69 research outputs found
Nucleosynthesis of s-elements in zero-metal AGB stars
Contrary to previous expectations, recent evolutionary models of
zero-metallicity stars show that the development of mixing episodes at the
beginning of the AGB phase allows low- and intermediate-mass stars to
experience thermal pulses. If these stars, like their metal-rich counterparts,
also experience partial mixing of protons from the H-rich envelope into the
C-rich layers at the time of the third dredge-up, an extensive neutron capture
nucleosynthesis leads to the production of s-process nuclei up to Pb and Bi.
Nucleosynthesis calculations based on stellar AGB models are performed assuming
a parameterized H-abundance profile below the convective envelope at the time
of the third dredge-up. Despite the absence of Fe-group elements, the large
neutron flux resulting from the 13C(alpha,n)16O reaction leads to an efficient
production of s-process elements starting from the neutron captures on the C-Ne
isotopes. Provided partial mixing of protons takes place, it is shown that
population III AGB stars should be enriched in s-process elementsand overall in
Pb and Bi.Comment: 4 pages, 3 Postscript figures, uses aa.sty. Accepted for publication
in A&A Letter
The survival of 205Pb in intermediate-mass AGB stars
The now extinct 205Pb is a pure s-process radionuclide (t_{1/2} = 1.5x10^7 y)
of possible substantial cosmochemical interest. As a necessary complement to
the detailed theoretical study of the nuclear physics and astrophysics aspects
of the 205Pb - 205Tl pair carried out by Yokoi et al. (1985), and to the recent
calculation of the 205Pb production in Wolf-Rayet stars by Arnould et al.
(1997), this paper addresses for the first time in some detail the question of
the survival of this radionuclide in thermally pulsing AGB stars. This problem
is made difficult by the high sensitivity to temperature and density of the
rates of the weak interaction processes that are able to produce or destroy
205Pb. In view of this sensitivity, a recourse to detailed stellar models is
mandatory. With the help of some simplifying assumptions concerning in
particular the third dredge-up characteristics, some of which (like its depth)
being considered as free parameters, predictions are made for the 205Pb
contamination of the stellar surface at the end of a pulse-interpulse cycle
following a series of a dozen of pulses in three different intermediate-mass
stars (M=3M_sun,Z=0.02; M=6M_sun,Z=0.02; M=3M_sun,Z=0.001). It is concluded
that the chances for a significant 205Pb surface enrichment are likely to
increase with M for a given Z, or to increase with decreasing Z for a given M.
More specifically, following the considered pulses at least, the enrichment
appears to be rather unlikely in the 3M_sun star with Z=0.02, while it seems to
be much more probable in the other two considered stars. It is also speculated
that the (3M_sun,Z=0.02) star could possibly experience some 205Pb enrichment
following later pulses than the ones considered in this paper.Comment: 10 pages, 10 figures, Latex A&A, ps file available at
ftp://obsftp.unige.ch/pub/mowlavi/pbtl.ps; accepted for publication in A&
On microscopic theory of radiative nuclear reaction characteristics
A survey of some results in the modern microscopic theory of properties of
nuclear reactions with gamma-rays is given. First of all, we discuss the impact
of phonon coupling (PC) on the photon strength function (PSF) because it
represents the most natural physical source of additional strength found for Sn
isotopes in recent experiments that could not be explained within the stan-
dard HFB+QRPA approach. The self-consistent version of the Extended Theory of
Finite Fermi Systems in the Quasiparticle Time Blocking Approximation, or
simply QTBA, is applied. It uses the HFB mean field and includes both the QRPA
and PC effects on the basis of the SLy4 Skyrme force. With our microscopic E1
PSFs, the following properties have been calculated for many stable and
unstable even-even semi-magic Sn and Ni isotopes as well as for double-magic
132Sn and 208Pb using the reaction codes EMPIRE and TALYS with several nuclear
level density (NLD) models: 1) the neutron capture cross sections, 2) the
corresponding neutron capture gamma spectra, 3) the av- erage radiative widths
of neutron resonances. In all the properties considered, the PC contribution
turned out to be significant, as compared with the standard QRPA one, and
necessary to explain the available experimental data. The results with the
phenomenological so-called generalized super- fluid NLD model turned out to be
worse, on the whole, than those obtained with the microscopic HFB+combinatorial
NLD model. Finally, we also discuss the modern microscopic NLD models based on
the self-consistent HFB method and show their relevance to explain experimental
data as compared with the phenomeno- logical models. The use of these
self-consistent microscopic approaches is of particular relevance for nuclear
astrophysics, but also for the study of double-magic nuclei.Comment: 13 pages, 14 figures, a survey given as a plenary talk to the Intern.
Conference "NUCLEUS 2015" (June 29 - July 3, 2015, Saint-Petersburg, Russia).
To be published in Phys. Atom. Nuc
Neutron-powered precursors of kilonovae
The merger of binary neutron stars (NSs) ejects a small quantity of neutron
rich matter, the radioactive decay of which powers a day to week long thermal
transient known as a kilonova. Most of the ejecta remains sufficiently dense
during its expansion that all neutrons are captured into nuclei during the
r-process. However, recent general relativistic merger simulations by Bauswein
and collaborators show that a small fraction of the ejected mass (a few per
cent, or ~1e-4 Msun) expands sufficiently rapidly for most neutrons to avoid
capture. This matter originates from the shocked-heated interface between the
merging NSs. Here we show that the beta-decay of these free neutrons in the
outermost ejecta powers a `precursor' to the main kilonova emission, which
peaks on a timescale of a few hours following merger at U-band magnitude ~22
(for an assumed distance of 200 Mpc). The high luminosity and blue colors of
the neutron precursor render it a potentially important counterpart to the
gravitational wave source, that may encode valuable information on the
properties of the merging binary (e.g. NS-NS versus NS-black hole) and the NS
equation of state. Future work is necessary to assess the robustness of the
fast moving ejecta and the survival of free neutrons in the face of neutrino
absorptions, although the precursor properties are robust to a moderate amount
of leptonization. Our results provide additional motivation for short latency
gravitational wave triggers and rapid follow-up searches with sensitive ground
based telescopes.Comment: 6 pages, 5 figures, accepted to MNRAS main journa
Nucleosynthesis constraints on the neutron star-black hole merger rate
We derive constraints on the time-averaged event rate of neutron star-black
hole (NS-BH) mergers by using estimates of the population-integrated production
of heavy rapid neutron-capture (r-process) elements with nuclear mass numbers A
> 140 by such events in comparison to the Galactic repository of these chemical
species. Our estimates are based on relativistic hydrodynamical simulations
convolved with theoretical predictions of the binary population. This allows us
to determine a strict upper limit of the average NS-BH merger rate of ~6*10^-5
per year. We quantify the uncertainties of this estimate to be within factors
of a few mostly because of the unknown BH spin distribution of such systems,
the uncertain equation of state of NS matter, and possible errors in the
Galactic content of r-process material. Our approach implies a correlation
between the merger rates of NS-BH binaries and of double NS systems.
Predictions of the detection rate of gravitational-wave signals from such
compact-object binaries by Advanced LIGO and Advanced Virgo on the optimistic
side are incompatible with the constraints set by our analysis.Comment: 5 pages, 3 figures; accepted for publication in ApJ
Impact of weak interactions of free nucleons on the r-process in dynamical ejecta from neutron-star mergers
We investigate beta-interactions of free nucleons and their impact on the
electron fraction (Y_e) and r-process nucleosynthesis in ejecta characteristic
of binary neutron star mergers (BNSMs). For that we employ trajectories from a
relativistic BNSM model to represent the density-temperature evolutions in our
parametric study. In the high-density environment, positron captures decrease
the neutron richness at the high temperatures predicted by the hydrodynamic
simulation. Circumventing the complexities of modelling three-dimensional
neutrino transport, (anti)neutrino captures are parameterized in terms of
prescribed neutrino luminosities and mean energies, guided by published results
and assumed as constant in time. Depending sensitively on the adopted
neutrino-antineutrino luminosity ratio, neutrino processes increase Y_e to
values between 0.25 and 0.40, still allowing for a successful r-process
compatible with the observed solar abundance distribution and a significant
fraction of the ejecta consisting of r-process nuclei. If the electron neutrino
luminosities and mean energies are relatively large compared to the
antineutrino properties, the mean Y_e might reach values >0.40 so that neutrino
captures seriously compromise the success of the r-process. In this case, the
r-abundances remain compatible with the solar distribution, but the total
amount of ejected r-material is reduced to a few percent, because the
production of iron-peak elements is favored. Proper neutrino physics, in
particular also neutrino absorption, have to be included in BNSM simulations
before final conclusions can be drawn concerning r-processing in this
environment and concerning observational consequences like kilonovae, whose
peak brightness and color temperature are sensitive to the
composition-dependent opacity of the ejecta.Comment: 12 pages, 9 figures; submitted to MNRA
Nucleosynthesis in dynamical and torus ejecta of compact binary mergers
We present a comprehensive study of r-process element nucleosynthesis in the
ejecta of compact binary mergers (CBMs) and their relic black-hole (BH)-torus
systems. The evolution of the BH-accretion tori is simulated for seconds with a
Newtonian hydrodynamics code including viscosity effects, pseudo-Newtonian
gravity for rotating BHs, and an energy-dependent two-moment closure scheme for
the transport of electron neutrinos and antineutrinos. The investigated cases
are guided by relativistic double neutron star (NS-NS) and NS-BH merger models,
producing ~3-6 Msun BHs with rotation parameters of A~0.8 and tori of 0.03-0.3
Msun. Our nucleosynthesis analysis includes the dynamical (prompt) ejecta
expelled during the CBM phase and the neutrino and viscously driven outflows of
the relic BH-torus systems. While typically ~20-25% of the initial
accretion-torus mass are lost by viscously driven outflows, neutrino-powered
winds contribute at most another ~1%, but neutrino heating enhances the viscous
ejecta significantly. Since BH-torus ejecta possess a wide distribution of
electron fractions (0.1-0.6) and entropies, they produce heavy elements from
A~80 up to the actinides, with relative contributions of A>130 nuclei being
subdominant and sensitively dependent on BH and torus masses and the exact
treatment of shear viscosity. The combined ejecta of CBM and BH-torus phases
can reproduce the solar abundances amazingly well for A>90. Varying
contributions of the torus ejecta might account for observed variations of
lighter elements with 40<Z<56 relative to heavier ones, and a considerable
reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly
asymmetric NS-BH mergers, might explain the composition of heavy-element
deficient stars.Comment: 7 pages, 4 figures, only changed title compared to previous version,
accepted for publication in Proceedings of Science (Nuclei in the Cosmos
XIII, Debrecen
NACRE II: an update of the NACRE compilation of charged-particle-induced thermonuclear reaction rates for nuclei with mass number
An update of the NACRE compilation [Angulo et al., Nucl. Phys. A 656 (1999)
3] is presented. This new compilation, referred to as NACRE II, reports
thermonuclear reaction rates for 34 charged-particle induced, two-body exoergic
reactions on nuclides with mass number , of which fifteen are
particle-transfer reactions and the rest radiative capture reactions. When
compared with NACRE, NACRE II features in particular (1) the addition to the
experimental data collected in NACRE of those reported later, preferentially in
the major journals of the field by early 2013, and (2) the adoption of
potential models as the primary tool for extrapolation to very low energies of
astrophysical -factors, with a systematic evaluation of uncertainties.
As in NACRE, the rates are presented in tabular form for temperatures in the
T K range. Along with the 'adopted'
rates, their low and high limits are provided. The new rates are available in
electronic form as part of the Brussels Library (BRUSLIB) of nuclear data. The
NACRE II rates also supersede the previous NACRE rates in the Nuclear Network
Generator (NETGEN) for astrophysics.
[http://www.astro.ulb.ac.be/databases.html.]Comment: 86 figure
Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers
We present the first comprehensive study of r-process element nucleosynthesis
in the ejecta of compact binary mergers (CBMs) and their relic black-hole
(BH)-torus systems. The evolution of the BH-accretion tori is simulated for
seconds with a Newtonian hydrodynamics code including viscosity effects,
pseudo-Newtonian gravity for rotating BHs, and an energy-dependent two-moment
closure scheme for the transport of electron neutrinos and antineutrinos. The
investigated cases are guided by relativistic double neutron star (NS-NS) and
NS-BH merger models, producing ~3-6 Msun BHs with rotation parameters of A~0.8
and tori of 0.03-0.3 Msun. Our nucleosynthesis analysis includes the dynamical
(prompt) ejecta expelled during the CBM phase and the neutrino and viscously
driven outflows of the relic BH-torus systems. While typically ~20-25% of the
initial accretion-torus mass are lost by viscously driven outflows,
neutrino-powered winds contribute at most another ~1%, but neutrino heating
enhances the viscous ejecta significantly. Since BH-torus ejecta possess a wide
distribution of electron fractions (0.1-0.6) and entropies, they produce heavy
elements from A~80 up to the actinides, with relative contributions of A>130
nuclei being subdominant and sensitively dependent on BH and torus masses and
the exact treatment of shear viscosity. The combined ejecta of CBM and BH-torus
phases can reproduce the solar abundances amazingly well for A>90. Varying
contributions of the torus ejecta might account for observed variations of
lighter elements with 40<Z<56 relative to heavier ones, and a considerable
reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly
asymmetric NS-BH mergers, might explain the composition of heavy-element
deficient stars.Comment: 30 pages, 22 figures; revised version, accepted by MNRAS; appendix
added with test results for neutrino transpor
Microscopic nature of the photon strength function: stable and unstable Ni and Sn isotopes
The pygmy-dipole resonances and photon strength functions in stable and
unstable Ni and Sn isotopes are calculated within the microscopic
self-consistent version of the extended theory of finite fermi systems which
includes the QRPA and phonon coupling effects and uses the known Skyrme forces
SLy4. The pygmy dipole resonance in is predicted with the mean energy
of 12.4 MeV and the energy-weighted sum rule exhausting 25.6\% of the total
strength. The microscopically obtained photon E1 strength functions are used to
calculate nuclear reaction properties, i.e the radiative neutron capture cross
section, gamma-ray spectra, and average radiative widths. Our main conclusion
is that in all these quantities it is necessary to take the phonon coupling
effects into account.Comment: 4 pages, 5 figures, 2 tables. Talk at 15-th International Symposium
on Capture Gamma-Ray Spectroscopy and Related Topics (CGS15), Dresden, August
2o1
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