On the mass of supernova progenitors: the role of the 12^{12}C+12+^{12}C reaction

A precise knowledge of the masses of supernova progenitors is essential to answer various questions of modern astrophysics, such as those related to the dynamical and chemical evolution of Galaxies. In this paper we revise the upper bound for the mass of the progenitors of CO white dwarfs (\mup) and the lower bound for the mass of the progenitors of normal type II supernovae (\mups). In particular, we present new stellar models with mass between 7 and 10 \msun, discussing their final destiny and the impact of recent improvements in our understanding of the low energy rate of the \c12c12 reaction.Comment: To be published on the proceedings of NIC 201

Asymptotic Giant Brach Stars as Astroparticle Laboratories

We show that the inclusion of axion emission during stellar evolution introduces important changes into the evolutionary behaviour of AGB stars. The mass of the resulting C/O white dwarf is much lower than the equivalent obtained from standard evolution. This implies a deficit in luminous AGB stars and in massive WDs. Moreover the total mass processed in the nuclear burning shells that is dredged-up to the surface ($3^{rd} D_{up}$) increases when axion emission is included, modifying the chemical composition of the photosphere. We conclude that the AGB is a promising phase to put constraints on particle physicsComment: 8 pages, 3 tables, 8 figures, accepted for publication in MNRA

Heavy elements in Globular Clusters: the role of AGB stars

Recent observations of heavy elements in Globular Clusters reveal intriguing deviations from the standard paradigm of the early galactic nucleosynthesis. If the r-process contamination is a common feature of halo stars, s-process enhancements are found in a few Globular Clusters only. We show that the combined pollution of AGB stars with mass ranging between 3 to 6 M$_\odot$ may account for most of the features of the s-process overabundance in M4 and M22. In these stars, the s process is a mixture of two different neutron-capture nucleosynthesis episodes. The first is due to the 13C(a,n)16O reaction and takes place during the interpulse periods. The second is due to the 22Ne(a,n)25Mg reaction and takes place in the convective zones generated by thermal pulses. The production of the heaviest s elements (from Ba to Pb) requires the first neutron burst, while the second produces large overabundances of light s (Sr, Y, Zr). The first mainly operates in the less-massive AGB stars, while the second dominates in the more-massive. From the heavy-s/light-s ratio, we derive that the pollution phase should last for $150\pm 50$ Myr, a period short enough compared to the formation timescale of the Globular Cluster system, but long enough to explain why the s-process pollution is observed in a few cases only. With few exceptions, our theoretical prediction provides a reasonable reproduction of the observed s-process abundances, from Sr to Hf. However, Ce is probably underproduced by our models, while Rb and Pb are overproduced. Possible solutions are discussed.Comment: Accepted by the Ap

Type Ia supernovae: differences due to progenitors within delayed detonation explosions

At this moment, the use of SNIa for cosmology lies on the assumption that the SNe at high redshifts are equal to the local ones. However, some observations indicate a correlation between the light curve (LC) properties and the morphological type of the host galaxy. This could indicate a dependence with the age (mass/composition) of the underlying population. In this work we have chosen the delayed detonation explosion model in CO Chandrasekhar mass WDs to explore the dependence of the SNIa LC and nucleosynthesis with the initial mass and composition of the WD progenitor. The progenitor influences the final SNIa via the mass of the CO core formed and the C/O ratio within it (1D explosion models). We have followed the evolution of stars with masses between 1.5 and 8 Mo and metallicity, Z=0, 1.E-05, 0.001 and 0.02, from the pre-main sequence to the TP-AGB phase. The differences obtained in the final C/O ratio within the explosive WD are smaller than 22%. This results in a difference at maximum of 0.03 mag and of 0.1 mag when the brightness-decline relation is applied.Comment: 4 pages, 1 figure, needs espcrc1.sty; conference "Nuclei in the Cosmos 2000", held in Arhus, Denmark, June 27-July 1, 2000; submitted to Nucl. Phys.