Aims. We present the first simulations of the full evolution of super-AGB stars through the entire thermally pulsing AGB phase. We analyse their structural and evolutionary properties and determine the first SAGB yields.Methods. Stellar models of various initial masses and metallicities were computed using standard physical assumptions which prevents the third dredge-up from occurring. A postprocessing nucleosynthesis code was used to compute the SAGB yields, to quantify the effect of the third dredge-up (3DUP), and to assess the uncertainties associated with the treatment of convection.Results. Owing to their massive oxygen-neon core, SAGB stars suffer weak thermal pulses, have very short interpulse periods and develop very high temperatures at the base of their convective envelope (up to 140 × 10 8 K), leading to very efficient hot bottom burning. SAGB stars are consequently heavy manufacturers of 4 He, 13 C, and 14 N. They are also able to inject significant amounts of 7 Li, 17 O, 25 Mg, and 26,27 Al in the interstellar medium. The 3DUP mainly affects the CNO yields, especially in the lower metallicity models. Our post-processing simulations also indicate that changes in the temperature at the base of the convective envelope, which would result from a change in the efficiency of convective energy transport, have a dramatic impact on the yields and represent another major source of uncertainty.

Context. Massive AGB (hereafter super-AGB or SAGB) stars ignite carbon off-center and have initial masses ranging between M up , the minimum initial mass for carbon ignition, and M mas the minimum mass for the formation of an iron core collapse supernova. In this mass interval, stars more massive than M n will undergo an electron capture supernova (EC-SN). Aims. We study the fate and selected evolutionary properties of SAGB stars up to the end of the carbon burning phase as a function of metallicity and core overshooting. Methods. The method is based on the analysis of a large set of stellar models covering the mass range 5−13 M and calculated for 7 different metallicities between Z = 10 −5 and twice solar. Core overshooting was considered in two subsets for Z = 10 −4 and 0.02. The models are available online at http://www-astro.ulb.ac.be/∼siess/database.html. The fate of SAGB stars is investigated through a parametric model which allows us to assess the role of mass loss and of the third dredge-up. Results. Our main results can be summarized as follows: a) prior to C-burning, the evolution of SAGB stars is very similar to that of intermediate-mass stars, being more luminous, b) SAGB stars suffer a large He enrichment at the end of the second dredge-up, c) the limiting masses M up , M n and M mas present a nonlinear behavior with Z, characterized by a minimum around Z = 10 −4 , d) the values of M up , M n and M mas are decreased by ∼2 M when core overshooting is considered, e) our models predict a minimum oxygenneon white dwarf mass of ∼1.05 M , f) the determination of M n is highly dependent on the mass loss and core growth rates, g) the evolutionary channel for EC-SN is limited to a very narrow mass range of < ∼ 1−1.5 M width and this mass window can be further decreased if some metallicity scaling factor is applied to the mass loss rate, h) the final fate of SAGB stars is connected to the second dredge-up and this property allowed us to refine the initial mass range for the formation of EC-SN. We find that if the ratio of the mass loss rate to the core growth rate averaged over the post carbon-burning evolution ζ = Ṁ loss /Ṁ core is greater than about 70−90, the evolutionary path to EC-SN is not accessible.

We explore the final fates of massive intermediate-mass stars by computing detailed stellar models from the zero age main sequence until near the end of the thermally pulsing phase. These super-AGB and massive AGB star models are in the mass range between 5.0 and 10.0 M ⊙ for metallicities spanning the range Z=0.02−0.0001. We probe the mass limits M up , M n and M mass , the minimum masses for the onset of carbon burning, the formation of a neutron star, and the iron core-collapse supernovae respectively, to constrain the white dwarf/electron-capture supernova boundary. We provide a theoretical initial to final mass relation for the massive and ultra-massive white dwarfs and specify the mass range for the occurrence of hybrid CO(Ne) white dwarfs. We predict electron-capture supernova (EC-SN) rates for lower metallicities which are significantly lower than existing values from parametric studies in the literature. We conclude the EC-SN channel (for single stars and with the critical assumption being the choice of mass-loss rate) is very narrow in initial mass, at most ≈ 0.2 M ⊙ . This implies that between ∼ 2−5 per cent of all gravitational collapse supernova are EC-SNe in the metallicity range Z=0.02 to 0.0001. With our choice for mass-loss prescription and computed core growth rates we find, within our metallicity range, that CO cores cannot grow sufficiently massive to undergo a Type 1.5 SN explosion.

We present new computations of the evolution of solar metallicity stars in the mass range 9−12 M . This first paper of a series focuses on the propagation of the carbon burning flame front and provides a detailed analysis of the structural evolution up to the formation of the neonoxygen core. Our calculations which do not include overshooting indicate that off-center carbon ignition is restricted to a small mass range between 9.0 and 11.3 M . The chemical imprints of the first and second dredge-ups on the surface composition are analyzed and compared to "standard" less massive stars. It results that, aside from being more luminous and slightly bluer in the HR diagram, massive AGB stars are almost indistinguishable from their lower mass counterparts, as far as the chemical composition is concerned. During the second dredge-up, we note however that the envelope penetrates deeper into the He burning shell than lower mass stars. Our simulations indicate that above ∼11.0 M , the depth of the second dredge up is considerably reduced, marking the transition between low and massive stars. We also investigate the effects of the nuclear uncertainties associated with 12 C + 12 C reactions and show that it has a little impact on the core composition. Finally we describe the nucleosynthesis and chemical structure of the newly formed neon-oxygen core.

Close-in planets are in jeopardy as their host stars evolve off the main sequence to the subgiant and red giant phases. In this paper, we explore the influences of the stellar mass (in the range 1.5-2M ⊙ ), mass-loss prescription, planet mass (from Neptune up to 10 Jupiter masses), and eccentricity, on the orbital evolution of planets as their parent stars evolve to become subgiants and Red Giants. We find that planet engulfment during the Red Giant Branch is not very sensitive to the stellar mass or mass-loss rates adopted in the calculations, but quite sensitive to the planetary mass. The range of initial separations for planet engulfment increases with decreasing mass-loss rates or stellar mass and increasing planetary masses. Regarding the planet's orbital eccentricity, we find that as the star evolves into the red giant phase, stellar tides start to dominate over planetary tides. As a consequence, a transient population of moderately eccentric close-in Jovian planets is created, that otherwise would have been expected to be absent from main sequence stars. We find that very eccentric and distant planets do not experience much eccentricity decay, and that planet engulfment is primarily determined by the pericenter distance and the maximum stellar radius.