In this article we study the Galactic evolution of the LiBeB elements within the framework of a detailed model of the chemical evolution of the Galaxy that includes Galactic cosmic-ray (GCR) nucleosynthesis by particles accelerated in superbubbles. The chemical composition of the superbubble consists of varying proportions of interstellar medium (ISM) and freshly supernova-synthesized material. The observational trends of 6LiBeB evolution are nicely reproduced by models in which GCRs come from a mixture of 25% supernova material with 75% ISM, except for 6Li, for which perhaps an extra source is required at low metallicities. To account for 7Li evolution, several additional sources have been considered (neutrino-induced nucleosynthesis, nova outbursts, and C stars). The model fulfills the energetic requirements for GCR acceleration.

A LiBeB evolution model including Galactic Cosmic Ray nucleosynthesis, the nu-process, novae, AGB and C-stars is presented.

We have developed a detailed standard chemi- cal evolution model to study the evolution of all the chem- ical elements up to the iron peak in the solar vicinity. We consider that the Galaxy was formed through two episodes of exponentially decreasing infall, out of extragalactic gas. In a first infall episode, with a duration of ∼ 1 Gyr, the halo and the thick disk were assembled out of primordial gas, while the thin disk formed in a second episode of in- fall of slightly enriched extragalactic gas, with much longer timescale. The model nicely reproduces the main observa- tional constraints of the solar neighborhood, and the cal- culated elemental abundances at the time of the solar birth are in excellent agreement with the solar abundances. By the inclusion of metallicity dependent yields for the whole range of stellar masses we follow the evolution of 76 iso- topes of all the chemical elements between hydrogen and zinc. Those results are confronted with a large and recent body of observational data, and we discuss in detail the implications for stellar nucleosynthesis.

We present the Light Element Evolution resulting from our new Chemical Evolution model. The LiBeB evolution is correctly fitted by taking into account several sources: Big Bang, Galactic Cosmic Ray Nucleosynthesis, the ν-process, novae and AGB and C-stars.

The final evolution of 8-10 M. stars has been a subject of debate for the last 15 years. In this work we show that the outcome of the electron-capture triggered explosion of an ONeMg electron-degenerate core is very likely a neutron star. When possible, the latest physical inputs have been used in our calculations. Specifically, we have used the most up to date electron capture rates and we have used Coulomb corrections to the electron capture thresholds. When no reliable theory exists for a specific physical input extreme simplified cases have been considered in order to determine limits to the NeO ignition density. This is the case of semiconvective mixing; both extreme efficiency and extreme inefficiency have been studied in this case. These two assumptions imply the use of either Schwarzschild's criterion or Ledoux's criterion for the growth rate of the convective stability, respectively. The effects of the chemical composition after C-12-burning, the other most important uncertainty, and its effects on the final evolution of the stars in this mass range are discussed in detail in the light of the newest available results.

The final stages of the evolution of electron-degenerate ONeMg cores, resulting from carbon burning in ''heavyweight'' intermediate-mass stars (8 M(circle dot) less than or similar to M less than or similar to 10 M(circle dot)) and growing in mass either from carbon burning in a shell or from accretion of matter in a close binary system, are examined. When due account is taken of the Coulomb corrections, both in the equation of state and in the electron capture threshold energies, explosive NeO ignition takes place at densities high enough to ensure gravitational collapse to nuclear matter densities. It is shown that this result holds for two extreme assumptions concerning mixing in the presence of an overstable temperature gradient: no mixing (Ledoux criterion) and ordinary convective entropy mixing according to the Schwarzschild criterion (the latter delaying explosive ignition to still higher densities). Discrepancies among earlier calculations, due to omission of Coulomb corrections, are clarified with the use of the most recent electron capture rates on the relevant nuclides plus very finely zoned models.

The dependence of the characteristics of the light curves of Type Ia supernovae on the ignition density of the progenitor white dwarf is studied with the aid of two models of propagation of the thermonuclear burning front: as a deflagration and as a delayed detonation. The light curve is computed from opacities which take into account the velocity gradients. The results show that in all cases the resulting light curves roughly agree with observations and that they are not sensitive to the ignition density of the white dwarf. Only the model corresponding to a deflagration starting at a density of 8 10(9) g/cm3 shows a deviation from the general behaviour, having a significantly lower luminosity at maximum. A dispersion of approximately 1000 km/s is found in the computed expansion velocities at maximum, which compares well with that found in the observations.

The white dwarf explosion model accounts fairly well for the global characteristics of type Ia supernovae. There are, however, several problems concerning the dynamics of the burning front (deflagration or detonation regime), initial conditions and nucleosynthesis, that still deserve attention. In particular, the overproduction of neutron-rich nuclides such as Fe-54 and Ni-58, due to electron captures and to the neutron excess trapped in Ne-22 nuclei, still poses a major problem. In this paper we examine the possibilities for Ne-22 to migrate towards the central regions of the star and we show that if the white dwarf has had enough time to solidify, the contribution of Ne-22 to the neutron excess becomes negligible.

The white dwarf explosion model accounts fairly well for the global characteristics of type Ia supernovae. There are, however, several problems concerning the dynamics of the burning front (deflagration or detonation regime), initial conditions and nucleosynthesis, that still deserve attention. In particular, the overproduction of neutron-rich nuclides such as 54 Fe and 58 Ni, due to electron captures and to the neutron excess trapped in 22 Ne nuclei, still poses a major problem. Here, the possibilities for 22 Ne to migrate towards the central regions of the star are examined and it is shown that if the white dwarf has had enough time to solidify, the contribution of 22 Ne to the neutron excess becomes negligible

The quasi-static evolution of an electron-degenerate ONeMg core is followed up to the point of explosive Ne-O ignition triggered by electron capture. Recently calculated electron-capture rates on Mg-24, Na-24, Ne-20, and F-20 are used, and the ignition density is found to be rho(ign) congruent-to 8.5 x 10(9) g cm-3 (when the Ledoux criterion for convection is adopted), instead of rho(ign) congruent-to 10(10) g cm-3 as in earlier calculations. Such a comparatively low ignition density should lead to complete explosive disruption of the core even if the thermonuclear burning propagated all the way as a conductive, laminar flame. The dependence of the ignition density on the treatment adopted for mixing in the semiconvective region and the astronomical implications of our results are briefly discussed.

The existence and the orbital parameters of low-mass binary x-ray sources suggest that a fraction at least of neutron stars have been produced by the accretion induced collapse (AIC) of a white dwarf (TAAM and VAN DEN HEUVEL [1]). Two types of candidates have been proposed thus far: carbon-oxygen white dwarfs (CANAL and SCHATZMANN [2]; CANAL and ISERN [3]; CANAL, ISERN and LABAY [4]), and oxygen-neon-magnesium white dwarfs (MIYAJI et al [5]; MIYAJI and NOMOTO [6]).

Electron-degenerate ONeMg cores result from the evolution of 8 M. less-than-or-equal-to M less-than-or-equal-to 12 M. stars. In the mass range 8 M. less-than-or-equal-to M less-than-or-equal-to 10 M., electron captures on Mg-24 and Na-24 first, and later on Ne-20, precede Ne-O ignition. The same occurs in mass-accreting ONeMg white dwarfs, formed in close binary systems by mass loss from stars in the above mass range. Electron captures on Ne-20 (or maybe on Mg-24 and Na-24) are the triggering mechanism of explosive ignition. Depending on the treatment of semiconvection, ignition density is most likely lower than 9.5 x 10(9) g cm-3. In this case, hydrodynamic burning propagation may lead to complete disruption of the core and not to core collapse, in contrast with the usual assumption. Thus, while neutron star formation by accretion-induced collapse (AIC) of ONeMg white dwarfs critically depends both on ignition density and on the velocity of the burning front, thermonuclear supernova production (either Ia or Ib/c), or even "Fe" white dwarf formation by milder outbursts are open possibilities for the outcome of the explosive ignition of ONeMg cores.