but is consistent with the present rate to within at .Comment: 32 pages (incl 6 figures), Nucl. Phys. in pres
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
Flame fronts in Supernovae Ia and their pulsational stability
The structure of the deflagration burning front in type Ia supernovae is
considered. The parameters of the flame are obtained: its normal velocity and
thickness. The results are in good agreement with the previous works of
different authors. The problem of pulsation instability of the flame, subject
to plane perturbations, is studied. First, with the artificial system with
switched-off hydrodynamics the possibility of secondary reactions to stabilize
the front is shown. Second, with account of hydrodynamics, realistic EOS and
thermal conduction we can obtain pulsations when Zeldovich number was
artificially increased. The critical Zeldovich numbers are presented. These
results show the stability of the flame in type Ia supernovae against
pulsations because its effective Zeldovich number is small.Comment: 12 pages, 11 figure
Thermonuclear Kinetics in Astrophysics
Over the billions of years since the Big Bang, the lives, deaths and
afterlives of stars have enriched the Universe in the heavy elements that make
up so much of ourselves and our world. This review summarizes the methods used
to evolve these nuclear abundances within astrophysical simulations. These
methods fall into 2 categories; evolution via rate equations and via
equilibria. Because the rate equations in nucleosynthetic applications involve
a wide range of timescales, implicit methods have proven mandatory, leading to
the need to solve matrix equations. Efforts to improve the performance of such
rate equation methods are focused on efficient solution of these matrix
equations, in particular by making best use of the sparseness of these
matrices, and finding methods that require less frequent matrix solutions.
Recent work to produce hybrid schemes which use local equilibria to reduce the
computational cost of the rate equations is also discussed. Such schemes offer
significant improvements in the speed of reaction networks and are accurate
under circumstances where calculations which assume complete equilibrium fail.Comment: 27 pages, 2 figures, a review for a special issue of Nuclear Physics
Comparison of low--energy resonances in 15N(alpha,gamma)19F and 15O(alpha,gamma)19Ne and related uncertainties
A disagreement between two determinations of Gamma_alpha of the astro-
physically relevant level at E_x=4.378 MeV in 19F has been stated in two recent
papers by Wilmes et al. and de Oliveira et al. In this work the uncertainties
of both papers are discussed in detail, and we adopt the value
Gamma_alpha=(1.5^{+1.5}_{-0.8})10^-9eV for the 4.378 MeV state. In addition,
the validity and the uncertainties of the usual approximations for mirror
nuclei Gamma_gamma(19F) approx Gamma_gamma(19Ne), theta^2_alpha(19F) approx
theta^2_alpha(19Ne) are discussed, together with the resulting uncertainties on
the resonance strengths in 19Ne and on the 15O(alpha,gamma)19Ne rate.Comment: 9 pages, Latex, To appear in Phys. Rev.
CLES, Code Liegeois d'Evolution Stellaire
Cles is an evolution code recently developed to produce stellar models
meeting the specific requirements of studies in asteroseismology. It offers the
users a lot of choices in the input physics they want in their models and its
versatility allows them to tailor the code to their needs and implement easily
new features. We describe the features implemented in the current version of
the code and the techniques used to solve the equations of stellar structure
and evolution. A brief account is given of the use of the program and of a
solar calibration realized with it.Comment: Comments: 8 pages, Astrophys. Space Sci. CoRoT-ESTA Volume, in the
pres
Primordial nucleosynthesis with a varying fine structure constant: An improved estimate
We compute primordial light-element abundances for cases with fine structure
constant alpha different from the present value, including many sources of
alpha dependence neglected in previous calculations. Specifically, we consider
contributions arising from Coulomb barrier penetration, photon coupling to
nuclear currents, and the electromagnetic components of nuclear masses. We find
the primordial abundances to depend more weakly on alpha than previously
estimated, by up to a factor of 2 in the case of ^7Li. We discuss the
constraints on variations in alpha from the individual abundance measurements
and the uncertainties affecting these constraints. While the present best
measurements of primordial D/H, ^4He/H, and ^7Li/H may be reconciled pairwise
by adjusting alpha and the universal baryon density, no value of alpha allows
all three to be accommodated simultaneously without consideration of systematic
error. The combination of measured abundances with observations of acoustic
peaks in the cosmic microwave background favors no change in alpha within the
uncertainties.Comment: Phys. Rev. D accepted version; minor changes in response to refere
An asteroseismic study of the beta Cephei star 12 Lacertae: multisite spectroscopic observations, mode identification and seismic modelling
We present the results of a spectroscopic multisite campaign for the beta
Cephei star 12 (DD) Lacertae. Our study is based on more than thousand
high-resolution high S/N spectra gathered with 8 different telescopes in a time
span of 11 months. In addition we make use of numerous archival spectroscopic
measurements. We confirm 10 independent frequencies recently discovered from
photometry, as well as harmonics and combination frequencies. In particular,
the SPB-like g-mode with frequency 0.3428 1/d reported before is detected in
our spectroscopy. We identify the four main modes as (l1,m1) = (1, 1), (l2,m2)
= (0, 0), (l3,m3) = (1, 0) and (l4,m4) = (2, 1) for f1 = 5.178964 1/d, f2 =
5.334224 1/d, f3 = 5.066316 1/d and f4 = 5.490133 1/d, respectively. Our
seismic modelling shows that f2 is likely the radial first overtone and that
the core overshooting parameter alpha_ov is lower than 0.4 local pressure scale
heights.Comment: 16 pages, 11 figures, accepted in MNRA
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