J. Labay

Galactic chemical abundance evolution in the solar neighborhood up to the iron peak

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.

Galactic Cosmic Rays from Superbubbles and the Abundances of Lithium, Beryllium, and Boron

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.

The collapse of carbon-oxygen white dwarfs

Carbon-oxygen white dwarfs formed in close binary systems may become unstable by mass accretion. Recent results concerning carbon-oxygen separation at the freezing point during the phase of cooling may have very important consequences for the problem of neutron star formation. The central, high-density regions of the star are then made of pure oxygen, the carbon being rejected to lower-density layers. When the star is compressed, carbon ignition can only happen after neutronization of the central (oxygen) regions. We show that, in this case, the chances of collapse to a neutron star are independent from the rate of mass accretion, in contrast with previous studies. A likely mechanism for neutron star formation emerges from this picture.

Nucleosynthesis in SNIa

The final products of a SNIa explosion critically depend on the degree of neutronization of the incinerated material and on the total amount of burned material. Here we study their dependence on the velocity of the burning front and on the density at which the thermonuclear runaway starts. The abundances of54Cr,54, 58Fe,58, 62Ni provide some constraints to the possible values of model parameters.

Neutron star formation by collapse of white dwarfs

Mass-accreting carbon-oxygen white dwarfs become thermally and dynamically unstable when they reach high enough central densities. Carbon ignition at the stars center likely propagates subsonically and, in the case of an initially solid core, leads to collapse if the rate of increase of the cores mass is sufficiently fast. Recent results indicate, however, that solidification of the core induces carbon-oxygen separation. The central regions are then made of pure oxygen while carbon is rejected to lower-density layers. Carbon ignition happens only after neutronization of the central (oxygen) regions. Collapse to a neutron star is then independent from the rate of mass increase and the only possible restrictions are set by the behaviour of the outer, accreted layers. X-ray sources, pulsars and Type I supernovae are likely outcomes of this process.

The Final Evolution of 8-10 M⊙ 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 semi-convective 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 12C-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.

On the Origin of LMXRBS: The ONEMG Case

The ignition of Ne-O in mass accreting white dwarfs, formed in close binary systems by mass loss from stars in the range of 8 M⊙ ≤ M ≤ 12 M⊙, is preceded by electron captures on 24Mg and 24Na. Electron captures on 20Ne (or maybe on 24Mg and 24Na) are the triggering mechanism of explosive ignition at densities in the vicinity of 9.5×109 g/cm3. The outcome depends on the adopted propagation velocity of the thermonuclear burning front. Burning fronts propagating with hydrodynamical velocities( ≥ 0.02cs) lead to the total disruption of the star or, in some cases, to the formation of an “iron” white dwarf. Burning fronts propagating conductively lead to the formation of a neutron star.

Dynamical instability in accreting white dwarfs

We study the evolution of solid, CO white dwarfs after explosive carbon ignition at central densities around 1010 g cm−3 triggered by steady accretion in a close binary system, in order to elucidate whether these stars can collapse to form a neutron star. We show that as long as the velocity of the burning front remains below a critical value of 0.006cs (∼60 km s−1), gravitational collapse is the final fate. These calculations support the accretion-induced collapse (AIC) scenario for the origin of a fraction of low-mass X-ray binaries.

PRECOLLAPSE EVOLUTION OF ACCRETING CO WHITE DWARFS

We calculate how accretion affects the evolution of carbon-oxygen white dwarfs up to thermonuclear runaway. The influence of internal temperature (including the possibility of solidification) as well as that of other initial conditions (accretion rate, total mass) are taken into account. We examine in some detail the influence of the adopted pycnonuclear reaction rate and of the lattice impurities on the ignition conditions. The consequences of having a completely ordered C-O crystal instead of a disordered one are also analyzed.

THE WHITE DWARF-NEUTRON STAR CONNECTION

The standard mechanism for neutron star formation is collapse of the cores of massive stars at the end of their thermonuclear evolution. However, this mechanism cannot account for the presence of neutron stars in binary systems where their companions are low-mass stars and no capture process (involving a previously isolated neutron star) can be invoked for the formation of the system. Gravitational collapse of a white dwarf into a neutron star is necessarily involved in those cases. Here we review the physical processes relevant to such collapses, for both C+0 white dwarfs and 0+Ne+Mg white dwarfs. We examine the various possibilities concerning the physical changes in C+0 white dwarf cores with cooling and we stress that all of them equally lead (for a suitable range of parameters) to collapsing structures. We discuss the different calculations done so far and we finally compare the case of C+0 white dwarfs with that of 0+Ne+Mg white dwarfs.