425 research outputs found
Fluorine in AGB Carbon Stars Revisited
A reanalysis of the fluorine abundance in three Galactic AGB carbon stars (TX
Psc, AQ Sgr and R Scl) has been performed from the molecular HF (1-0) R9 line
at 2.3358 m. High-resolution (R) and high signal to noise
spectra obtained with the CRIRES spectrograph and the VLT telescope or from the
NOAO archive (for TX Psc) have been used. Our abundance analysis uses the
latest generation of MARCS model atmospheres for cool carbon rich stars. Using
spectral synthesis in LTE we derive for these stars fluorine abundances that
are systematically lower by dex in average with respect to the sole
previous estimates by Jorissen, Smith & Lambert (1992). The possible reasons of
this discrepancy are explored. We conclude that the difference may rely on the
blending with C-bearing molecules (CN and C) that were not properly taken
into account in the former study. The new F abundances are in better agreement
with the prediction of full network stellar models of low mass AGB stars. These
models also reproduce the -process elements distribution in the sampled
stars. This result, if confirmed in a larger sample of AGB stars, might
alleviate the current difficulty to explain the largest [F/O] ratios found by
Jorissen et al. In particular, it may not be necessary to search for
alternative nuclear chains affecting the production of F in AGB stars.Comment: 25 pages, 3 figures. to be appear in The Astrophysical Journal (Jan
2009 issue
On the Origin of the Early Solar System Radioactivities. Problems with the AGB and Massive Star Scenarios
Recent improvements in stellar models for intermediate-mass and massive stars
are recalled, together with their expectations for the synthesis of radioactive
nuclei of lifetime Myr, in order to re-examine the origins
of now extinct radioactivities, which were alive in the solar nebula. The
Galactic inheritance broadly explains most of them, especially if -process
nuclei are produced by neutron star merging according to recent models.
Instead, Al, Ca, Cs and possibly Fe require
nucleosynthesis events close to the solar formation. We outline the persisting
difficulties to account for these nuclei by Intermediate Mass Stars (2
M/M). Models of their final stages now
predict the ubiquitous formation of a C reservoir as a neutron capture
source; hence, even in presence of Al production from Deep Mixing or Hot
Bottom Burning, the ratio Al/Pd remains incompatible with
measured data, with a large excess in Pd. This is shown for two recent
approaches to Deep Mixing. Even a late contamination by a Massive Star meets
problems. In fact, inhomogeneous addition of Supernova debris predicts
non-measured excesses on stable isotopes. Revisions invoking specific low-mass
supernovae and/or the sequential contamination of the pre-solar molecular cloud
might be affected by similar problems, although our conclusions here are
weakened by our schematic approach to the addition of SN ejecta. The limited
parameter space remaining to be explored for solving this puzzle is discussed.Comment: Accepted for publication on Ap
Chemical evolution with rotating massive star yields II. A new assessment of the solar s- and r- process components
The decomposition of the Solar system abundances of heavy isotopes into their sand r- components plays a key role in our understanding of the corresponding nuclear
processes and the physics and evolution of their astrophysical sites. We present a new
method for determining the s- and r- components of the Solar system abundances,
fully consistent with our current understanding of stellar nucleosynthesis and galactic chemical evolution. The method is based on a study of the evolution of the solar
neighborhood with a state-of-the-art 1-zone model, using recent yields of low and intermediate mass stars as well as of massive rotating stars. We compare our results with
previous studies and we provide tables with the isotopic and elemental contributions
of the s- and r-processes to the Solar system compositionThis article is based upon work partially supported from
the “ChETEC” COST Action (CA16117) of COST (European Cooperation in Science and Technology). C.A. acknowledges in part to the Spanish grants AYA2015-63588-P
and PGC2018-095317-B-C21 within the European Founds
for Regional Development (FEDER)
Theoretical estimate of the half-life for the radioactive Cs and Cs in astrophysical scenarios
We analyze the CsBa and
CsBa decays, which are crucial
production channels for Ba isotopes in Asymptotic Giant Branch (AGB) stars. We
reckon, from relativistic quantum mechanis, the effects of multichannel
scattering onto weak decays, including nuclear and electronic excited states
(ES) populated above 10 keV, for both parent and daughter nuclei. We
find increases in the half-lives for K (by more than a factor 3 for
Cs) as compared to previous works based on systematics. We also discuss
our method in view of these previous calculations. An important impact on
half-lives comes from nuclear ES decays, while including electronic
temperatures yields further increases of about 20\% at energies 10-30 keV,
typical of AGB stars of moderate mass (). Despite
properly considering these effects, the new rates remain sensitively lower than
the TY values, implying longer half-lives at least above 8-9 keV. Our rate
predictions are in substantial accord with recent results based on the shell
model, and strongly modify branching ratios along the -process path
previously adopted. With our new rate, nucleosynthesis models well account for
the isotopic admixtures of Ba in presolar SiC grains and in the Sun.Comment: 15 pages, 3 figures, 5 tables. Accepted for publication in Ap
Environmental Impact of Meals: How Big Is the Carbon Footprint in the School Canteens?
The inhabitants of the world are expected to grow by two billion in the next two decades; as population increases, food demand rises too, leading to more intensive resource exploitation and greater negative externalities related to food production. In this paper the environmental impact of meals provided in school canteens are analysed through the Life Cycle Assessment methodology, in order to evaluate the GHGs emissions released by food production. Meals, and not just individual foods, have been considered so as to include in the analysis the nutritional aspects on which meals are based. Results shows that meat, fish and dairy products are the most impacting in terms of greenhouse gas emissions, with values that shift from 31.7 and 24.1 kg CO2 eq for butter and veal, to 2.37 kg CO2 eq for the octopus, while vegetables, legumes, fruit and cereals are less carbon intensive (average of 3.71 kg CO2 eq for the considered vegetables). When the environmental impact is related to the food energy, the best option are first courses because they combine a low carbon footprint with a high energy content. The results of the work can be used both by the consumer, who can base the meal choice on environmental impact information, and by food services, who can adjust menus to achieve a more sustainable production
Measurement of 1323 and 1487 keV resonances in 15N({\alpha}, {\gamma})19F with the recoil separator ERNA
The origin of fluorine is a widely debated issue. Nevertheless, the
^{15}N({\alpha},{\gamma})^{19}F reaction is a common feature among the various
production channels so far proposed. Its reaction rate at relevant temperatures
is determined by a number of narrow resonances together with the DC component
and the tails of the two broad resonances at E_{c.m.} = 1323 and 1487 keV.
Measurement through the direct detection of the 19F recoil ions with the
European Recoil separator for Nuclear Astrophysics (ERNA) were performed. The
reaction was initiated by a 15N beam impinging onto a 4He windowless gas
target. The observed yield of the resonances at Ec.m. = 1323 and 1487 keV is
used to determine their widths in the {\alpha} and {\gamma} channels. We show
that a direct measurement of the cross section of the
^{15}N({\alpha},{\gamma})^{19}F reaction can be successfully obtained with the
Recoil Separator ERNA, and the widths {\Gamma}_{\gamma} and {\Gamma}_{\alpha}
of the two broad resonances have been determined. While a fair agreement is
found with earlier determination of the widths of the 1487 keV resonance, a
significant difference is found for the 1323 keV resonance {\Gamma}_{\alpha} .
The revision of the widths of the two more relevant broad resonances in the
15N({\alpha},{\gamma})19F reaction presented in this work is the first step
toward a more firm determination of the reaction rate. At present, the residual
uncertainty at the temperatures of the ^{19}F stellar nucleosynthesis is
dominated by the uncertainties affecting the Direct Capture component and the
364 keV narrow resonance, both so far investigated only through indirect
experiments.Comment: 8 pages, 11 figures. Accepted for publication in PR
Thermonuclear oubursts of AM CVn stars
We consider initial stage of the evolution of AM CVn type stars with white dwarf donors, which is characterized by thermonuclear explosions in the layer of accreted He. It is shown that accretion never results in detonation of He and accretors in AM CVn stars finish their evolution as massive WD. We found, for the first time, that in the outbursts the synthesis of n-rich isotopes, initiated by reaction 22 Ne(α, n) 25 Mg, becomes possible
The First Fluorine Abundance Determinations in Extragalactic AGB Carbon Stars
Fluorine (19F) abundances (or upper limits) are derived in six extragalactic
AGB carbon stars from the HF(1-0) R9 line at 2.3358 mu in high resolution
spectra. The stars belong to the Local Group galaxies LMC, SMC and Carina dwarf
spheroidal, spanning more than a factor 50 in metallicity. This is the first
study to probe the behaviour of F with metallicity in intrinsic extragalactic
C-rich AGB stars. Fluorine could be measured only in four of the target stars,
showing a wide range in F-enhancements. Our F abundance measurements together
with those recently derived in Galactic AGB carbon stars show a correlation
with the observed carbon and s-element enhancements. The observed correlations
however, display a different dependence on the stellar metallicity with respect
to theoretical predictions in low mass, low metallicity AGB models. We briefly
discuss the possible reasons for this discrepancy. If our findings are
confirmed in a larger number of metal-poor AGBs, the issue of F production in
AGB stars will need to be revisited.Comment: 15 pages, 4 figures, accepted in ApJ Letter
Constraints of the Physics of Low-mass AGB Stars from CH and CEMP Stars
We analyze a set of published elemental abundances from a sample of CH stars which are based on high resolution spectral analysis of ELODIE and SUBARU/HDS spectra. All the elemental abundances were derived from local thermodynamic equilibrium analysis using model atmospheres, and thus they represent the largest homogeneous abundance data available for CH stars to date. For this reason, we can use the set to constrain the physics and the nucleosynthesis occurring in low mass asymptotic giant branch (AGB) s.tars. CH stars have been polluted in the past from an already extinct AGB companion and thus show s-process enriched surfaces. We discuss the effects induced on the surface AGB s-process distributions by different prescriptions for convection and rotation. Our reference theoretical FRUITY set fits only part of the observations. Moreover, the s-process observational spread for a fixed metallicity cannot be reproduced. At [Fe/H] > -1, a good fit is found when rotation and a different treatment of the inner border of the convective envelope are simultaneously taken into account. In order to increase the statistics at low metallicities, we include in our analysis a selected number of CEMP stars and, therefore, we compute additional AGB models down to [Fe/H] = -2.85. Our theoretical models are unable to attain the large [hs/ls] ratios characterizing the surfaces of those objects. We speculate on the reasons for such a discrepancy, discussing the possibility that the observed distribution is a result of a proton mixing episode leading to a very high neutron density (the so-called I-process)
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