113 research outputs found
Luminosities of Carbon-rich Asymptotic Giant Branch stars in the Milky Way
Stars evolving along the Asymptotic Giant Branch can become Carbon-rich in
the final part of their evolution. They replenish the inter-stellar medium with
nuclear processed material via strong radiative stellar winds. The
determination of the luminosity function of these stars, even if far from being
conclusive, is extremely important to test the reliability of theoretical
models. In particular, strong constraints on the mixing treatment and the
mass-loss rate can be derived.
We present an updated Luminosity Function of Galactic Carbon Stars obtained
from a re-analysis of available data already published in previous papers.
Starting from available near- and mid-infrared photometric data, we
re-determine the selection criteria. Moreover, we take advantage from updated
distance estimates and Period-Luminosity relations and we adopt a new
formulation for the computation of Bolometric Corrections. This leads us to
collect an improved sample of carbon-rich sources from which we construct an
updated Luminosity Function.
The Luminosity Function of Galactic Carbon Stars peaks at magnitudes around
-4.9, confirming the results obtained in a previous work. Nevertheless, the
Luminosity Function presents two symmetrical tails instead of the larger high
luminosity tail characterizing the former Luminosity Function. The derived
Luminosity Function of Galactic Carbon Stars matches the indications coming
from recent theoretical evolutionary Asymptotic Giant Branch models, thus
confirming the validity of the choices of mixing treatment and mass-loss
history. Moreover, we compare our new Luminosity Function with its counterpart
in the Large Magellanic Cloud finding that the two distributions are very
similar for dust-enshrouded sources, as expected from stellar evolutionary
models. Finally, we derive a new fitting formula aimed to better determine
Bolometric Corrections for C-stars.Comment: 7 pages, 5 figures. To be published in Astronomy and Astrophysic
Nucleation of small silicon carbide dust clusters in AGB stars
Silicon carbide (SiC) grains are a major dust component in carbon-rich AGB
stars. The formation pathways of these grains are, however, not fully
understood.\ We calculate ground states and energetically low-lying structures
of (SiC), clusters by means of simulated annealing (SA) and Monte
Carlo simulations of seed structures and subsequent quantum-mechanical
calculations on the density functional level of theory. We derive the infrared
(IR) spectra of these clusters and compare the IR signatures to observational
and laboratory data.\ According to energetic considerations, we evaluate the
viability of SiC cluster growth at several densities and temperatures,
characterising various locations and evolutionary states in circumstellar
envelopes.\ We discover new, energetically low-lying structures for
SiC, SiC, SiC and SiC, and
new ground states for SiC and SiC. The clusters
with carbon-segregated substructures tend to be more stable by 4-9 eV than
their bulk-like isomers with alternating Si-C bonds. However, we find ground
states with cage ("bucky"-like) geometries for SiC and
SiC and low-lying, stable cage structures for n 12. The
latter findings indicate thus a regime of clusters sizes that differs from
small clusters as well as from large-scale crystals. Thus, and owing to their
stability and geometry, the latter clusters may mark a transition from a
quantum-confined cluster regime to crystalline, solid bulk-material.
The calculated vibrational IR spectra of the ground-state SiC clusters shows
significant emission. They include the 10-13 m wavelength range and the
11.3 m feature inferred from laboratory measurements and observations,
respectively, though the overall intensities are rather low.Comment: 16 pages, 25 figures, 3 tables, accepted for publication in Ap
The chemical composition of carbon stars: The R-type stars
The aim of this work is to shed some light on the problem of the formation of
carbon stars of R-type from a detailed study of their chemical composition. We
use high-resolution and high signal-to-noise optical spectra of 23 R-type stars
selected from the Hipparcos catalogue. The chemical analysis is made using
spectral synthesis in LTE and state-of-the-art carbon-rich spherical model
atmospheres. We derive their CNO content (including the carbon isotopic ratio),
average metallicity, lithium, and light (Sr, Y, Zr) and heavy (Ba, La, Nd, Sm)
s-element abundances. The observed properties of the stars (galactic
distribution, kinematics, binarity, photometry and luminosity) are also
discussed. Our analysis shows that late-R stars are carbon stars with identical
chemical and observational characteristics than the normal (N-type) AGB carbon
stars. We confirm the results of the sole previous abundance analysis of
early-R stars by Dominy (1984, ApJS, 55, 27), namely: they are carbon stars
with near solar metallicity showing enhanced nitrogen, low carbon isotopic
ratios and no s-element enhancements. In addition, we have found that early-R
stars have Li abundances larger than expected for post RGB tip giants. We also
find that a significant number (aprox. 40 %) of the early-R stars in our sample
are wrongly classified, being probably classical CH stars and normal K giants.
In consequence, we suggest that the number of true R stars is considerably
lower than previously believed. We briefly discuss the different scenarios
proposed for the formation of early-R stars. The mixing of carbon during an
anomalous He-flash is favoured, although no physical mechanism able to trigger
that mixing has been found yet. The origin of these stars still remains a
mystery.Comment: 15 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
New constraints on the major neutron source in low-mass AGB stars
We compare updated Torino postprocessing asymptotic giant branch (AGB)
nucleosynthesis model calculations with isotopic compositions of mainstream SiC
dust grains from low-mass AGB stars. Based on the data-model comparison, we
provide new constraints on the major neutron source, 13C({\alpha},n)16O in the
He-intershell, for the s-process. We show that the literature Ni, Sr, and Ba
grain data can only be consistently explained by the Torino model calculations
that adopt the recently proposed magnetic-buoyancy-induced 13C-pocket. This
observation provides strong support to the suggestion of deep mixing of H into
the He-intershell at low 13C concentrations as a result of efficient transport
of H through magnetic tubes.Comment: ApJ, accepte
The s-Process Nucleosynthesis in Low Mass Stars : Impact of the Uncertainties in the Nuclear Physics Determined by Monte Carlo Variations
© Springer Nature Switzerland AG 2019We investigated the impact of uncertainties in neutron-capture and weak reactions (on heavy elements) on the s-process nucleosynthesis in low-mass stars using a Monte-Carlo based approach. We performed extensive nuclear reaction network calculations that include newly evaluated temperature-dependent upper and lower limits for the individual reaction rates. Our sophisticated approach is able to evaluate the reactions that impact more significantly the final abundances. We found that -decay rate uncertainties affect typically nuclides near s-process branchings, whereas most of the uncertainty in the final abundances is caused by uncertainties in neutron capture rates, either directly producing or destroying the nuclide of interest. Combined total nuclear uncertainties due to reactions on heavy elements are approximately 50%
Oxygen and Aluminum-Magnesium Isotopic Systematics of Presolar Nanospinel Grains from CI Chondrite Orgueil
Presolar oxide grains have been previously divided into several groups (Group
1 to 4) based on their isotopic compositions, which can be tied to several
stellar sources. Much of available data was acquired on large grains, which may
not be fully representative of the presolar grain population present in
meteorites. We present here new O isotopic data for 74 small presolar oxide
grains (~200 nm in diameter on average) from Orgueil and Al-Mg isotopic
systematics for 25 of the grains. Based on data-model comparisons, we show that
(i) Group 1 and Group 2 grains more likely originated in low-mass first-ascent
(red giant branch; RGB) and/or second-ascent (asymptotic giant branch; AGB) red
giant stars and (ii) Group 1 grains with (26Al/27Al)0 >= 5x10^-3 and Group 2
grains with (26Al/27Al)0 <= 1x10^-2 all likely experienced extra circulation
processes in their parent low-mass stars but under different conditions,
resulting in proton-capture reactions occurring at enhanced temperatures. We do
not find any large 25Mg excess in Group 1 oxide grains with large 17O
enrichments, which provides evidence that 25Mg is not abundantly produced in
low-mass stars. We also find that our samples contain a larger proportion of
Group 4 grains than so far suggested in the literature for larger presolar
oxide grains (~400 nm). We also discuss our observations in the light of
stellar dust production mechanisms
n_TOF: Measurements of Key Reactions of Interest to AGB Stars
In the last 20 years, the neutron time-of-flight facility n_TOF at CERN has been providing relevant data for the astrophysical slow neutron capture process (s process). At n_TOF, neutron-induced radiative capture (n,γ) as well as (n,p) and (n,α) reaction cross sections are measured as a function of energy, using the time-of-flight method. Improved detection systems, innovative ideas and collaborations with other neutron facilities have lead to a considerable contribution of the n_TOF collaboration to studying the s process in asymptotic giant branch stars. Results have been reported for stable and radioactive samples, i.e.,24,25,26Mg,26Al,33S,54,57Fe,58,59,62,63Ni,70,72,73Ge,90,91,92,93,94,96Zr,139La,140Ce,147Pm,151Sm,154,155,157Gd,171Tm,186,187,188Os,197Au,203,204Tl,204,206,207Pb and209Bi isotopes, while others are being studied or planned to be studied in the near future. In this contribution, we present an overview of the most successful achievements, and an outlook of future challenging measurements, including ongoing detection system developments
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