150 research outputs found
Nucleosynthesis and mixing on the Asymptotic Giant Branch. III. Predicted and observed s-process abundances
We present the results of s-process nucleosynthesis calculations for AGB
stars of different metallicities and initial masses. The computations were
based on previously published stellar evolutionary models that account for the
III dredge up phenomenon occurring late on the AGB. Neutron production is
driven by the 13C(alpha,n)16O reaction during the interpulse periods in a tiny
layer in radiative equilibrium at the top of the He- and C-rich shell. The
s-enriched material is subsequently mixed with the envelope by the III dredge
up, and the envelope composition is computed after each thermal pulse. We
follow the changes in the photospheric abundance of the Ba-peak elements (heavy
s, or `hs') and that of the Zr-peak ones (light s, or `ls'), whose logarithmic
ratio [hs/ls] has often been adopted as an indicator of the s-process
efficiency. The theoretical predictions are compared with published abundances
of s elements for Galactic AGB giants of classes MS, S, SC, post-AGB
supergiants, and for various classes of binary stars. The observations in
general confirm the complex dependence of n captures on metallicity. They
suggest that a moderate spread exists in the abundance of 13C that is burnt in
different stars. Although additional observations are needed, a good
understanding has been achieved of s-process operation in AGB. The detailed
abundance distribution including the light elements (CNO) of a few s-enriched
stars at different metallicity are examined.Comment: Accepted for ApJ, 59 pages, 19 figures, 5 table
Galactic chemical evolution of heavy elements: from Barium to Europium
We follow the chemical evolution of the Galaxy for elements from Ba to Eu,
using an evolutionary model suitable to reproduce a large set of Galactic
(local and non local) and extragalactic constraints. Input stellar yields for
neutron-rich nuclei have been separated into their s-process and r-process
components. The production of s-process elements in thermally pulsing
asymptotic giant branch stars of low mass proceeds from the combined operation
of two neutron sources: the dominant reaction 13C(alpha,n)16O, which releases
neutrons in radiative conditions during the interpulse phase, and the reaction
22Ne(alpha,n)25Mg, marginally activated during thermal instabilities. The
resulting s-process distribution is strongly dependent on the stellar
metallicity. For the standard model discussed in this paper, it shows a sharp
production of the Ba-peak elements around Z = Z_sun/4. Concerning the r-process
yields, we assume that the production of r-nuclei is a primary process
occurring in stars near the lowest mass limit for Type II supernova
progenitors. The r-contribution to each nucleus is computed as the difference
between its solar abundance and its s-contribution given by the Galactic
chemical evolution model at the epoch of the solar system formation. We compare
our results with spectroscopic abundances of elements from Ba to Eu at various
metallicities (mainly from F and G stars) showing that the observed trends can
be understood in the light of the present knowledge of neutron capture
nucleosynthesis. Finally, we discuss a number of emerging features that deserve
further scrutiny.Comment: 34 pages, 13 figures. accepted by Ap
Type Ia Supernova Nucleosynthesis: Metallicity-dependent Yields
Type Ia supernova explosions (SN Ia) are fundamental sources of elements for the chemical evolution of galaxies. They efficiently produce intermediate-mass (with Z between 11 and 20) and iron group elements - for example, about 70% of the solar iron is expected to be made by SN Ia. In this work, we calculate complete abundance yields for 39 models of SN Ia explosions, based on three progenitors - a 1.4 M ⊙ deflagration detonation model, a 1.0 M ⊙ double detonation model, and a 0.8 M ⊙ double detonation model - and 13 metallicities, with 22Ne mass fractions of 0, 1 × 10-7, 1 × 10-6, 1 × 10-5, 1 × 10-4, 1 × 10-3, 2 × 10-3, 5 × 10-3, 1 × 10-2, 1.4 × 10-2, 5 × 10-2, and 0.1, respectively. Nucleosynthesis calculations are done using the NuGrid suite of codes, using a consistent nuclear reaction network between the models. Complete tables with yields and production factors are provided online at Zenodo:Yields (https://doi.org/10.5281/zenodo.8060323). We discuss the main properties of our yields in light of the present understanding of SN Ia nucleosynthesis, depending on different progenitor mass and composition. Finally, we compare our results with a number of relevant models from the literature
Type Ia Supernova Nucleosynthesis: Metallicity-Dependent Yields
Type Ia supernova explosions (SNIa) are fundamental sources of elements for
the chemical evolution of galaxies. They efficiently produce intermediate-mass
(with Z between 11 and 20) and iron group elements - for example, about 70% of
the solar iron is expected to be made by SNIa. In this work, we calculate
complete abundance yields for 39 models of SNIa explosions, based on three
progenitors - a 1.4M deflagration detonation model, a 1.0 double detonation
model and a 0.8 M double detonation model - and 13 metallicities, with 22Ne
mass fractions of 0, 1x10-7, 1x10-6, 1x10-5, 1x10-4, 1x10-3, 2x10-3, 5x10-3,
1x10-2, 1.4x10-2, 5x10-2, and 0.1 respectively. Nucleosynthesis calculations
are done using the NuGrid suite of codes, using a consistent nuclear reaction
network between the models. Complete tables with yields and production factors
are provided online at Zenodo: Yields. We discuss the main properties of our
yields in the light of the present understanding of SNIa nucleosynthesis,
depending on different progenitor mass and composition. Finally, we compare our
results with a number of relevant models from the literature.Comment: 42 pages, 21 figures. Accepted for publication in ApJS 21-06-2
Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with M~0.9 M_sun
Type Ia supernovae (SNe Ia) are thought to result from thermonuclear
explosions of carbon-oxygen white dwarf stars. Existing models generally
explain the observed properties, with the exception of the sub-luminous
1991-bg-like supernovae. It has long been suspected that the merger of two
white dwarfs could give rise to a type Ia event, but hitherto simulations have
failed to produce an explosion. Here we report a simulation of the merger of
two equal-mass white dwarfs that leads to an underluminous explosion, though at
the expense of requiring a single common-envelope phase, and component masses
of ~0.9 M_sun. The light curve is too broad, but the synthesized spectra, red
colour and low expansion velocities are all close to what is observed for
sub-luminous 1991bg-like events. While mass ratios can be slightly less than
one and still produce an underluminous event, the masses have to be in the
range 0.83-0.9 M_sun.Comment: Accepted to Natur
Lead: Asymptotic Giant Branch production and Galactic Chemical Evolution
The enrichment of Pb in the Galaxy is followed in the framework of a detailed
model of Galactic chemical evolution that already proved adequate to reproduce
the chemical enrichment of O and of the elements from Ba to Eu. The stellar
yields are computed through nucleosynthesis calculations in the Asymptotic
Giant Branch (AGB) phase of low- and intermediate-mass stars, covering a wide
range of metallicities. The physical parameters of the stellar structure were
derived from full stellar evolutionary models previously computed. We show that
low-mass AGB stars are the main producers of Pb in the Galaxy, with a complex
dependence on metallicity and a maximum efficiency at [Fe/H] ~ -1. Our
calculations succeed in reproducing the abundances of Pb isotopes in the solar
system: the role attributed by the classical analysis of the s-process to the
'strong component', in order to explain more than 50% of solar 208Pb, is
actually played by the high production of Pb in low-mass and low-metallicity
AGB stars. We then follow the Galactic chemical evolution of Pb isotopes and
give our expectations on the s-process contribution to each of them at the
epoch of the solar system formation. Finally, we present new spectroscopic
estimates of Pb abundance on a sample of field stars and compare them, together
with a few other determinations available, with the predicted trend of [Pb/Fe]
in the Galaxy.Comment: Accepted for ApJ, 19 pages, 4 figure
Rubidium, zirconium, and lithium production in intermediate-mass asymptotic giant branch stars
A recent survey of a large sample of Galactic intermediate-mass (>3 Msun)
asymptotic giant branch (AGB) stars shows that they exhibit large
overabundances of rubidium (Rb) up to 100--1000 times solar. These observations
set constraints on our theoretical notion of the slow neutron capture process
(s process) that occurs inside intermediate-mass AGB stars. Lithium (Li)
abundances are also reported for these stars. In intermediate-mass AGB stars,
Li can be produced by proton captures occuring at the base of the convective
envelope. For this reason the observations of Rb, Zr, and Li set complementary
constraints on different processes occurring in the same stars. We present
predictions for the abundances of Rb, Zr, and Li as computed for the first time
simultaneously in intermediate-mass AGB star models and compare them to the
current observational constraints. We find that the Rb abundance increases with
increasing stellar mass, as is inferred from observations but we are unable to
match the highest observed [Rb/Fe] abundances. Inclusion of a partial mixing
zone (PMZ) to activate the 13C(a,n)16O reaction as an additional neutron source
yields significant enhancements in the Rb abundance. However this leads to Zr
abundances that exceed the upper limits of the current observational
constraints. If the third dredge-up (TDU) efficiency remains as high during the
final stages of AGB evolution as during the earlier stages, we can match the
lowest values of the observed Rb abundance range. We predict large variations
in the Li abundance, which are observed. Finally, the predicted Rb production
increases with decreasing metallicity, in qualitative agreement with
observations of Magellanic Cloud AGB stars. However stellar models of Z=0.008
and Z=0.004 intermediate-mass AGB stars do not produce enough Rb to match the
observed abundances.Comment: 11 pages, 7 figures, accepted for publication on Astronomy &
Astrophysic
Origin and evolution of the light nuclides
After a short historical (and highly subjective) introduction to the field, I
discuss our current understanding of the origin and evolution of the light
nuclides D, He-3, He-4, Li-6, Li-7, Be-9, B-10 and B-11. Despite considerable
observational and theoretical progress, important uncertainties still persist
for each and every one of those nuclides. The present-day abundance of D in the
local interstellar medium is currently uncertain, making it difficult to infer
the recent chemical evolution of the solar neighborhood. To account for the
observed quasi-constancy of He-3 abundance from the Big Bang to our days, the
stellar production of that nuclide must be negligible; however, the scarce
observations of its abundance in planetary nebulae seem to contradict this
idea. The observed Be and B evolution as primaries suggests that the source
composition of cosmic rays has remained quasi-constant since the early days of
the Galaxy, a suggestion with far reaching implications for the origin of
cosmic rays; however, the main idea proposed to account for that constancy,
namely that superbubbles are at the source of cosmic rays, encounters some
serious difficulties. The best explanation for the mismatch between primordial
Li and the observed "Spite-plateau" in halo stars appears to be depletion of Li
in stellar envelopes, by some yet poorly understood mechanism. But this
explanation impacts on the level of the recently discovered early ``Li-6
plateau'', which (if confirmed), seriously challenges current ideas of cosmic
ray nucleosynthesis.Comment: 18 pages, 9 figs. Invited Review in "Symposium on the Composition of
Matter", honoring Johannes Geiss on the occasion of his 80th birthday
(Grindelwald, Switzerland, Sept. 2006), to be published in Space Science
Series of ISS
Analysis of 26 Barium Stars II. Contributions of s-, r- and p-processes in the production of heavy elements
Barium stars show enhanced abundances of the slow neutron capture (s-process)
heavy elements, and for this reason they are suitable objects for the study of
s-process elements. The aim of this work is to quantify the contributions of
the s-, r- and p-processes for the total abundance of heavy elements from
abundances derived for a sample of 26 barium stars. The abundance ratios
between these processes and neutron exposures were studied. The abundances of
the sample stars were compared to those of normal stars thus identifying the
fraction relative to the s-process main component. The fittings of the sigmaN
curves (neutron capture cross section times abundance, plotted against atomic
mass number) for the sample stars suggest that the material from the companion
asymptotic giant branch star had approximately the solar isotopic composition
as concerns fractions of abundances relative to the s-process main component.
The abundance ratios of heavy elements, hs, ls and s and the computed neutron
exposure are similar to those of post-AGB stars. For some sample stars, an
exponential neutron exposure fits well the observed data, whereas for others, a
single neutron exposure provides a better fit. The comparison between barium
and AGB stars supports the hypothesis of binarity for the barium star
formation. Abundances of r-elements that are part of the s-process path in
barium stars are usually higher than those in normal stars,and for this reason,
barium stars seemed to be also enriched in r-elements, although in a lower
degree than s-elements. No dependence on luminosity classes was found in the
abundance ratios behaviour among the dwarfs and giants of the sample barium
stars.Comment: 30 pages including 24 figures, accepted to A&
SNLS Spectroscopy: Testing for Evolution in Type Ia Supernovae
Aims: We present a quantitative study of a new data set of high redshift Type
Ia supernovae spectra, observed at the Gemini telescopes during the first 34
months of the Supernova Legacy Survey. During this time 123 supernovae
candidates were observed, of which 87 have been identified as SNe Ia at a
median redshift of z=0.720. Spectra from the entire second year of the survey
and part of the third year (59 total SNe candidates with 46 confirmed SNe Ia)
are published here for the first time. The spectroscopic measurements made on
this data set are used determine if these distant SNe comprise a population
similar to those observed locally. Methods: Rest-frame equivalent width and
ejection velocity measurements are made on four spectroscopic features.
Corresponding measurements are presented for a set of 167 spectra from 24 low-z
SNe Ia from the literature. Results: We show that there exists a sample at high
redshift with properties similar to nearby SNe. No significant difference was
found between the distributions of measurements at low and high redsift for
three of the features. The fourth feature displays a possible difference that
should be investigated further. Correlations between Type Ia SNe properties and
host galaxy morphology were also found to be similar at low and high z, and
within each host galaxy class we see no evidence for redshift-evolution in SN
properties. A new correlation between SNe Ia peak magnitude and the equivalent
width of SiII absorption is presented. We demonstrate that this correlation
reduces the scatter in SNe Ia luminosity distances in a manner consistent with
the lightcurve shape-luminosity corrections that are used for Type Ia SNe
cosmology. Conclusions: We show that this new sample of SNLS SNe Ia has
spectroscopic properties similar to nearby objects. (Abridged)Comment: Accepted for publication in Astronomy and Astrophysic
- …