IXTH TORINO WORKSHOP ON EVOLUTION AND NUCLEOSYNTHESIS IN AGB STARS AND THE IIND PERUGIA WORKSHOP ON NUCLEAR ASTROPHYSICS
1001(2008); http://dx.doi.org/10.1063/1.2916987View Description Hide Description
The full understanding of final stellar evolution phases is a fundamental request to properly investigate the Universe at any temporal and spatial scale. While the theoretical scenarios of H‐ and He‐burning have been deeply investigated in the last 30 years, the modelling of stellar evolution beyond the core‐helium burning phase and the related nucleosynthesis still present problems related to the physics and to the numerical methods. Low mass AGB Stars are among the most important polluters of the Milky Way, because of the strong winds eroding their chemically enriched envelopes. In the AGB phase, the material processed during the alternating series of H and He burnings is recurrently mixed within the convective zones generated by Thermal Pulses and then partially mixed with the surface by the so called Third Dredge Up episodes. As a matter of fact, MS, S, C(N) and some post‐AGB stars are enriched in C and s‐process elements and the spectro‐scopic detection of unstable Tc demonstrates that the s process is at workin these stars . Within this context, we started a longstanding program devoted to the study of the physical and chemical evolution of low mass AGB stars by means on the FRANEC stellar evolutionary code [2, 3].
Among the most important results we achieved, we highlight the explanation of the mechanism of neutron production, which controls the s‐process nucleosynthesis in low‐mass AGB, the coupling of the code with a full nuclear network, from H up to Pb‐Bi (at the termination point of the s‐process path) and the introduction of C‐enhanced low temperature opacities, whose effects are particularly important at low metallicities. In this paper, we describe the current state of the art of our modelling and we present detailed models of low mass AGB stars at different metallicities (from to ). The final elemental distributions we find are representative of the ones expected for the intrinsic carbon stars observed in the disk and in the halo of the Milky Way.
1001(2008); http://dx.doi.org/10.1063/1.2916953View Description Hide Description
Asymptotic Giant Branch (AGB) stars undergo a change in their chemical composition during their evolution. This in turn leads to an alteration of the radiative opacities, especially in the cool layers of the envelope and the atmosphere, where molecules are the dominant opacity sources. A key parameter in this respect is the number ratio of carbon to oxygen atoms (C/O). In terms of low temperature mean opacities, a variation of this parameter usually cannot be followed in stellar evolution models, because up to now tabulated values were only available for scaled solar metal mixtures (with ). We thus present a set of newly generated tables containing Rosseland mean opacity coefficients covering both the oxygen‐rich and the carbon‐rich regime. We compare our values to existing tabular data and investigate the relevant molecular opacity contributors.
1001(2008); http://dx.doi.org/10.1063/1.2916964View Description Hide Description
We briefly discuss the influence of mass loss on the internal evolutionary properties of massive stars. In particular we show that the C abundance left by the central He burning depends significantly on the adopted mass loss rate after the H rich mantle has been completely ejected.
1001(2008); http://dx.doi.org/10.1063/1.2916973View Description Hide Description
Type Ia supernovae are identified as exploding degenerate stars. Their luminosity is due to the radioactive decay of about a solar mass of through to Although it is generally accepted that a degenerate carbon/oxygen white dwarf explodes as it accretes material from a binary companion, the progenitors of type Ia supernovae have not been categorically identified. The cores of intermediate mass asymptotic giant branch stars are composed of carbon and oxygen in a degenerate state. We discuss conditions under which carbon can ignite at the centre of such a core and when the ensuing explosion would appear as a type Ia supernova.
1001(2008); http://dx.doi.org/10.1063/1.2916984View Description Hide Description
The R stars are a rare class of K‐type giant carbon stars. Canonical stellar evolutionary theory cannot explain their existence, yet they have been observed for more than a century. The early‐R stars, the warmest in the R class, are enhanced in and relative to the Sun, but not in s‐processes elements or oxygen, and are all single stars. We test the idea that binary mergers lead to the formation of the early‐R stars by a comparison of binary population synthesis model results with observations.
1001(2008); http://dx.doi.org/10.1063/1.2916988View Description Hide Description
The possible sites of hot CNO processing for the gas which allowed the formation of “second generation”, chemically anomalous, stars in Globular Clusters are either the envelopes of massive rotating stars or the envelopes of massive asymptotic giant branch (AGB) stars. As the massive stars destroy Lithium, while the massive AGBs can also produce it, the Lithium abundance of unevolved stars of GCs could be a powerful discriminant between the two scenarios. We show the status of art of the computation of Lithium production in massive AGBs for different population II metallicities, provide explanations for the trends with metallicity and mass loss rate of the models, and compare the results with observations. Unfortunately, today's determinations of Lithium abundance in GC stars are not sufficient, and not enough precise, to allow a choice between self–enrichment by AGB or massive star models.
1001(2008); http://dx.doi.org/10.1063/1.2916989View Description Hide Description
We test the reliability of the self‐enrichment scenario for Globular Clusters of intermediate metallicity, i.e. To this scope, we calculated new models of intermediate mass stars evolved during the Asymptotic giant branch phase, with the latest updates of the input physics and with an α–enhanced mixture We compare the theoretical yields with the observed patterns of stars in Globular Clusters, and confirm that when convection is modelled efficiently, the chemical composition of the ejecta of the most massive models are in agreement with the observed abundance patterns.
1001(2008); http://dx.doi.org/10.1063/1.2916990View Description Hide Description
In this contribution we present the first measurements of the C/O ratio for a sample of AGB stars in the LMC globular cluster NGC 1846. From a variability study we set strong constraints on the mass of the cluster stars, which is an important quantity for comparison with evolutionary models. Variations of the surface abundance both with luminosity and pulsation properties were found and the various relevant factors are discussed. We find a good agreement with our synthetic models if we assume The F abundance seems to increase with the C/O ratio. As variability plays a key role on the AGB we give some preliminary results from our study of abundance determination in the dynamical case at the end of this paper.
1001(2008); http://dx.doi.org/10.1063/1.2916991View Description Hide Description
We propose an explanation for the considerable scatter of the abundances of neutron capture elements observed in low metallicity stars in the solar vicinity, compared to the small star to star scatter observed for the α‐elements. We have developed a stochastic chemical evolution model, in which the main assumption is a random formation of new stars, subject to the condition that the cumulative mass distribution follows a given initial mass function. With our model we are able to reproduce the different spreads of neutron capture elements and α‐elements in low metallicity stars. The reason for different observed spread in neutron capture elements and α‐elements resides in the random birth of stars coupled with different stellar mass ranges from which α‐elements and neutron capture elements originate. In particular, the site of production of α‐elements is the whole range of the massive stars, from 10 to whereas the mass range of production for neutron capture elements lies between 12 and
1001(2008); http://dx.doi.org/10.1063/1.2916992View Description Hide Description
We have computed and analyzed the evolution of intermediate‐mass stars of metallicity from the main sequence until the early stages of the TP‐(S)AGB phase. In order to check the influence of mixing we have performed our calculations using two different evolutionary codes, EVOLVE—which does not include diffusion but allows for overshooting—and LPCODE, which includes diffusion and overshooting “à‐la‐Herwig”. Important differences appear during the TP‐AGB phase, as the calculations done with EVOLVE lead to an almost negligible third dredge‐up, whereas the sequences computed with LPCODE show important third dredge‐up and, therefore, significant enrichment in metals of the envelope. This fact has consequences on the strength of the winds during the TP‐AGB phase and, ultimately, on the final fate of the considered stars. Also, the models computed with LPCODE show strong double flashes produced by the ingestion of protons on the helium burning shell.
1001(2008); http://dx.doi.org/10.1063/1.2916993View Description Hide Description
The history of nuclear physics data for s‐process calculations are briefly reviewed with emphasis on the actual status of the neutron capture cross sections. The remaining challenges will be illustrated and discussed in the light of new or optimized methods and state‐of‐the‐art facilities, indicating the potential for accurate measurements and the possibility to study cross sections of radioactive isotopes. These opportunities will be considerably enriched by the enormous improvements provided by upcoming new facilities.
1001(2008); http://dx.doi.org/10.1063/1.2916994View Description Hide Description
The neutron Time‐of‐Flight facility at CERN has been used for a series of neutron capture and fission cross sections measurements. The facility, detectors and data analysis techniques are described, followed by a brief review of the capture results and of the preliminary results for selected fission cross sections. The problem of photon strength functions is discussed and preliminary first results are presented. All these subjects are discussed with respect to their implications in Nuclear Astrophysics. The last section is dedicated to the future perspectives concerning the next experimental campaign and the upgrade of the facility.
1001(2008); http://dx.doi.org/10.1063/1.2916995View Description Hide Description
The reaction is considered as the main neutron source for s‐process in low‐mass asymptotic giant branch stars. At low energies of astrophysical interest, the contribution of the subthreshold state 6.356 MeV of to the cross section should be taken into account. However, the results of previous studies of this contribution lead to different conclusions.
Hence, we investigated the effect of this resonance on the astrophysical S‐factor through a new precise measurement of the alpha spectroscopic factor, of the 6.356 MeV state using the transfer reaction at two different incident energies. The measured angular distributions and the obtained spectroscopic factors will be presented as well as their impact on cross section and reaction rate.
1001(2008); http://dx.doi.org/10.1063/1.2916951View Description Hide Description
The Trojan Horse Method has been recently applied to the study of reactions involved in fluorine nucleosynthesis inside AGB stars. Fluorine abundance is important since it allows to constrain mixing models from the comparison of the observed fluorine abundances with the ones predicted by models. Anyway direct measurements of the cross section do not extend down to the Gamow peak, which is the astrophysically relevant energy region. In particular the study focuses on the and the reactions which can influence fluorine yield as they are part of production/destruction network.
1001(2008); http://dx.doi.org/10.1063/1.2916952View Description Hide Description
Light elements lithium, beryllium and boron (LiBeB) were used in the last years as “possible probe” for a deeper understanding of some extra‐mixing phenomena occurring in young Main‐Sequence stars. They are mainly destroyed by (p,α) reactions and cross section measurements for such channels are then needed. The Trojan Horse Method (THM) allows one to extract the astrophysical S(E)‐factor without the experience of tunneling through the Coulomb barrier. In this work a résumé of the recent results about the and reactions is shown.
1001(2008); http://dx.doi.org/10.1063/1.2916954View Description Hide Description
The study of post‐AGB stars gives important informations on the s‐process inside AGB stars, as the s‐enrichment of post‐AGB stars is due to an intrinsic process. A sample of thirteen C‐and s‐rich post‐AGB stars were analyzed with AGB nucleosynthetic models at different masses, metallicities and s‐process efficiencies. We present and discuss the theoretical fits obtained for 3 selected stars of our sample. A summary of the results obtained by comparing spectroscopic abundances and AGB theoretical models for all the considered stars is reported.
1001(2008); http://dx.doi.org/10.1063/1.2916955View Description Hide Description
A sample of 74 very metal‐poor, C‐ and s‐rich rich stars were collected from the literature. We give here a summary of the results obtained by comparing spectroscopic abundances of CEMP‐s stars and AGB theoretical models, using different ‐pocket efficiencies, initial masses and initial r‐enrichments. They acquired the C and s enrichments by mass transfer in a close binary system from the more massive companion while on the AGB (now a white dwarf). In general, reasonable solutions are obtained for all the stars. About half of these objects have a relevant number of elements detected at high resolution spectroscopy, while only 10 of them were obtained at moderate resolution. When few elements were detected, several solutions are possible. For most of the main‐sequence turnoff stars, due to the low Na and, in some cases, the low ls peak (Sr‐Y‐Zr), a good interpretation can only be obtained with lower initial AGB masses and the absence of dilution factors suggests that no efficient thermohaline mixing had occurred. Among the stars with r‐elements measured, an important number is r‐enhanced including six objects which need a strong initial r‐enrichment We show here some examples.
1001(2008); http://dx.doi.org/10.1063/1.2916956View Description Hide Description
Barium stars are stars of Population I. They are considered extrinsic AGB stars as their enhancement in carbon and s‐process elements is due to mass transfer in binary systems from the more massive companion, when it reaches the Thermal Pulsing Asymptotic Giant Branch. The accreted material from the AGB donor is mixed with the envelope of the secondary, resulting in a dilution of the s‐element abundances. We have made an analysis of the s‐element formation inside the AGB donor and the subsequent dilution inside the observed star, for a sample of barium dwarfs and giants. We obtained theoretical fits for all the stars of our sample. We discuss the intrinsic aspects of these stars and comment on the dilution factors obtained from our fits.
1001(2008); http://dx.doi.org/10.1063/1.2916957View Description Hide Description
A recent survey of a large sample of massive Galactic asymptotic giant branch (AGB) stars shows that significant overabundances of rubidium (up to 100 times solar), but merely solar zirconium, are present in these stars. These observations can set constraints on our theoretical notion of the slow neutron capture process (the s process) in AGB stars, as well as on the rates of the neutron capture reactions involved in the production of Rb and Zr. We use the Monash nucleosynthesis code with a recently extended network to try to reproduce these observations. We present results for AGB stars of masses 5, 6, and and solar metallicity. We also show results for different available choices of the neutron capture rates, as well as for the possible inclusion of a partial mixing zone (PMZ), leading to the activation of the neutron source. We find increasing Rb overabundances with increasing stellar mass, as observed, but we are far from matching the highest observed Rb enhancements. Inclusion of a PMZ increases the Rb abundance, but also produces an overabundance of Zr, contrary to what is observed. Only if the third dredge up efficiency remains as high as before the onset of the superwind phase during the final few pulses of a massive AGB star, can we match the highest [Rb/Fe] ratios observed by García‐Hernández et al. [l]. A better understanding of the third dredge up efficiency with decreasing envelope mass for massive AGB stars is essential for further investigation of this issue.