1,721,190 research outputs found
Short-lived radioactivities in the early solar system: A fossil record of our origins in stars
No full textThe measurements, revealing that several radioactive nuclei, with half-lives from 0.1 to 20 Myr, were present, alive, in the Early Solar System, are reviewed. The literature recently published for attempting an astrophysical explanation of their concentrations is then discussed, underlining how several uncertainties still hamper our capability of explaining this crucial data set, informing us on our origins. Galactic evolution can broadly account for most of the above nuclei, but the shortest-lived ones (26Al, 41Ca, 135Cs and perhaps 60Fe) require some nucleosynthesis event very close in time and in space to the forming Sun. The difficulties met by stellar nucleosynthesis in intermediate-mass stars (IMSs: 2 ≤ M/M_{⊙} ≤ 7-8) and in massive stars (MSs; M/M_{⊙} ≤ 10) are then presented and it is illustrated how also the expected pollution of the interstellar medium by multiple previously estinguished stars in a possible cluster or association in which the Sun might have formed may not solve the problems, so that short-lived radioactivities remain as a challenging constraint for nuclear astrophysics
The Be()Li reaction and the Cosmological Lithium Problem: measurement of the cross section in a wide energy range at n_TOF (CERN)
No full textWe report on the measurement of the Be()Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n_TOF facility at CERN. This reaction plays a key role in the lithium yield of the Big Bang Nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and showed a large discrepancy between each other. The measurement was performed with a Si-telescope, and a high-purity sample produced by implantation of a Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low-energy, relative to current evaluations, in the region of BBN interest the present results are consistent with the values inferred from the time-reversal Li()Be reaction, thus yielding only a relatively minor improvement on the so-called Cosmological Lithium Problem (CLiP). The relevance of these results on the near-threshold neutron production in the p+Li reaction is also discussed. ..
Technetium in Galactic Bulge AGB Stars
No full textTechnetium has been of high interest in AGB studies for many decades due to its radioactive nature. Here we report on observations of technetium in galactic bulge AGB variables. ..
La produzione degli elementi chimici nelle stelle. I - L'evoluzione stellare e la nucleosintesi primaria.
No full textNot Availabl
Chemical evolution of the galactic halo. I - Effects of possible mass segregation mechanisms
No full textThe distribution of metals in globular clusters and in the stars of the
bulge component of the Galaxy are reproduced by means of galactic halo
chemical evolution models giving particular attention to the observed
Z(N) relation. Following Hartwick's (1976) demonstration that chemical
distributions of the type observed for population II stars can be
theoretically reproduced if mass is temporarily removed from the
star-forming regions, two alternative mass segregation mechanisms are
proposed to account for this phenomenon: the first being derived from
the different dynamical behavior of the gas and the stars, and the
second from consideration of a star formation law conditioned by the
ionization of early-type stars. Models based on simple schematizations
of the phenomena to which the two mass segregation mechanisms give rise,
as well as on an initial mass function allowing for high M/L ratios, are
found to reproduce observational data for both metal abundances and mass
distributions
On the luminosity of AGB stars as observed in the infrared from space and from the ground
No full textAs a part of a reanalysis of galactic Asymptotic Giant Branch stars (hereafter AGB stars), we discuss here two samples for which photometry in the near- and mid-infrared and distance estimates exist. Whenever possible we searched also for mass loss rates. Spectral energy distributions were always up to 20 mu m and for the best observed sources up to 45 mu m. For the first sample (carbon-rich stars) we present rather complete results, while for the second sample (S stars) we give only the preliminary results. In particular, for carbon stars the wide wavelength coverage allows us to obtain reliable bolometric magnitudes. Moreover, we show that mid-IR fluxes are crucial to estimate the magnitudes of stars with dusty envelopes. The resulting brightness of C stars is relatively high (brighter than Mbol = -4.8), so that the long-claimed luminosity problem of C-rich stars does not appear to exist. We examine the different properties of the carbon rich stars according to their type of variability and find that the relevance of Miras and Semiregulars varies with evolution, with Miras dominating the final stages. We present here also the first results of a similar analysis that we are doing for a sample of galactic S stars. Finally, we show here a first comparison between mid-IR measurements made by space observatoriess (ISO, MSX) and ground observations performed by us with the IR camera TIRCAM2...
Buoyant magnetic flux tubes as a site for 26Al production in AGB stars
No full textWe address the diffusive circulation of matter occurring in AGB stars above the H-burning shell, which triggers the production of p-rich isotopes like 13C and 17O and of the unstable nucleus 26Al, observed in presolar grains and in Early Solar System materials. As a physical mechanism for these phenomena we consider the buoyancy of magnetic flux tubes formed below the convective envelope and we arrive at expressions for the relevant mixing parameters in terms of the required strength of the magnetic field |/rightarrowB|. We show how values of |/rightarrowB| like those normally expected in the radiative layers above the H-burning shell of a red giant can indeed trigger chemical mixing at the required efficiency and should therefore be considered for explaining the abundance peculiarities induced in such stars by proton captures. The technique discussed here is suitable for chemically stratified regions. A different formalism should be used for studying the more internal, well-mixed He layers, where similar circulation phenomena induce the formation of the neutron source 13Cb during dredge-up. ..
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