131 research outputs found
The Structure of 1-\u3cem\u3etert\u3c/em\u3e-butyl-2,2,3,4,4-pentamethylphosphetane 1-oxide, C\u3csub\u3e12\u3c/sub\u3eH\u3csub\u3e25\u3c/sub\u3eOP
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
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
Influence of two updated nuclear reaction rates on the evolution of low and intermediate mass stars
Two key reactions of hydrostatic nuclear burning in stars have recently been
revised by new experimental data - the 14N(p,gamma)15O and triple alpha
reaction rates. We investigate how much the new rates influence the evolution
of low-mass, metal-poor and metal-free, stars and of an intermediate-mass star
of solar-type composition. We concentrate on phases of helium ignition or
thermally unstable helium burning. Our global result is that the new triple
alpha rate has no significant influence on such stars, but that there is a
noticable, though small, effect of the new 14N(p,gamma)15O rate, in particular
on the core helium flash and the blue loop during core helium burning in the
intermediate-mass star.Comment: shortened version; accepted as a Research Note by Astronomy &
Astrophysic
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
Non-conservative Evolution of Cataclysmic Variables
We suggest a new mechanism to account for the loss of angular momentum in
binaries with non-conservative mass exchange. It is shown that in some cases
the loss of matter can result in increase of the orbital angular momentum of a
binary. If included into consideration in evolutionary calculations, this
mechanism appreciably extends the range of mass ratios of components for which
mass exchange in binaries is stable. It becomes possible to explain the
existence of some observed cataclysmic binaries with high donor/accretor mass
ratio, which was prohibited in conservative evolution models.Comment: LaTeX, 32 pages, to be published in Astron. Z
Evolutionary and pulsational properties of low-mass white dwarf stars with oxygen cores resulting from close binary evolution
The present work is designed to explore the evolutionary and pulsational
properties of low-mass white dwarfs with carbon/oxygen cores. In particular, we
follow the evolution of a 0.33 Msun white dwarf remnant in a self-consistent
way with the predictions of nuclear burning, element diffusion and the history
of the white dwarf progenitor. Attention is focused on the occurrence of
hydrogen shell flashes induced by diffusion processes during cooling phases.
The evolutionary stages prior to the white dwarf formation are also fully
accounted for by computing the conservative binary evolution of an initially
2.5-Msun Pop. I star with a 1.25 Msun companion, and period P_i= 3 days.
Evolution is followed down to the domain of the ZZ Ceti stars on the white
dwarf cooling branch. We find that chemical diffusion induces the occurrence of
an additional hydrogen thermonuclear flash which leads to stellar models with
thin hydrogen envelopes. As a result, a fast cooling is encountered at advanced
stages of evolution. In addition, we explore the adiabatic pulsational
properties of the resulting white dwarf models. As compared with their
helium-core counterparts, low-mass oxygen-core white dwarfs are characterized
by a pulsational spectrum much more featured, an aspect which could eventually
be used for distinguishing both types of stars if low-mass white dwarfs were in
fact found to pulsate as ZZ Ceti-type variables. Finally, we perform a
non-adiabatic pulsational analysis on the resulting carbon/oxygen low-mass
white dwarf models.Comment: 13 Pages, including 16 Postscript figures. Accepted for publication
in MNRA
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
Constraining fundamental stellar parameters using seismology. Application to Alpha Centauri AB
We apply the Levenberg-Marquardt minimization algorithm to seismic and
classical observables of the Alpha Cen binary system in order to derive the
fundamental parameters of Alpha Cen A+B and to analyze the dependence of these
parameters on the chosen observables, on their uncertainty and on the physics
used in stellar modelling. The seismological data are those by Bouchy & Carrier
(2002) for Alpha Cen A, and those by Carrier & Bourban (2003) for Alpha Cen B.
We show that while the fundamental stellar parameters do not depend on the
treatment of convection adopted (Mixing Length Theory -- MLT -- or ``Full
Spectrum of Turbulence'' -- FST), the age of the system depends on the
inclusion of gravitational settling, and is deeply biased by the small
frequency separation of component B.
We try to answer the question of the universality of the mixing length
parameter, and we find a statistically reliable dependence of the
alpha--parameter on the HR diagram location (with a trend similar to the one
predicted by Ludwig et al.1999).
We propose the frequency separation ratios introduced by Roxburgh &
Voronstsov (2003) as better observables to determine the fundamental stellar
parameters, and to use the large frequency separation and frequencies to
extract information about the stellar structure. The effects of diffusion and
equation of state on the oscillation frequencies are also studied, but present
seismic data do not allow their detection.Comment: 15 pages, 8 figures, accepted by A&
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.
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