2,436 research outputs found
From stars to nuclei
We recall the basic physical principles governing the evolution of stars with
some emphasis on the role played by the nuclear reactions. We argue that in
general it is not possible from observations of stars to deduce constraints on
the nuclear reaction rates. This is the reason why precise measurements of
nuclear reaction rates are a necessity in order to make progresses in stellar
physics, nucleosynthesis and chemical evolution of galaxies. There are however
some stars which provides useful constraint on nuclear processes. The
Wolf-Rayet stars of the WN type present at their surface CNO equilibrium
patterns. There is also the particular case of the abundance of Ne at
the surface of WC stars. The abundance of this element is a measure of the
initial CNO content. Very interestingly, recent determinations of its abundance
at the surface of WC stars tend to confirm that massive stars in the solar
neighborhood have initial metallicities in agreement with the Asplund et al.
(2005) solar abundances.Comment: 8 pages, 2 figures, be published in "European Physical Journal:
Special Topics
The production of short-lived radionuclides by new non-rotating and rotating Wolf-Rayet model stars
It has been speculated that WR winds may have contaminated the forming solar
system, in particular with short-lived radionuclides (half-lives in the
approximate 10^5 - 10^8 y range) that are responsible for a class of isotopic
anomalies found in some meteoritic materials. We revisit the capability of the
WR winds to eject these radionuclides using new models of single non-exploding
WR stars with metallicity Z = 0.02. The earlier predictions for non-rotating WR
stars are updated, and models for rotating such stars are used for the first
time in this context. We find that (1) rotation has no significant influence on
the short-lived radionuclide production by neutron capture during the core
He-burning phase, and (2) 26Al, 36Cl, 41Ca, and 107Pd can be wind-ejected by a
variety of WR stars at relative levels that are compatible with the meteoritic
analyses for a period of free decay of around 10^5 y between production and
incorporation into the forming solar system solid bodies. We confirm the
previously published conclusions that the winds of WR stars have a radionuclide
composition that can meet the necessary condition for them to be a possible
contaminating agent of the forming solar system. Still, it remains to be
demonstrated from detailed models that this is a sufficient condition for these
winds to have provided a level of pollution that is compatible with the
observations.Comment: 8 pages, 8 figure
Stellar evolution with rotation and magnetic fields II: General equations for the transport by Tayler--Spruit dynamo
We further develop the Tayler--Spruit dynamo theory, based on the most
efficient instability for generating magnetic fields in radiative layers of
differentially rotating stars. We avoid the simplifying assumptions that either
the -- or the --gradient dominates, but we treat the general case and
we also account for the nonadiabatic effects, which favour the growth of the
magnetic field. Stars with a magnetic field rotate almost as a solid body.
Several of their properties (size of the core, MS lifetimes, tracks,
abundances) are closer to those of models without rotation than with rotation
only. In particular, the observed N/C or N/H excesses in OB stars are better
explained by our previous models with rotation only than by the present models
with magnetic fields that predict no nitrogen excesses. We show that there is a
complex feedback loop between the magnetic instability and the thermal
instability driving meridional circulation. This opens the possibility for
further magnetic models, but at this stage we do not know the relative
importance of the magnetic fields due to the Tayler instability in stellar
interiors.Comment: 14 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
Stellar Evolution in the Early Universe
Massive stars played a key role in the early evolution of the Universe. They
formed with the first halos and started the re-ionisation. It is therefore very
important to understand their evolution. In this paper, we describe the strong
impact of rotation induced mixing and mass loss at very low . The strong
mixing leads to a significant production of primary nitrogen 14, carbon 13 and
neon 22. Mass loss during the red supergiant stage allows the production of
Wolf-Rayet stars, type Ib,c supernovae and possibly gamma-ray bursts (GRBs)
down to almost Z=0 for stars more massive than 60 solar masses. Galactic
chemical evolution models calculated with models of rotating stars better
reproduce the early evolution of N/O, C/O and C12/C13. We calculated the weak
s-process production induced by the primary neon 22 and obtain overproduction
factors (relative to the initial composition, Z=1.e-6) between 100-1000 in the
mass range 60-90.Comment: 8 pages, 4 figures, proceedings of IAU Symposium 255,
"Low-Metallicity Star Formation: From the First stars to Dwarf Galaxies",
L.K. Hunt, S. Madden & R. Schneider, ed
The first stars: a classification of CEMP-no stars
We propose and apply a new classification for the CEMP-no stars, which are
"carbon-enhanced metal-poor" stars with no overabundance of s-elements and with
[Fe/H] generally inferior or equal to -2.5. This classification is based on the
changes in abundances for the elements and isotopes involved in the CNO, Ne-Na,
and Mg-Al nuclear cycles. These abundances change very much owing to successive
back and forth mixing motions between the He- and H-burning regions in massive
stars (the "source stars" responsible for the chemical enrichment of the
CEMP-no stars). The wide variety of the ratios [C/Fe], 12C/13C, [N/Fe], [O/Fe],
[Na/Fe], [Mg/Fe], [Al/Fe], [Sr/Fe], and [Ba/Fe], which are the main
characteristics making the CEMP-no and low s stars so peculiar, is described
well in terms of the proposed nucleosynthetic classification. We note that the
[(C+N+O)/Fe] ratios significantly increase for lower values of [Fe/H]. The
classification of CEMP-no stars and the behavior of [(C+N+O)/Fe] support the
presence, in the first stellar generations of the Galaxy, of fast-rotating
massive stars experiencing strong mixing and mass loss (spinstars). This result
has an impact on the early chemical and spectral evolution of the Galaxy.Comment: 10 pages, 7 figures, 2 tables, accepted for publication in Astronomy
and Astrophysic
The thermonuclear production of F19 by Wolf-Rayet stars revisited
New models of rotating and non-rotating stars are computed for initial masses
between 25 and 120 Msun and for metallicities Z = 0.004, 0.008, 0.020 and 0.040
with the aim of reexamining the wind contribution of Wolf-Rayet (WR) stars to
the F19 enrichment of the interstellar medium. Models with an initial rotation
velocity vini = 300 km/s are found to globally eject less F19 than the
non-rotating models. We compare our new predictions with those of Meynet &
Arnould (2000), and demonstrate that the F19 yields are very sensitive to the
still uncertain F19(alpha,p)Ne22 rate and to the adopted mass loss rates. Using
the recommended mass loss rate values that take into account the clumping of
the WR wind and the NACRE reaction rates when available, we obtain WR F19
yields that are significantly lower than predicted by Meynet & Arnould (2000),
and that would make WR stars non-important contributors to the galactic F19
budget. In view, however, of the large nuclear and mass loss rate
uncertainties, we consider that the question of the WR contribution to the
galactic F19 remains quite largely open.Comment: 9 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Wind anisotropies and GRB progenitors
We study the effect of wind anisotropies on the stellar evolution leading to
collapsars. Rotating models of a 60 M star with on the ZAMS, accounting for shellular rotation and a magnetic
field, with and without wind anisotropies, are computed at =0.002 until the
end of the core He-burning phase. Only the models accounting for the effects of
the wind anisotropies retain enough angular momentum in their core to produce a
Gamma Ray Burst (GRB). The chemical composition is such that a type Ic
supernova event occurs. Wind anisotropies appear to be a key physical
ingredient in the scenario leading to long GRBs.Comment: 5 pages, 4 figures, accepted for publication in A&A Lette
On the Origin of the High Helium Sequence in Centauri
The blue Main Sequence (bMS) of Cen implies a ratio of helium to
metal enrichment , which is a major enigma.
We show that rotating models of low metallicity stars, which account for the
anomalous abundance ratios of extremely metal poor stars, are also useful for
understanding the very high ratio in Cen. Models
of massive stars with moderate initial rotation velocities produce stellar
winds with large He-- and N--excesses, but without the large C-- (and O--)
excesses made by very fast rotation, in agreement with the observed chemical
abundance ratios in Cen. It is still uncertain whether the abundance
peculiarities of Cen result from the fact that the high velocity
contributions of supernovae escaped the globular cluster, usually considered as
a tidally stripped core of a dwarf galaxy. Another possibility is a general
dominance of wind ejecta at very low , due to the formation of black holes.
Some abundance and isotopic ratios like , , ,
, and may allow us to further
discriminate between these scenarios and between the AGB and massive star
contributions.Comment: 5 pages, 3 figures, accepted for publication in A&
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