286 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 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
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&
Stellar evolution with rotation and magnetic fields:III: The interplay of circulation and dynamo
We examine the effects of the magnetic field created by the Tayler--Spruit
dynamo in differentially rotating stars. Magnetic fields of the order of a few
G are present through most of the stellar envelope, with the exception
of the outer layers. The diffusion coefficient for the transport of angular
momentum is very large and it imposes nearly solid body rotation during the MS
phase. In turn, solid body rotation drives meridional circulation currents
which are much faster than usual and leads to much larger diffusion
coefficients than the magnetic diffusivity for the chemical species. The
consequence is that the interplay of the thermal and magnetic instabilities
favours the chemical transport of elements, while there would be no transport
in models with magnetic field only. We also discuss the effects on the stellar
interior, lifetimes and HR diagram.Comment: 11 pages, 10 figures, accepted by Astronomy & Astrophysic
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