170 research outputs found
Quantum corrections to microscopic diffusion constants
We review the state of the art regarding the computation of the resistance
coefficients in conditions typical of the stellar plasma, and compare the
various results studying their effect on the solar model. We introduce and
discuss for the first time in an astrophysical context the effect of quantum
corrections to the evaluation of the resistance coefficients, and provide
simple yet accurate fitting formulae for their computation. Although the
inclusion of quantum corrections only weakly modifies the solar model, their
effect is growing with density, and thus might be of relevance in case of
denser objects like, e.g., white dwarfs.Comment: 8 pages, 5 figures, accepted for publication in A&
New solar opacities, abundances, helioseismology, and neutrino fluxes
We construct solar models with the newly calculated radiative opacities from
the Opacity Project (OP) and recently determined (lower) heavy element
abundances. We compare results from the new models with predictions of a series
of models that use OPAL radiative opacities, older determinations of the
surface heavy element abundances, and refinements of nuclear reaction rates.
For all the variations we consider, solar models that are constructed with the
newer and lower heavy element abundances advocated by Asplund et al. (2005)
disagree by much more than the estimated measuring errors with
helioseismological determinations of the depth of the solar convective zone,
the surface helium composition, the internal sound speeds, and the density
profile. Using the new OP radiative opacities, the ratio of the 8B neutrino
flux calculated with the older and larger heavy element abundances (or with the
newer and lower heavy element abundances) to the total neutrino flux measured
by the Sudbury Neutrino Observatory is 1.09 (0.87) with a 9% experimental
uncertainty and a 16% theoretical uncertainty, 1 sigma errors.Comment: ApJ Letters (in press), added 3 references, detailed numerical solar
models and distributions of neutrino fluxes available at
http://www.sns.ias.edu/~jnb (models go back to 1982
On the helium flash in low-mass Population III Red Giant stars
We investigate the evolution of initially metal-free, low-mass Red Giant
stars through the He core flash at the tip of the Red Giant Branch. The low
entropy barrier between the helium- and hydrogen-rich layers enables a
penetration of the helium flash driven convective zone into the inner tail of
the extinguishing H-burning shell. As a consequence, protons are mixed into
high-temperature regions triggering a H-burning runaway. The subsequent
dredge-up of matter processed by He and H burning enriches the stellar surface
with large amounts of helium, carbon and nitrogen. Extending previous results
by Hollowell et al. (1990) and Fujimoto et al. (2000), who claimed that the
H-burning runaway is an intrinsic property of extremely metal-poor low-mass
stars, we found that its occurrence depends on additional parameters like the
initial composition and the treatment of various physical processes.
We perform some comparisons between predicted surface chemical abundances and
observational measurements for extremely metal-deficient stars. As in previous
investigations, our results disclose that although the described scenario
provides a good qualitative agreement with observations, considerable
discrepancies still remain. They may be due to a more complex evolutionary path
of `real' stars, and/or some shortcomings in current evolutionary models.
In addition, we analyze the evolutionary properties after the He core flash,
during both the central and shell He-burning phases, allowing us to deduce some
interesting differences between models whose Red Giant Branch progenitor has
experienced the H-flash and canonical models. In particular, the Asymptotic
Giant Branch evolution of extremely metal-deficient stars and the occurrence of
thermal pulses are strongly affected by the previous RGB evolution.Comment: 7 figures, AASTeX, submitted to Ap
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&
Evolution of low-mass metal-free stars including effects of diffusion and external pollution
We investigate the evolution of low-mass metal-free Population III stars.
Emphasis is laid upon the question of internal and external sources for
CNO-elements, which - if present in sufficient amounts in the hydrogen-burning
regions - lead to a strong modification of the stars' evolutionary behavior.
For the production of carbon due to nuclear processes inside the stars, we use
an extended nuclear network, demonstrating that hot pp-chains do not suffice to
produce enough carbon or are less effective than the triple3-alpha-process. As
an external source of CNO-elements we test the efficiency of pollution by a
nearby massive star combined with particle diffusion. For all cases
investigated, the additional metals fail to reach nuclear burning regions
before deep convection on the Red Giant Branch obliterates the previous
evolution. The surface abundance history of the polluted Pop III stars is
presented. The possibilities to discriminate between a Pop II and a polluted
Pop III field star are also discussed.Comment: Accepted for publication in Ap
Temporal variation of coupling constants and nucleosynthesis
We investigate the triple-alpha process and the Oklo phenomenon to obtain
constraints on possible cosmological time variations of fundamental constants.
Specifically we study cosmological temporal constraints for the fine structure
constant and nucleon and meson masses.Comment: 4 pages. Proceedings of the Nuclear Physics in Astrophysics
Conference, Debrecen, Hungary, September 30 - October 3, 2002. To be
published in Nuc. Phys.
Parameters' domain in three flavour neutrino oscillations
We consider analytically the domain of the three mixing angles
and the CP phase for three flavour neutrino oscillations both in
vacuum and matter. Similarly to the quark sector, it is necessary and
sufficient to let all the mixing angles
and be in the range and ,
respectively. To exploit the full range of will be important in future
when more precise fits are possible, even without CP violation measurements.
With the above assumption on the angles we can restrict ourselves to the
natural order of masses . Considerations of the mass schemes with
some negative 's, though for some reasons useful, are not necessary
from the point of view of neutrino oscillation parametrization and cause double
counting only. These conclusions are independent of matter effects.Comment: references added, to appear in PL
Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars II. s-process nucleosynthesis during the core He flash
Models of primordial and hyper-metal-poor stars with masses similar to the
Sun experience an ingestion of protons into the hot core during the core helium
flash phase at the end of their red giant branch evolution. This produces a
concurrent secondary flash powered by hydrogen burning that gives rise to
further nucleosynthesis in the core. We perform post-process nucleosynthesis
calculations on a one-dimensional stellar evolution calculation of a star of 1
solar mass and metallicity [Fe/H] = -6.5 that suffers a proton ingestion
episode. Our network includes 320 nuclear species and 2,366 reactions and
treats mixing and burning simultaneously. The mixing and burning of protons
into the hot convective core leads to the production of 13C, which then burns
via the 13C(alpha,n)16O reaction releasing a large number of free neutrons.
During the first two years of neutron production the neutron poison 14N
abundance is low, allowing the prodigious production of heavy elements such as
strontium, barium, and lead via slow neutron captures (the s process). These
nucleosynthetic products are later mixed to the stellar surface and ejected via
stellar winds. We compare our results with observations of the hyper-metal-poor
halo star HE 1327-2326, which shows a strong Sr overabundance. Our model
provides the possibility of self-consistently explaining the Sr overabundance
in HE 1327-2326 together with its C, N, and O overabundances (all within a
factor of ~4) if the material were heavily diluted, for example, via mass
transfer in a wide binary system. The model produces at least 18 times too much
Ba than observed, but this may be within the large modelling uncertainties. In
this scenario, binary systems of low mass must have formed in the early
Universe. If true then this puts constraints on the primordial initial mass
function.Comment: Accepted for publication on Astronomy & Astrophysics Letter
Solar models and electron screening
We investigate the sensitivity of the solar model to changes in the nuclear
reaction screening factors. We show that the sound speed profile as determined
by helioseismology certainly rules out changes in the screening factors
exceeding more than 10%. A slightly improved solar model could be obtained by
enhancing screening by about 5% over the Salpeter value. We also discuss how
envelope properties of the Sun depend on screening, too. We conclude that the
solar model can be used to help settling the on-going dispute about the
``correct'' screening factors.Comment: accepted for publication by Astron. Astrophy
Life Products of Stars
We attempt to document complete energetic transactions of stars in their
life. We calculate photon and neutrino energies that are produced from stars in
their each phase of evolution from 1 to 8 M_sun, using the state-of-the-art
stellar evolution code, tracing the evolution continuously from pre-main
sequence gravitational contraction to white dwarfs. We also catalogue
gravitational and thermal energies and helium, and heavier elements that are
stored in stars and those ejected into interstellar space in each evolutionary
phase.Comment: 26 pages, including 8 figures and 3 tables. Submitted to ApJ
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