75 research outputs found
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
Recent Advances in Modeling Stellar Interiors
Advances in stellar interior modeling are being driven by new data from
large-scale surveys and high-precision photometric and spectroscopic
observations. Here we focus on single stars in normal evolutionary phases; we
will not discuss the many advances in modeling star formation, interacting
binaries, supernovae, or neutron stars. We review briefly: 1) updates to input
physics of stellar models; 2) progress in two and three-dimensional evolution
and hydrodynamic models; 3) insights from oscillation data used to infer
stellar interior structure and validate model predictions (asteroseismology).
We close by highlighting a few outstanding problems, e.g., the driving
mechanisms for hybrid gamma Dor/delta Sct star pulsations, the cause of giant
eruptions seen in luminous blue variables such as eta Car and P Cyg, and the
solar abundance problem.Comment: Proceedings for invited talk at conference High Energy Density
Laboratory Astrophysics 2010, Caltech, March 2010, submitted for special
issue of Astrophysics and Space Science; 7 pages; 5 figure
Helium accreting CO white dwarfs with rotation: helium novae instead of double detonation
We present evolutionary models of helium accreting carbon-oxygen white dwarfs
in which we include the effects of the spin-up of the accreting star induced by
angular momentum accretion, rotationally induced chemical mixing and rotational
energy dissipation. Initial masses of 0.6 Msun and 0.8 Msun and constant
accretion rates of a few times 10^{-8} Msun/yr of helium rich matter have been
considered, which is typical for the sub-Chandrasekhar mass progenitor scenario
for Type Ia supernovae. It is found that the helium envelope in an accreting
white dwarf is heated efficiently by friction in the differentially rotating
spun-up layers. As a result, helium ignites much earlier and under much less
degenerate conditions compared to the corresponding non-rotating case.
Consequently, a helium detonation may be avoided, which questions the
sub-Chandrasekhar mass progenitor scenario for Type Ia supernovae. We discuss
implications of our results for the evolution of helium star plus white dwarf
binary systems as possible progenitors of recurrent helium novae.Comment: 8 pages, 5 figures, Accepted to A&
Effects of rotation on the helium burning shell source in accreting white dwarfs
We investigate the effects of rotation on the behavior of the helium burning
shell source in accreting carbon-oxygen white dwarfs, in the context of the
single degenerate Chandrasekhar mass progenitor scenario for Type Ia supernovae
(SNe Ia). We model the evolution of helium accreting white dwarfs of initially
1 Msun, assuming four different constant accretion rates (2, 3, 5 and 10
times10^{-7} Msun/yr). In a one-dimensional approximation, we compute the mass
accretion and subsequent nuclear fusion of helium into carbon and oxygen, as
well as angular momentum accretion, angular momentum transport inside the white
dwarf, and rotationally induced chemical mixing. Our models show two major
effects of rotation: a) The helium burning nuclear shell source in the rotating
models is much more stable than in corresponding non-rotating models -- which
increases the likelihood of accreting white dwarfs to reach the stage of
central carbon ignition. This effect is mainly due to rotationally induced
mixing at the CO/He interface which widens the shell source, and due to the
centrifugal force lowering the density and degeneracy at the shell source
location. b) The C/O-ratio in the layers which experience helium shell burning
-- which may affect the energy of a SN Ia explosion -- is strongly decreased by
the rotationally induced mixing of alpha-particles into the carbon-rich layers.
We discuss implications of our results for the evolution of SNe Ia progenitors.Comment: 12 pages, 9 figures, to appear in A&
Thermohaline instability and rotation-induced mixing. I - Low- and intermediate-mass solar metallicity stars up to the end of the AGB
(abridged) Numerous spectroscopic observations provide compelling evidence
for non-canonical processes that modify the surface abundances of low- and
intermediate-mass stars beyond the predictions of standard stellar theory. We
study the effects of thermohaline instability and rotation-induced mixing in
the 1-4 Msun range at solar metallicity. We present evolutionary models by
considering both thermohaline and rotation-induced mixing in stellar interior.
We discuss the effects of these processes on the chemical properties of stars
from the zero age main sequence up to the end of the second dredge-up on the
early-AGB for intermediate-mass stars and up to the AGB tip for low-mass stars.
Model predictions are compared to observational data for
lithium,12C/13C,[N/C],[Na/Fe],16O/17O, and 16O/18O in Galactic open clusters
and in field stars with well-defined evolutionary status,as well as in
planetary nebulae. Thermohaline mixing simultaneously accounts for the observed
behaviour of 12C/13C,[N/C], and lithium in low-mass stars that are more
luminous than the RGB bump, and its efficiency is increasing with decreasing
initial stellar mass. On the TP-AGB,thermohaline mixing leads to lithium
production, although the 7Li yields remain negative. Although the 3He stellar
yields are much reduced thanks to this process, we find that solar-metallicity,
low-mass stars remain net 3He producers. Rotation-induced mixing is found to
change the stellar structure so that in the mass range between \sim 1.5 and 2.2
Msun the thermohaline instability occurs earlier on the red giant branch than
in non-rotating models. Finally rotation accounts for the observed star-to-star
abundance variations at a given evolutionary status, and is necessary to
explain the features of CN-processed material in intermediate-mass stars.Comment: 18 pages, 22 figures, accepted for publication in A&
Nuclear Reaction Network for Primordial Nucleosynthesis: a detailed analysis of rates, uncertainties and light nuclei yields
We analyze in details the standard Primordial Nucleosynthesis scenario. In
particular we discuss the key theoretical issues which are involved in a
detailed prediction of light nuclide abundances, as the weak reaction rates,
neutrino decoupling and nuclear rate modeling. We also perform a new analysis
of available data on the main nuclear processes entering the nucleosynthesis
reaction network, with particular stress on their uncertainties as well as on
their role in determining the corresponding uncertainties on light nuclide
theoretical estimates. The current status of theoretical versus experimental
results for 2H, 3He, 4He and 7Li is then discussed using the determination of
the baryon density as obtained from Cosmic Microwave Background anisotropies.Comment: LaTeX, 83 pages, 30 .pdf figures. Some typos in the units of
R-functions in appendix D and relative plots fixe
Explosive Nucleosynthesis: What we learned and what we still do not understand
This review touches on historical aspects, going back to the early days of
nuclear astrophysics, initiated by BFH and Cameron, discusses (i) the
required nuclear input from reaction rates and decay properties up to the
nuclear equation of state, continues (ii) with the tools to perform
nucleosynthesis calculations and (iii) early parametrized nucleosynthesis
studies, before (iv) reliable stellar models became available for the late
stages of stellar evolution. It passes then through (v) explosive environments
from core-collapse supernovae to explosive events in binary systems (including
type Ia supernovae and compact binary mergers), and finally (vi) discusses the
role of all these nucleosynthesis production sites in the evolution of
galaxies. The focus is put on the comparison of early ideas and present, very
recent, understanding.Comment: 11 pages, to appear in Springer Proceedings in Physics (Proc. of
Intl. Conf. "Nuclei in the Cosmos XV", LNGS Assergi, Italy, June 2018
Nuclear Reaction Rates and Primordial Li
We examine the possibility that Big Bang Nucleosynthesis (BBN) may produce
non-trivial amounts of Li. If a primordial component of this isotope could
be observed, it would provide a new fundamental test of Big-Bang cosmology, as
well as new constraints on the baryon density of the universe. At present,
however, theoretical predictions of the primordial Li abundance are
extremely uncertain due to difficulties in both theoretical estimates and
experimental determinations of the deuterium-plus-alpha radiative capture
reaction cross-section. We also argue that present observational capabilities
do not yet allow the detection of primeval Li in very metal-poor stars of
the galactic halo. However, if the critical cross section is towards the upper
end of its plausible range, then improvements in Li detection capabilities
may allow the establishment of Li as another product of BBN. It is also
noted that a primordial Li detection could help resolve current concerns
about the extragalactic D/H determination.Comment: 10 pages, REVTeX, 5 PostScript figures with psfig. Submitted to
Physical Review
The quest for the solar g modes
Solar gravity modes (or g modes) -- oscillations of the solar interior for
which buoyancy acts as the restoring force -- have the potential to provide
unprecedented inference on the structure and dynamics of the solar core,
inference that is not possible with the well observed acoustic modes (or p
modes). The high amplitude of the g-mode eigenfunctions in the core and the
evanesence of the modes in the convection zone make the modes particularly
sensitive to the physical and dynamical conditions in the core. Owing to the
existence of the convection zone, the g modes have very low amplitudes at
photospheric levels, which makes the modes extremely hard to detect. In this
paper, we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation of the
g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the
techniques that have been used to try to detect g modes. We review results in
the literature, and finish by looking to the future, and the potential advances
that can be made -- from both data and data-analysis perspectives -- to give
unambiguous detections of individual g modes. The review ends by concluding
that, at the time of writing, there is indeed a consensus amongst the authors
that there is currently no undisputed detection of solar g modes.Comment: 71 pages, 18 figures, accepted by Astronomy and Astrophysics Revie
- …