on the fluorine nucleosynthesis in agb stars in the light of the 19f p α 16o and 19f α p 22ne reaction rate measured via thm

In the last years the [Formula: see text]O and the [Formula: see text]F([Formula: see text],p)[Formula: see text]Ne reactions have been studied via the Trojan Horse Method in the energy range of interest for astrophysics. These are the first experimental data available for the main channels of [Formula: see text]F destruction that entirely cover the energy regions typical of the stellar H- and He- burning. In both cases the reaction rates are significantly larger than the previous estimations available in the literature. We present here a re-analysis of the fluorine nucleosynthesis in Asymptotic Giant Branch stars by employing in state-of-the-art models of stellar nucleosynthesis the THM reaction rates for [Formula: see text]F destruction

effects of the s process on fe group elements in meteorites

In the present paper we investigate the possible connection between s-process nucleosynthesis occurring during the asymptotic giant branch (AGB) phase of low-mass stars (LMS) and the isotopic anomalies of the "Fe-group" elements observed in several macroscopic samples of meteorites or in grains formed as circumstellar condensates (hereafter CIRCONs). The available measurements of chromium, iron, and nickel are well reproduced by stellar models, which account for the largest shifts in the heaviest isotopes of each element: in particular ^(54)Cr, ^(58)Fe, and ^(64)Ni. Moreover, many circumstellar condensates reflect ^(50)Ti excesses and some production of ^(46, 47, 49)Ti, as predicted by slow-neutron captures in AGB stars. Nevertheless, some difficulties are found in comparing theoretical calculations of s-process nucleosynthesis with calcium, silicon, and zinc isotopic anomalies

The temperature and chronology of heavy-element synthesis in low-mass stars

Roughly half of the heavy elements (atomic mass greater than that of iron) are believed to be synthesized in the late evolutionary stages of stars with masses between 0.8 and 8 solar masses. Deep inside the star, nuclei (mainly iron) capture neutrons and progressively build up (through the slow-neutron-capture process, or s-process) heavier elements that are subsequently brought to the stellar surface by convection. Two neutron sources, activated at distinct temperatures, have been proposed: 13C and 22Ne, each releasing one neutron per alpha-particle (4He) captured. To explain the measured stellar abundances, stellar evolution models invoking the 13C neutron source (which operates at temperatures of about one hundred million kelvin) are favoured. Isotopic ratios in primitive meteorites, however, reflecting nucleosynthesis in the previous generations of stars that contributed material to the Solar System, point to higher temperatures (more than three hundred million kelvin), requiring at least a late activation of 22Ne. Here we report a determination of the s-process temperature directly in evolved low-mass giant stars, using zirconium and niobium abundances, independently of stellar evolution models. The derived temperature supports 13C as the s-process neutron source. The radioactive pair 93Zr-93Nb used to estimate the s-process temperature also provides, together with the pair 99Tc-99Ru, chronometric information on the time elapsed since the start of the s-process, which we determine to be one million to three million years.Comment: 30 pages, 10 figure

The Treiman-Yang Criterion: validating the Trojan Horse Method by experimentally probing the reaction mechanism

Proper selection of the quasi-free (QF) break-up channel in a three-body reaction is a key aspect for the applicability of the Trojan Horse Method (THM). The Treiman-Yang (TY) Criterion is a model-independent experimental test for the dominance of the QF mechanism, and hence constitutes one of the strongest validity tests of the THM. An experiment was performed at LNS to apply the test to the d(10B, 7Be α)n reaction. Here, the criterion is described and some preliminary data from the experiment are shown

The 19F(α, p)22Ne and 23Na(p,α)20Ne reaction in AGB nucleosynthesis via THM

In AGB environment, fluorine and sodium abundances are still matter of debate. About 19F (only stable isotope of fluorine), its abundance in the universe is strictly related to standard and extra-mixing processes taking place inside AGB-stars, that are considered to be the most important sites for its production. Nevertheless the way in which it is destroyed is far from being well understood. On the other hand, 23Na presence in Globular Clusters, along with is well-known anticorrelation with oxygen has made clear that this element must be produced in previous generations stars, and intermediatemass AGB stars are one of the possible candidates for its production. For this reason we studied the 19F(α,p)22Ne and 23Na(p,α)20Ne reactions in the energy range of relevance for astrophysics via the Trojan Horse Method (THM), using the three-body reactions 6Li(19F, p22Ne)d and 23Na(d, pn)20Ne

Oxygen isotopic analysis of Mineo (Sicily, Italy) pallasite

Pallasites are stony-iron meteorites essentially composed of olivine [(Mg,Fe)2SiO4] with a metallic portion covering one-third of the total volume (Boesenberg et al., 2012) and principally consisting of Fe-Ni metal. The sample studied in the present work is a piece of the Mineo pallasite, named as the place where it fell in the South-Eastern part of Sicily (Italy) in 1826. The sole sample available in the world belongs to the Department of Physics and Geology at the University of Perugia. Preliminary chemical analysis allowed to locate Mineo meteorite in among the Main Group (MG) pallasites. A peculiar compositional variability of olivines was observed in terms of both iron (11-14 wt%) and trace elements. This observation is quite interesting because the composition of olivines is normally rather uniform within an individual pallasite (Buseck & Goldstein,1969; Boesenberg et al., 2012). The study of oxygen isotopes and trace elements is needed to definitely classify the Mineo meteorite. The Mineo pallasite was sampled and the collected fragments were embedded in epoxy resin and polished up to 1 μm. The isotopic oxygen analysis was obtained by means of Time of Flight – Secondary Ion Mass Spectrometry (ToF- SIMS), which is a static technique used for surface analysis. Samples are bombarded with primary ions at typical energies of 10-30 keV. The corresponding secondary ions are than accelerated into a flight tube and their mass is determined measuring their time of flight. Major advantages of this technique are: a) the high lateral resolution; b) only a limited destruction of the sample, i.e., the erosion is very small due to the use of a pulsed beam; c) parallel detection of all secondary ions with one polarity in a single measurement; d) an adequate mass resolution for the separation of major mass interferences (Stephan, 2001). Five measurements were performed on Mineo fragments: 4 points in olivine and 1 in the metal phase. Oxygen isotopic ratios were used to calculate the δ18O values. Four measurements have been done on a sample of the Brenham meteorite, used as internal standard. The ToF-SIMS analysis showed a variability of the oxygen isotopic composition in the Mineo olivines, on average δ18O = 2.3±0.3, in agreement with the previous findings on the chemical composition. Such a value lies in the lower part of the range of variation of MG pallasites and allows us to definitely classify Mineo among the MG pallasites. Boesenberg, J.S., Delaney, G.S., Hewins, R.H.J. (2012): A petrological and chemical reexamination of Main Group pallasite formation. Geochim. Cosmochim. Acta, 89, 134-158. Buseck, P.R. & Goldstein, J.I. (1969): Olivine compositions and cooling rates of pallasitic meteorites. Geol. Soc. Am. Bull., 80, 2141-2158. Stephan, T. (2001): TOF-SIMS in cosmochemistry. Planet. Space Sci., 49, 859-906