194 research outputs found
Modelling the evolution and nucleosynthesis of carbon-enhanced metal-poor stars
We present the results of binary population simulations of carbon-enhanced
metal-poor (CEMP) stars. We show that nitrogen and fluorine are useful tracers
of the origin of CEMP stars, and conclude that the observed paucity of very
nitrogen-rich stars puts strong constraints on possible modifications of the
initial mass function at low metallicity. The large number fraction of CEMP
stars may instead require much more efficient dredge-up from low-metallicity
asymptotic giant branch stars.Comment: 6 pages, 1 figure, to appear in the proceedings of IAU Symposium 252
"The Art of Modelling Stars in the 21st Century", April 6-11, 2008, Sanya,
Chin
Do meteoritic silicon carbide grains originate from asymptotic giant branch stars of super-solar metallicity?
We compare literature data for the isotopic ratios of Zr, Sr, and Ba from
analysis of single meteoritic stardust silicon carbide (SiC) grains to new
predictions for the slow neutron-capture process (the s process) in metal-rich
asymptotic giant branch (AGB) stars. The models have initial metallicities Z =
0.014 (solar) and Z = 0.03 (twice-solar) and initial masses 2 - 4.5 Msun,
selected such as the condition C/O>1 for the formation of SiC is achieved.
Because of the higher Fe abundance, the twice-solar metallicity models result
in a lower number of total free neutrons released by the 13C({\alpha},n)16O
neutron source. Furthermore, the highest-mass (4 - 4.5 Msun) AGB stars of
twice-solar metallicity present a milder activation of the 22Ne({\alpha},n)25Mg
neutron source than their solar metallicity counterparts, due to cooler
temperatures resulting from the effect of higher opacities. They also have a
lower amount of the 13C neutron source than the lower-mass models, following
their smaller He-rich region. The combination of these different effects allows
our AGB models of twice-solar metallicity to provide a match to the SiC data
without the need to consider large variations in the features of the 13C
neutron source nor neutron-capture processes different from the s process. This
raises the question if the AGB parent stars of meteoritic SiC grains were in
fact on average of twice-solar metallicity. The heavier-than-solar Si and Ti
isotopic ratios in the same grains are in qualitative agreement with an origin
in stars of super-solar metallicity because of the chemical evolution of the
Galaxy. Further, the SiC dust mass ejected from C-rich AGB stars is predicted
to significantly increase with increasing the metallicity.Comment: 40 pages, 7 figures, 1 table. Accepted for publication on Geochimica
Cosmochimica Act
New determination of the 13C(a, n)16O reaction rate and its influence on the s-process nucleosynthesis in AGB stars
We present a new measurement of the -spectroscopic factor
() and the asymptotic normalization coefficient (ANC) for the 6.356
MeV 1/2 subthreshold state of O through the C(B,
Li)O transfer reaction and we determine the -width of this
state. This is believed to have a strong effect on the rate of the
C(, )O reaction, the main neutron source for {\it
slow} neutron captures (the -process) in asymptotic giant branch (AGB)
stars. Based on the new width we derive the astrophysical S-factor and the
stellar rate of the C(, )O reaction. At a temperature
of 100 MK our rate is roughly two times larger than that by \citet{cau88} and
two times smaller than that recommended by the NACRE compilation. We use the
new rate and different rates available in the literature as input in
simulations of AGB stars to study their influence on the abundances of selected
-process elements and isotopic ratios. There are no changes in the final
results using the different rates for the C(, )O
reaction when the C burns completely in radiative conditions. When the
C burns in convective conditions, as in stars of initial mass lower than
2 M_\sun and in post-AGB stars, some changes are to be expected, e.g.,
of up to 25% for Pb in our models. These variations will have to be carefully
analyzed when more accurate stellar mixing models and more precise
observational constraints are available
Current hot questions on the s process in AGB stars
The version of record, M. Lugaro et al., 2016, 'Current Hot Questions on the s process in AGB stars', Journal of Physics: Conference Series, Vol. 665, 012021, published under licence by IOP Publishing Ltd, is available on line at doi: 10.1088/1742-6596/665/1/012021 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Asymptotic giant branch (AGB) stars are a main site of production of nuclei heavier than iron via the s process. In massive (>4 Mâ) AGB stars the operation of the 22Ne neutron source appears to be confirmed by observations of high Rb enhancements, while the lack of Tc in these stars rules out 13C as a main source of neutrons. The problem is that the Rb enhancements are not accompanied by Zr enhancements, as expected by s-process models. This discrepancy may be solved via a better understanding of the complex atmospheres of AGB stars. Second- generation stars in globular clusters (GCs), on the other hand, do not show enhancements in any s-process elements, not even Rb. If massive AGB stars are responsible for the composition of these GC stars, they may have evolved differently in GCs than in the field. In AGB stars of lower masses, 13C is the main source of neutrons and we can potentially constrain the effects of rotation and proton-ingestion episodes using the observed composition of post-AGB stars and of stardust SiC grains. Furthermore, independent asteroseismology observations of the rotational velocities of the cores of red giants and of white dwarves will play a fundamental role in helping us to better constrain the effect of rotation. Observations of carbon-enhanced metal-poor stars enriched in both Ba and Eu may require a neutron flux in-between the s and the r process, while the puzzling increase of Ba as function of the age in open clusters, not accompanied by increase in any other element heavier than iron, require further observational efforts. Finally, stardust SiC provides us high-precision constraints to test nuclear inputs such as neutron-capture cross sections of stable and unstable isotopes and the impact of excited nuclear states in stellar environments.Peer reviewe
Ultra-heavy cosmic-ray science--Are r-process nuclei in the cosmic rays produced in supernovae or binary neutron star mergers?
The recent detection of 60Fe in the cosmic rays provides conclusive evidence
that there is a recently synthesized component (few MY) in the GCRs (Binns et
al. 2016). In addition, these nuclei must have been synthesized and accelerated
in supernovae near the solar system, probably in the Sco-Cen OB association
subgroups, which are about 100 pc distant from the Sun. Recent theoretical work
on the production of r-process nuclei appears to indicate that it is difficult
for SNe to produce the solar system abundances relative to iron of r-process
elements with high atomic number (Z), including the actinides (Th, U, Np, Pu,
and Cm). Instead, it is believed by many that the heaviest r-process nuclei, or
perhaps even all r-process nuclei, are produced in binary neutron star mergers.
Since we now know that there is at least a component of the GCRs that has been
recently synthesized and accelerated, models of r-process production by SNe and
BNSM can be tested by measuring the relative abundances of these ultra-heavy
r-process nuclei, and especially the actinides, since they are radioactive and
provide clocks that give the time interval from nucleosynthesis to detection at
Earth. Since BNSM are believed to be much less frequent in our galaxy than SNe
(roughly 1000 times less frequent, the ratios of the actinides, each with their
own half-life, will enable a clear determination of whether the heaviest
r-process nuclei are synthesized in SNe or in BNSM. In addition, the r-process
nuclei for the charge range from 34 to 82 can be used to constrain models of
r-process production in BNSM and SNe. Thus, GCRs become a multi-messenger
component in the study of BNSM and SNe.Comment: Astro2020 Science White Pape
The s process in AGB stars as constrained by a large sample of barium stars
© ESO 2018. Context. Barium (Ba) stars are dwarf and giant stars enriched in elements heavier than iron produced by the slow neutron-capture process (s process). These stars belong to binary systems in which the primary star evolved through the asymptotic giant branch (AGB) phase. During this phase the primary star produced s-process elements and transferred them onto the secondary, which is now observed as a Ba star. Aims. We compare the largest homogeneous set of Ba giant star observations of the s-process elements Y, Zr, La, Ce, and Nd with AGB nucleosynthesis models to reach a better understanding of the s process in AGB stars. Methods. By considering the light-s (ls: Y and Zr) heavy-s (hs: La, Ce, and Nd) and elements individually, we computed for the first time quantitative error bars for the different hs-element to ls-element abundance ratios, and for each of the sample stars. We compared these ratios to low-mass AGB nucleosynthesis models. We excluded La from our analysis because the strong La lines in some of the sample stars cause an overestimation and unreliable abundance determination, as compared to the other observed hs-Type elements. Results. All the computed hs-Type to ls-Type element ratios show a clear trend of increasing with decreasing metallicity with a small spread (less than a factor of 3). This trend is predicted by low-mass AGB models in which 13C is the main neutron source. The comparison with rotating AGB models indicates the need for the presence of an angular momentum transport mechanism that should not transport chemical species, but significantly reduces the rotational speed of the core in the advanced stellar evolutionary stages. This is an independent confirmation of asteroseismology observations of the slow down of core rotation in giant stars, and of rotational velocities of white dwarfs lower than predicted by models without an extra angular momentum transport mechanism
Aluminium-26 from Massive Binary Stars. I. Nonrotating Models
Aluminium-26 is a short-lived radionuclide with a half-life of 0.72 Myr, which is observed today in the Galaxy via Îł-ray spectroscopy and i inferred to have been present in the early solar system via analysis o meteorites. Massive stars are considered the main contributors o 26Al. Although most massive stars are found in binar systems, the effect, however, of binary interactions on th 26Al yields has not been investigated since Braun & Langer. Here we aim to fill this gap. We have used the MESA stella evolution code to compute massive (10 M â †M †80 â) nonrotating single and binary stars of solar metallicit (Z = 0.014). We computed the wind yields for the single stars and fo the binary systems where mass transfer plays a major role. Depending o the initial mass of the primary star and orbital period, th 26Al yield can either increase or decrease in a binar system. For binary systems with primary masses up to âŒ35â40 â, the yield can increase significantly, especially at th lower mass end, while above âŒ45 M â the yield becomes simila to the single-star yield or even decreases. Our preliminary results sho that compared to supernova explosions, the contribution of mass loss i binary systems to the total 26Al abundance produced by stellar population is minor. On the other hand, if massive star mas loss is the origin of 26Al in the early solar system, ou results will have significant implications for the identification of th potential stellar, or stellar population, source. This paper i dedicated to the celebration of the 100th birthday of Margaret Burbidge in recognition of the outstanding contributions she has made to nuclea astrophysics (Burbidge et al. 1957
The impact of the 18F(a,p)21Ne reaction on asymptotic giant branch nucleosynthesis
We present detailed models of low and intermediate-mass asymptotic giant
branch (AGB) stars with and without the 18F(a,p)21Ne reaction included in the
nuclear network, where the rate for this reaction has been recently
experimentally evaluated for the first time. The lower and recommended measured
rates for this reaction produce negligible changes to the stellar yields,
whereas the upper limit of the rate affects the production of 19F and 21Ne. The
stellar yields increase by ~50% to up to a factor of 4.5 for 19F, and by
factors of ~2 to 9.6 for 21Ne. While the 18}F(a,p)21Ne reaction competes with
18O production, the extra protons released are captured by 18O to facilitate
the 18O(p,a)15N(a,g)19F chain. The higher abundances of 19F obtained using the
upper limit of the rate helps to match the [F/O] ratios observed in AGB stars,
but only for large C/O ratios. Extra-mixing processes are proposed to help to
solve this problem. Some evidence that the 18F(a,p)21Ne rate might be closer to
its upper limit is provided by the fact that the higher calculated 21Ne/22Ne
ratios in the He intershell provide an explanation for the Ne isotopic
composition of silicon-carbide grains from AGB stars. This needs to be
confirmed by future experiments of the 18F(a,p)21Ne reaction rate. The
availability of accurate fluorine yields from AGB stars will be fundamental for
interpreting observations of this element in carbon-enhanced metal-poor stars.Comment: 9 pages, accepted for publication in Ap
Women Scientists Who Made Nuclear Astrophysics
Female role models reduce the impact on women of stereotype threat, i.e., of being at risk of conforming to a negative stereotype about one's social, gender, or racial group [1,2]. This can lead women scientists to underperform or to leave their scientific career because of negative stereotypes such as, not being as talented or as interested in science as men. Sadly, history rarely provides role models for women scientists; instead, it often renders these women invisible [3]. In response to this situation, we present a selection of twelve outstanding women who helped to develop nuclear astrophysics
The Most Metal-Poor Stars. II. Chemical Abundances of 190 Metal-Poor Stars Including 10 New Stars With [Fe/H] < -3.5
We present a homogeneous chemical abundance analysis of 16 elements in 190
metal-poor Galactic halo stars (38 program and 152 literature objects). The
sample includes 171 stars with [Fe/H] < -2.5, of which 86 are extremely metal
poor, [Fe/H] < -3.0. Our program stars include ten new objects with [Fe/H] <
-3.5. We identify a sample of "normal" metal-poor stars and measure the trends
between [X/Fe] and [Fe/H], as well as the dispersion about the mean trend for
this sample. Using this mean trend, we identify objects that are chemically
peculiar relative to "normal" stars at the same metallicity. These chemically
unusual stars include CEMP-no objects, one star with high [Si/Fe], another with
high [Ba/Sr], and one with unusually low [X/Fe] for all elements heavier than
Na. The Sr and Ba abundances indicate that there may be two nucleosynthetic
processes at lowest metallicity that are distinct from the main r-process.
Finally, for many elements, we find a significant trend between [X/Fe] versus
Teff which likely reflects non-LTE and/or 3D effects. Such trends demonstrate
that care must be exercised when using abundance measurements in metal-poor
stars to constrain chemical evolution and/or nucleosynthesis predictions.Comment: Accepted for publication in Ap
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