1,847 research outputs found
From the microcosm of the atomic nuclei to the macrocosm of the stars
A necessary condition for the reliable modelling of the structure or
evolution of the stars and of their concomitant nucleosynthesis is the
availability of good quality nuclear data in a very wide area of the chart of
nuclides. This short review presents a non-exhaustive list of nuclear data of
astrophysics interest (masses, -decays, thermonuclear and
non-thermonuclear reaction rates) for nuclides at the bottom of the valley of
nuclear stability (mainly involved in the modelling of non-explosive phases of
stellar evolution), or for more or less highly exotic nuclides (to be
considered in the description of stellar explosions). Special emphasis is put
on the importance of providing quality nuclear data bases that can be easily
used by astrophysicists.Comment: 6 pages, To appear in Proceedings of International Conference on
Nuclear Data for Science and Technology 2007 (ND2007), Nice, Franc
Synthesis of 19F in Wolf-Rayet stars
Meynet and Arnould (1993) have suggested that Wolf-Rayet (WR) stars could
significantly contaminate the Galaxy with 19F. In their scenario, 19F is
synthesized at the beginning of the He-burning phase from the 14N left over by
the previous CNO-burning core, and is ejected in the interstellar medium when
the star enters its WC phase. Recourse to CNO seeds makes the 19F yields
metallicity-dependent. These yields are calculated on grounds of detailed
stellar evolutionary sequences for an extended range of initial masses (from 25
to 120 Msol) and metallicities (Z = 0.008, 0.02 and 0.04). The adopted mass
loss rate prescription enables to account for the observed variations of WR
populations in different environments. The 19F abundance in the WR winds of 60
Msol model stars is found to be about 10 to 70 times higher than its initial
value, depending on the metallicity. This prediction is used in conjunction
with a very simple model for the chemical evolution of the Galaxy to predict
that WR stars could be significant (dominant?) contributors to the solar system
fluorine content. We also briefly discuss the implications of our model on the
possible detection of fluorine at high redshift.Comment: 2 figures; requires LaTeX A&A class file; accepted for publication in
Astron. Astrophy
Hope and Inquietudes in Nucleo-cosmochronology
Critical views are presented on some nucleo-cosmochronological questions.
Progress has been made recently in the development of the 187Re-187Os
cosmochronometry. From this, there is good hope for this clock to become of the
highest quality for the nuclear dating of the Universe. The simultaneous
observation of Th and U in ultra-metal-poor stars would also be a most
interesting prospect. In contrast, a serious inquietude is expressed about the
reliability of the chronometric attempts based on the classical 232Th-238U and
235U-238U pairs, as well as on the Th (without U) abundance determinations in
ultra-metal poor stars.Comment: 9 pages, no figures; ASP Conference Series: "Astrophysical Ages and
Time Scales
Nuclear Astrophysics
Nuclear astrophysics is that branch of astrophysics which helps understanding
some of the many facets of the Universe through the knowledge of the microcosm
of the atomic nucleus. In the last decades much advance has been made in
nuclear astrophysics thanks to the sometimes spectacular progress in the
modelling of the structure and evolution of the stars, in the quality and
diversity of the astronomical observations, as well as in the experimental and
theoretical understanding of the atomic nucleus and of its spontaneous or
induced transformations. Developments in other sub-fields of physics and
chemistry have also contributed to that advance. Many long-standing problems
remain to be solved, however, and the theoretical understanding of a large
variety of observational facts needs to be put on safer grounds. In addition,
new questions are continuously emerging, and new facts endanger old ideas. This
review shows that astrophysics has been, and still is, highly demanding to
nuclear physics in both its experimental and theoretical components. On top of
the fact that large varieties of nuclei have to be dealt with, these nuclei are
immersed in highly unusual environments which may have a significant impact on
their static properties, the diversity of their transmutation modes, and on the
probabilities of these modes. In order to have a chance of solving some of the
problems nuclear astrophysics is facing, the astrophysicists and nuclear
physicists are obviously bound to put their competence in common, and have
sometimes to benefit from the help of other fields of physics, like particle
physics, plasma physics or solid-state physics.Comment: LaTeX2e with iopart.cls, 84 pages, 19 figures (graphicx package), 374
updated references. Published in Reports on Progress in Physics, vol.62, pp.
395-464 (1999
News from the p-process: is the s-process a troublemaker?
The most detailed calculations of the p-process call for its development in
the O/Ne layers of Type II supernovae. In spite of their overall success in
reproducing the solar system content of p-nuclides, they suggest a significant
underproduction of the light Mo and Ru isotopes. On grounds of a model for the
explosion of a 25 solar mass star with solar metallicity, we demonstrate that
this failure might just be related to the uncertainties left in the rate of the
22Ne(alpha,n)25Mg neutron producing reaction. The latter indeed has a direct
impact on the distribution of the s-process seeds for the p-process.Comment: 8 pages, 3 figures, Review talk at Nuclei in the Cosmos 2000, Aarhus,
June 27 - July 1, 200
The p- and r-processes: reviews and other views
A review is presented of the p-process in Type II supernovae, one of its
goals being to enlighten the changes in views on this nucleosynthesis mechanism
since the work of Jean and Jim on the subject in 1975. Specific discussions are
also devoted to cases of particular interest, like the light Mo and Ru stable
isotopes, the rare nuclide 138La or the radionuclide 146Sm. Some comments of
diverse natures are also made on the r-process. These considerations do not aim
at really providing an exhaustive review of the many nuclear physics and
astrophysics intricacies of this process. In contrast, they are hoped to
complement or to put in perspective other views that are often expressed in
relation with this nucleosynthesis mechanismComment: 7 pages, 2 figures. To appear in the conference proceedings of
"Cosmic evolution" (in the honor of J. Audouze and J. Truran), Institut
d'Astrophysique de Paris, November 200
Fluorine production in intermediate-mass stars
The 19F production during the first dozen thermal pulses of AGB stars with
(M=3,Z=0.02), (M=6,Z=0.02) and (M=3,Z=0.001) is investigated on grounds of
detailed stellar models and of revised rates for 15N(a,g)19F and 18O(a,g)22Ne.
These calculations confirm an early expectation that 19F {\it is} produced in
AGB thermal pulses. They also enlarge substantially these previous results by
showing that the variations of the level of 19F production during the evolution
is very sensitive to the maximum temperature reached at the base of the pulse.
These variations are analyzed in detail, and are shown to result from a subtle
balance between different nuclear effects (mainly 19F production or destruction
in a pulse, and 15N synthesis during the interpulse), possibly super-imposed on
dilution effects in more or less extended pulse convective tongues. Our
calculations, as most others, do not predict the third dredge-up self-
consistently. When parametrized, it appears that our models of
intermediate-mass AGB stars are able to account only for the lowest 19F
overabundances observed in solar-metallicity MS, S and C stars. That conclusion
is expected to hold true for low-mass stars when F production results from
secondary 13C only. Massive AGB stars, on the other hand, are not expected to
build up large surface F abundanc- es. Therefore, the large F overabundance
reported for the super Li-rich star WZ Cas (where HBB is supposed to be
operating) remains unexplained so far. Our results for the (M=3,Z=0.001) star
indicate that F surface overabundances can also be expected in low-metallicity
stars provided that third dredge-ups occur after the early cool pulses. The
relative increase in the surface 19F/12C ratio is, however, lower in the
low-metallicity than in the solar-metallicity star. No observations areComment: 27 pages, includes figures, postcript file (A&A format, 15 pages
including figures) can be found via anonymous ftp at
ftp://obsftp.unige.ch/pub/mowlavi/fluor.ps.gz ; accepted by A&
Non-explosive hydrogen and helium burnings: Abundance predictions from the NACRE reaction rate compilation
The abundances of the isotopes of the elements from C to Al produced by the
non-explosive CNO, NeNa and MgAl modes of hydrogen burning, as well as by
helium burning, are calculated with the thermonuclear rates recommended by the
European compilation of reaction rates for astrophysics (NACRE: details about
NACRE may be found at http://astro.ulb.ac.be. This electronic address provides
many data of nuclear astrophysics interest and also offers the possibility of
generating interactively tables of reaction rates for networks and temperature
grids selected by the user). The impact of nuclear physics uncertainties on the
derived abundances is discussed in the framework of a simple parametric
astrophysical model. These calculations have the virtue of being a guide in the
selection of the nuclear uncertainties that have to be duly analyzed in
detailed model stars, particularly in order to perform meaningful
confrontations between abundance observations and predictions. They are also
hoped to help nuclear astrophysicists pinpointing the rate uncertainties that
have to be reduced most urgently.Comment: 13 pages, 13 figures, Latex, accepted for publication in Astronomy
and Astrophysics main journal. Also available at
http://astro.ulb.ac.be/Htm/iaa0.ht
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