We review the current state of modeling convective mixing in AGB stars. The focus is on results obtained through multi-dimensional hydrodynamic simulations of AGB convection, both in the envelope and the unstable He-shell. Using two different codes and a wide range of resolutions and modeling assumptions we find that mixing across convective boundaries is significant for He-shell flash convection. We present a preliminary quantitative analysis of this convectively induced extra mixing, based on a sub-set of our simulations. Other non-standard mixing will be discussed briefly.
We present results from new, detailed computations of lithium production by hot bottom burning (HBB) in asymptotic giant branch (AGB) stars of intermediate mass. The dependance of lithium production on stellar mass, metallicity, mass loss rate, convection and overshooting are discussed. In particular, nuclear burning, turbulent mixing and convective overshooting (if any) are self-consistently coupled by a diffusive algorythm, and the Full Spectrum of Turbulence (FST) model of convection is adopted, with test comparisons to Mixing Length Theory (MLT) stellar models.
The influence of extended convective mixing (overshoot) on asymptotic giant branch stellar evolution is investigated in detail. The extended mixing is treated time-dependently, and the efficiency declines exponentially with the geometric distance from the convective boundary. It has been considered at all convective boundaries, including the He-flash convection zone in the intershell region which forms during the thermal pulses. Both the structural and the chemical evolution are affected by the inclusion of overshoot. The main results include a very efficient third dredge-up which leads to the…
The focus of this study is on the treatment of those stellar regions immediately adjacent to convective zones. The results of hydrodynamical simulations by Freytag et al. (1996, A&A313,497) show that the motion of convective elements extends well beyond the boundary of the convectively unstable region. We have applied their parametrized description of the corresponding velocities to the treatment of overshoot in stellar evolution calculations up to the AGB (Pop.I, M_zams=3M_sun).
We have extended our published set of low mass AGB stellar models to lower metallicity. Different mass loss rates have been explored. Interpolation formulae for luminosity, effective temperature, core mass, mass of dredge up material and maximum temperature in the convective zone generated by thermal pulses are provided. Finally, we discuss the modifications of these quantities as obtained when an appropriate treatment of the inward propagation of the convective instability, caused by the steep rise of the radiative opacity occurring when the convective envelope penetrates the H-depleted regio…
A thermal pulse during the post-AGB phase of stellar evolution may lead to a unique mode of light element nucleosynthesis. The stage is set by the ingestion of the unprocessed envelope material into the hot He-flash convection zone below. If the temperature is sufficiently large and the C12 abundance high enough (e.g. T_8 > 0.8, X(C12) ~ 0.4 and X(H) ~ 1E-3) protons react faster with C12 and form C13 than destroying Be7 The latter forms by alpha-capture of He3 after an initial reduction of the He3 abundance to about 3E-5 X(He4) by the ppI reaction He3(He3,2p)He4 (for T_8 ~ 1). All He3 is burne…
Recent advances in constructing stellar evolution models of hydrogen-deficient post-asymptotic giant branch (AGB) stars are presented. Hydrogen-deficiency can originate from mixing and subsequent convective burning of protons in the deeper layers during a thermal pulse on the post-AGB (VLTP). Dredge-up alone may also be responsible for hydrogen-deficiency of post-AGB stars. Models of the last thermal pulse on the AGB with very small envelope masses have shown efficient third dredge-up. The hydrogen content of the envelope is diluted sufficiently to produce H-deficient post-AGB stars (AFTP). Mo…
It is known that there must be some weak form of transport (called cool bottom processing, or CBP) acting in low mass RGB and AGB stars, adding nuclei, newly produced near the hydrogen-burning shell, to the convective envelope. We assume that this extra-mixing originates in a stellar dynamo operated by the differential rotation below the envelope, maintaining toroidal magnetic fields near the hydrogen-burning shell. We use a phenomenological approach to the buoyancy of magnetic flux tubes, assuming that they induce matter circulation as needed by CBP models. This establishes requirements on th…
We report on the discovery of a new kind of thermal pulse in intermediate mass AGB stars. Deep dredge-up during normal thermal pulses on the AGB leads to the formation of a long, unburnt tail to the helium profile. Eventually the tail ignites under partially degenerate conditions producing a strong shell flash with very deep subsequent dredge-up. The carbon content of the intershell convective region (X_C ~ 0.6) is substantially higher than in a normal thermal pulse (X_C ~ 0.25) and about 4 times more carbon is dredged-up than in a normal pulse.
We present the first evolutionary models of intermediate mass stars up to their thermal pulses which include effects of rotation on the stellar structure as well as rotationally induced mixing of chemical species and angular momentum. We find a significant angular momentum transport from the core to the hydrogen-rich envelope and obtain a white dwarf rotation rate comparable to current observational upper limits of 50 km/s. Large angular momentum gradients at the bottom of the convective envelope and the tip of the pulse driven convective shell are shown to produce marked chemical mixing betwe…
We model the nucleosynthesis during a radiative interpulse phase of a rotating 3Msun Asymptotic Giant Branch (AGB) star. We find an enhanced production of the neutron source species C13 compared to non-rotating models due to shear mixing of protons and C12 at the core-envelope interface. We estimate that the resulting total production of heavy elements by slow neutron capture s-process is too low to account for most observations. This due to the fact that rotationally induced mixing during the interpulse phase causes a pollution of the C13 pocket layer with the neutron poison N14. As a result …
Planetary nebulae (PN) represent the evolutionary fate of the asymptotic giant branch (AGB) stellar envelopes, thus are ideally suited to study the chemical impact of AGB stars. Stellar evolution predict elemental enrichment through the AGB evolution, and convective dredge-up episodes allow the products of stellar evolution to reach the stellar outer layers. Planetary nebulae are probes of these processes, and are also probes of the environment at the time of formation of their progenitors, through the elements not affected by AGB evolution. Ultimately PN may be used to test AGB stars as actor…
New stellar models with mass ranging between 4 and 8 Mo, Z=0 and Y=0.23 are presented. The models have been evolved from the pre Main Sequence up to the Asymptotic Giant Branch (AGB). At variance with previous claims, we find that these updated stellar models do experience thermal pulses in the AGB phase. In particular we show that: a) in models with mass larger than 6 Mo, the second dredge up is able to raise the CNO abundance in the envelope enough to allow a "normal" AGB evolution, in the sense that the thermal pulses and the third dredge up settle on; b) in models of lower mass, the effici…
Structural and chemical changes during the AGB and post-AGB evolution are discussed with respect to two recent observational and theoretical findings. On the one hand, high-resolution infrared observations revealed details of the dynamical evolution of the fragmented, bipolar dust shell around the far-evolved carbon star IRC+10216 giving evidence for rapid changes of an already PPN-like structure during the very end of the AGB evolution. On the other hand, stellar evolution calculations considering convective overshoot have shown how thermal pulses during the post-AGB stage lead to the formati…
We describe our first attempt at modelling nucleosynthesis in massive AGB stars which have undergone core carbon burning, the super-AGB stars. We fit a synthetic model to detailed stellar evolution models in the mass range 9<=M/Msun<=11.5 (Z=0.02), and extrapolate these fits to the end of the AGB. We determine the number of thermal pulses and AGB lifetime as a function of mass and mass-loss prescription. Our preliminary nucleosynthesis calculations show that, for a reasonable mass-loss rate, the effect of hot-bottom burning in super-AGB stars on the integrated yield of a stellar population is …
We consider the stability of a compressible shear flow separating two streams of different speeds and temperatures. The velocity and temperature profiles in this mixing layer are hyperbolic tangents. The normal mode analysis of the flow stability reduces to an eigenvalue problem for the pressure perturbation. We briefly describe the numerical method that we used to solve this problem. Then, we introduce the notions of the absolute and convective instabilities and examine the effects of Mach number, and the velocity and temperature ratios of each stream on the transition between convective and …
We discuss the synthesis of some short-lived isotopes and of 23Na in thermally pulsing AGB stars with initial mass of 2 Msun and two different metallicities (Z=1.5e-2, corresponding to the metal amount in the Sun, and Z=1e-4), representative of disk and halo stars, respectively. The different nucleosynthesis channels are illustrated in some details. As previously found, the 13C formed after each third dredge up episode is usually completely consumed by alpha captures before the onset of the subsequent thermal pulse, releasing neutrons. This is the most efficient neutron source in low mass AGB …
We study prospects for seismic sounding the layer of a partial mixing above the convective core in main-sequence stars with masses in the 1.2 -- 1.9 solar mass range. There is an initial tendency to an increase of convective core mass in such stars and this leads to ambiguities in modeling. Solar-like oscillations are expected to be excited in such objects. Frequencies of such oscillations provide diagnostics, which are sensitive to the structure of the innermost part of the star and they are known as the small separations. We construct evolutionary models of stars in this mass range assuming …
We have studied the evolution of intermediate mass (M.ge.5Mo) zero metal (Z=0) stars with particular attention to the AGB phase. At variance with previous claims we find that these stars experience thermal instability (the so called thermal pulses). The critical quantity which controls the onset of a thermally pulsing phase is the amount of CNO in the envelope during the AGB. For these stars the central He burning starts in the blue side of the HR diagram and the 1^{st} dredge up does not take place. Then the envelope maintains its initial composition up to the beginning of the AGB phase. Howe…
Contrary to previous expectations, recent evolutionary models of zero-metallicity stars show that the development of mixing episodes at the beginning of the AGB phase allows low- and intermediate-mass stars to experience thermal pulses. If these stars, like their metal-rich counterparts, also experience partial mixing of protons from the H-rich envelope into the C-rich layers at the time of the third dredge-up, an extensive neutron capture nucleosynthesis leads to the production of s-process nuclei up to Pb and Bi. Nucleosynthesis calculations based on stellar AGB models are performed assuming…
We constructed hydrodynamical model atmospheres for mid M-type main-, as well as pre-main-sequence objects. Despite the complex chemistry encountered in such cool atmospheres a reasonably accurate representation of the radiative transfer is possible. The detailed treatment of the interplay between radiation and convection in the hydrodynamical models allows to study processes usually not accessible within the framework conventional model atmospheres. In particular, we determined the efficiency of the convective energy transport, and the efficiency of mixing by convective overshoot. The convect…
We constructed hydrodynamical model atmospheres for mid M-type main-, as well as pre-main-sequence (PMS) objects. Despite the complex chemistry encountered in these cool atmospheres a reasonably accurate representation of the radiative transfer is possible, even in the context of time-dependent and three-dimensional models. The models provide detailed information about the morphology of M-type granulation and statistical properties of the convective surface flows. In particular, we determined the efficiency of the convective energy transport, and the efficiency of mixing by convective overshoo…
The evolution of the first generations of stars at zero or extremly low metallicity, and especially some crucial properties like the primary N14 production, is charactarized by convective-reactive mixing events that are mostly absent from similar evolution phases at solar-like metallicity. These episodes occur when unprocessed H-rich material is mixed accross a convective boundary into C12 rich He-burning material, as for example in He-shell flashes of extremely-low metallicity AGB stars. In this paper we describe the astrophysical context of such convective-reactive events, including the diff…
We calculated models of massive AGB stars with a self-consistent coupling of time-dependent mixing and nuclear burning for 30 isotopes and 74 reactions. Overshoot with an exponentially declining velocity field was considered and applied during all stages of evolution and in all convective regions. Very efficient 3rd dredge-up was found even overcompensating the growth of the hydrogen-exhausted core after a few thermal pulses. Hot bottom burning occurs for M>4Msol within the sequences with overshoot. Carbon star formation in these more massive AGB stars is delayed or even prevented by hot botto…
A super-solar fluorine abundance was observed in the carbon-enhanced metal-poor (CEMP) star HE 1305+0132 ([F/Fe] = +2.90, [Fe/H] = -2.5). We propose that this observation can be explained using a binary model that involve mass transfer from an asymptotic giant branch (AGB) star companion and, based on this model, we predict F abundances in CEMP stars in general. We discuss wether F can be used to discriminate between the formation histories of most CEMP stars: via binary mass transfer or from the ejecta of fast-rotating massive stars. We compute AGB yields using different stellar evolution and…
The oldest stars hide information about the chemical evolution of the early universe. In this contect new models of the AGB stellar evolution phase at very low and zero metallicity are presented. Due to the deficiency or absence of CNO catalytic material hydrogen burning operates at significantly higher temperatures than in stars of higher metallicity. As a result convective mixing plays a very important role since the nuclear burning shells are not well separated by an entropy barrier. These stars can host flash-like burning events induced by mixing of C12 and protons at the core-envelope int…
We examine the convective core helium burning phase of intermediate mass stars, and investigate the role of coupling of nuclear burning and mixing on the extension and duration of the blue loop phase. We compare the theoretical scenario with the distribution of stars in the colour-magnitude (CM) diagram of the Large Magellanic Cloud (LMC) cluster NGC1866, whose largely populated clump of He-burning stars is equally populated in the blue and red side. We compare the distributions expected by adopting either a diffusive scheme within the instability regions, in which nuclear burning and mixing a…
We present a critical analysis of the nature of the so-called Li-rich RGB stars. For a majority of the stars, we have used Hipparcos parallaxes to determine masses and evolutionary states by comparing their position on the Hertzsprung-Russell diagram with theoretical evolutionary tracks. Among the twenty Li-rich giants whose location on the HR diagram we were able to determine precisely, eight should be re-classified as Li-normal. For the remaining stars, the high Li abundance must be a result of fresh synthesis of this fragile element. We identify two distinct episodes of Li production which …
We determine the ratio of helium- to hydrogen-atmosphere white dwarf stars as a function of effective temperature from a model atmosphere analysis of the infrared photometric data from the Two Micron All Sky Survey combined with available visual magnitudes. Our study surpasses any previous analysis of this kind both in terms of the accuracy of the Teff determinations as well as the size of the sample. We observe that the ratio of helium- to hydrogen-atmosphere white dwarfs increases gradually from a constant value of ~0.25 between Teff = 15,000 K and 10,000 K to a value twice as large in the r…
Analytic formulae are presented to construct detailed secular lightcurves of both early asymptotic giant branch (AGB) and thermally pulsing AGB stars. They are based on an extensive grid of evolutionary calculations, performed with an updated stellar evolution code. Basic input parameters are the initial mass between 0.8 and 7 solar mass, metallicity of 0.0001, 0.008, 0.02, and the mixing length theory (MLT) parameter. The formulae allow for two important effects, namely that the first pulses do not reach the full amplitude, and hot bottom burning (HBB) in massive stars, which are both not acc…
We examine the stability and observational consequences of mixing induced by 3He burning in the envelopes of first ascent red giants. We demonstrate that there are two unstable modes: a rapid, nearly adiabatic mode that we cannot identify with an underlying physical mechanism, and a slow, nearly radiative mode that can be identified with thermohaline convection. We present observational constraints that make the operation of the rapid mode unlikely to occur in real stars. Thermohaline convection turns out to be fast enough only if fluid elements have finger-like structures with a length to dia…
In order to understand the composition of planetary nebulae we first need to study the nucleosynthesis occurring in the progenitor star during the thermally-pulsing Asymptotic Giant Branch (AGB) phase. I present an overview of single AGB evolution, with an emphasis on the mixing processes that alter the envelope composition, followed by a discussion of the stellar yields available from single AGB stellar models.
We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. We find that the production of fluorine depends on the temperature of the convective pulses, the amount of primary 12C mixed into the envelope by third dredge up and the extent of the partial mixing zone. Then we perform a detailed analysis of the reaction rates involved in the production of 19F and the effects of their uncertainties. We find that the major uncertainties are associated with the 14C(alpha,gamma)18O and the 19F(alpha,p)22Ne reaction rates.…
(abridged) We model the nucleosynthesis during the thermal pulse phase of a rotating, solar metallicity AGB star of 3M_sun. Rotationally induced mixing during the thermal pulses produces a layer (~2E-5M_sun) on top of the CO-core where large amounts of protons and C12 co-exist. We follow the abundance evolution in this layer, in particular that of the neutron source C13 and of the neutron poison N14. In our AGB model mixing persists during the entire interpulse phase due to the steep angular velocity gradient at the core-envelope interface. We follow the neutron production during the interpuls…
Aims: We investigate the change in the surface abundance of $^{12}$C during the evolution along the AGB, aiming to constrain third dredge-up models. Methods: High-resolution, near-infrared spectra of a sample of AGB stars in the LMC cluster NGC 1846 were obtained. A cluster sample ensures a high level of homogeneity with respect to age, metallicity, and distance. The C/O ratio and the ratio of $^{12}$C/$^{13}$C were measured and compared with our evolutionary models. Results: For the first time, we show the evolution of the C/O and $^{12}$C/$^{13}$C ratios along a cluster AGB. Our findings all…
Type I planetary nebulae (PNe) have high He/H and N/O ratios and are thought to be descendants of stars with initial masses of ~3-8Msun. These characteristics indicate that the progenitor stars experienced proton-capture nucleosynthesis at the base of the convective envelope, in addition to the slow neutron capture process operating in the He-shell (the s-process). We compare the predicted abundances of elements up to Sr from models of intermediate-mass asymptotic giant branch (AGB) stars to measured abundances in Type I PNe. In particular, we compare predictions and observations for the light…
We present an evolutionary sequence of a low mass star from the Asymptotic Giant Branch (AGB) through its post-AGB stage, during which its surface chemical composition changes from hydrogen-rich to strongly hydrogen-deficient as consequence of a very late thermal pulse, following the so-called born-again scenario. The internal structure and abundance changes during this pulse are computed with a numerical method which allows the physically consistent calculation of stellar layers where thermonuclear and mixing time scale are comparable - a situation which occurs when the helium flash driven co…
We suggest to use the shape of the Red Giant Branch (RGB) Bump in metal-rich globular clusters as a diagnostic of partial mixing processes between the base of the convective envelope and the H-burning shell. The Bump located along the differential luminosity function of cluster RGB stars is a key observable to constrain the H-profile inside these structures. In fact, standard evolutionary models that account for complete mixing in the convective unstable layers and radiative equilibrium in the innermost regions do predict that the first dredge-up lefts over a very sharp H-discontinuity at the …
The unveiled main-sequence splitting in omega Centauri as well as NGC 2808 suggests that matter highly-enriched in He (in terms of its mass fraction Y~0.4) was produced and made the color of some main-sequence stars bluer in these globular clusters (GCs). The potential production site for the He-rich matter is generally considered to be massive AGB stars that experience the second dredge-up. However, it is found that massive AGB stars provide the matter with Y~0.35 at most, while the observed blue-shift requires the presence of Y~0.4 matter. Here, we show that extra mixing, which operates in t…
FUSE spectroscopy has proved that extremely hot hydrogen-deficient post-AGB stars (PG1159 stars) display matter on their surface that usually remains hidden in the region between the H- and He-burning shells of the former AGB star. Hence, the spectral analysis of PG1159 stars allows to study directly the chemistry of this intershell region which is the outcome of complicated burning and mixing processes during AGB evolution. Detailed abundance determinations provide constraints for these processes which are still poorly understood. With FUSE we have discovered high neon and fluorine overabunda…
It is well appreciated that the description of overadiabatic convection affects the structure of the envelopes of luminous asymptotic giant branch (AGB) stars in the phase of ``hot bottom burning '' (HBB). We stress that this important uncertainty in the modeling plays a role which is much more dramatic than the role which can be ascribed, e.g., to the uncertainty in the nuclear cross-sections. Due to the role tentatively attributed today to the HBB nucleosynthesis as the site of self-enrichment of Globular Clusters stars, it is necessary to explore the difference in nucleosynthesis obtained b…
[abridged] We study the s-process in AGB stars using three different stellar evolutionary models computed for a 3Msun and solar metallicity star. First we investigate the formation and the efficiency of the main neutron source. We parametrically vary the number of protons mixed from the envelope into the C12 rich core. For p/C12 > 0.3, mainly N14 is produced, which represent a major neutron poison. The amount of C12 in the He intershell and the maximum value of the time-integrated neutron flux are proportional. Then we generate detailed s-process calculations on the basis of stellar evolutiona…
We study the response of the structure of an asymptotic giant branch (AGB) star to the accretion of a brown dwarf or planet in its interior. In particular, we examine the case in which the brown dwarf spirals-in, and the accreted matter is deposited at the base of the convective envelope and in the thin radiative shell surrounding the hydrogen burning shell. In our spherically symmetric simulations, we explore the effects of different accretion rates and we follow two scenarios in which the amounts of injected mass are equal to $\sim 0.01$ and $\sim 0.1 M_\odot$. The calculations show that for…
We discuss the occurrence of the s-process during the radiative interpulse phase of rotating AGB stars. Due to differential rotation, protons are mixed into C12-rich layers after thermal pulses, in the course of the so called third dredge up episode. We follow the time evolution of key isotope abundances in the relevant layers with a post-processing code which includes time dependant mixing and nucleosynthesis. In rotating AGB models, the mixing persists during the entire interpulse phase due to the steep gradient of angular velocity at the envelope-core interface. As the layers containing pro…
(abridged) Observations clearly show that low-mass AGB stars can provide a nucleosynthesis site of the s-process. Recent stellar evolution models indicate that radiative burning of C13 between thermal pulses in low-mass AGB stars may indeed provide the needed neutrons. Some mixing between the proton-rich envelope and the carbon-rich core may lead to the production of C13. However, the responsible physical mechanism is not yet unambiguously identified. We present stellar model calculations with overshoot and rotation. Overshoot, with a time-dependent and exponentially decaying efficiency, leads…
The concept of overshoot has already been considered for numerous cases in stellar evolution calculations. We explore the consequences of overshoot at the convection zone which forms during the He-flash (thermal pulse) in AGB stars. We find dramatic changes for the abundances within the intershell region as well as for the mechanism of the 3rd dredge-up. That means that both the predicted evolution of structural quantities as well as the chemical evolution at the surface will be different, if overshoot is considered. We also present evidence for the presence of overshoot during the He-flash fr…
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted element abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agree…
We present the results of s-process nucleosynthesis calculations for AGB stars of different metallicities and initial masses. The computations were based on previously published stellar evolutionary models that account for the III dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13C(alpha,n)16O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He- and C-rich shell. The s-enriched material is subsequently mixed with the envelope by the III dredge up, and the envelope composition is computed after each thermal pul…
The structure and evolution of central stars of planetary nebulae (CSPNe) is reviewed. CSPNe represent the rapid transitional stage between the Asymptotic Giant Branch (AGB) and the white-dwarf domain. It is shown that the whole evolution off the AGB through the central-star regime depends on the evolutionary history. The detailed evolution into a white dwarf is controlled by the internal stellar structure which, in turn, is determined by the duration of the preceding AGB evolution and therefore by the AGB mass-loss history. The evolution of hydrogen-deficient central stars has been a matter o…
The third dredge-up phenomenon in asymptotic giant branch (AGB) stars is analyzed through evolutionary model calculations of a \mass{3}, solar metallicity star. The Schwarzschild criterion is used to test the stability of a given layer against convection, and the calculations are performed either with or without extra-mixing below the convective envelope. Based on these calculations, several questions are addressed regarding the occurrence of the third dredge-up in AGB star models, the laws governing that phenomenon, and some of its implications on the structural and chemical evolution of thos…
Adpoting new s-process nucleosynthesis scenario and branch s-process path, we calculate the heavy-element abundances and C/O ratio of solar metallicity 3M_sun TP-AGB stars. The evolutionary sequence from M to S to C stars of AGB stars is explained naturally by the calculated results. Then combining the angular momentum conservation model of wind accretion with the heavy-element abundances on the surface of TP-AGB stars, we calculate the heavy-element overabundances of barium stars via successive pulsed accreting and mixing. Our results support that the barium stars with longer orbital period, …
Vortices have been postulated at a range of size scales in the universe including at the stellar size-scale. Whilst hydrodynamically simulating the wind from an asymptotic giant branch (AGB) star moving through and sweeping up its surrounding interstellar medium (ISM), we have found vortices on the size scale of 10^-1 pc to 10^1 pc in the wake of the star. These vortices appear to be the result of instabilities at the head of the bow shock formed upstream of the AGB star. The instabilities peel off downstream and form vortices in the tail of AGB material behind the bow shock, mixing with the s…
We present detailed evolutionary calculations focused on the evolution of intermediate mass stars with 3 Msun < M < 9 Msun of metallicity typical of the Large Magellanic Cloud (LMC), i.e. Z=0.008. We compare carefully the models calculated by adopting a diffusive scheme for chemical mixing, in which nuclear burning and mixing are self-consistently coupled, while the eddy velocities beyond the formal convective core boundary are treated to decay exponentially, and those calculated with the traditional instantaneous mixing approximation. We find that: i) the physical and chemical behaviour of th…
We demonstrate that a massive asymptotic giant branch (AGB) star is a good candidate as the main source of short-lived radionuclides in the early solar system. Recent identification of massive (4-8 solar masses) AGB stars in the Galaxy, which are both lithium- and rubidium-rich, demonstrates that these stars experience proton captures at the base of the convective envelope (hot bottom burning), together with high-neutron density nucleosynthesis with 22Ne as a neutron source in the He shell and efficient dredge-up of the processed material. A model of a 6.5 solar masses star of solar metallicit…
The Low Mach Number Approximation (LMNA) is applied to 2D hydrodynamical modeling of Type I X-ray bursts on a rectangular patch on the surface of a non-rotating neutron star. Because such phenomena involve decidedly subsonic flows, the timestep increase offered by the LMNA makes routine simulations of these deflagrations feasible in an environment where strong gravity produces significant stratification, while allowing for potentially significant lateral differences in temperature and density. The model is employed to simulate the heating, peak, and initial cooling stages in the deep envelope …
In previous work, we have shown that recent updated standard solar models cannot reproduce the radial profile of the sound speed at the base of the convective zone (CZ) and fail to predict the Li7 depletion. In parallel, helioseismology has shown that the transition from differential rotation in the CZ to almost uniform rotation in the radiative solar interior occurs in a shallow layer called the tachocline. This layer is presumably the seat of large scale circulation and of turbulent motions. Here, we introduce a macroscopic transport term in the structure equations, which is based on a hydro…
In low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars, anomalous mixing must transport material near the hydrogen-burning shell to the convective envelope. Recently, it was suggested that buoyant magnetic flux tubes could supply the necessary transport rate (Busso et al. 2007). The fields are assumed to originate from a dynamo operating in the stellar interior. Here, we show what is required of an $α-Ω$ dynamo in the envelope of an AGB star to maintain these fields. Differential rotation and rotation drain via turbulent dissipation and Poynting flux, so if shear can be res…
The available information on isotopic abundances in the atmospheres of low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars requires that episodes of extensive mixing occur below the convective envelope, reaching down to layers close to the hydrogen burning shell (Cool Bottom Processing). Recently \cite{Busso:2007jw} suggested that dynamo-produced buoyant magnetic flux tubes could provide the necessary physical mechanisms and also supply sufficient transport rates. Here, we present an $α-Ω$ dynamo in the envelope of an RGB/AGB star in which shear and rotation drain via turbu…
The main scope of this paper is to investigate the possible existence of a metallicity dependence of the overshooting from main sequence stars turbulent cores. We focus on objects with masses in the range ~2.5 Msol - ~25 Msol. Basically, evolutionary time scale ratios are compared with star numbers ratios on the main sequence. Star populations are synthesized using grids of evolutionary tracks computed with various overshooting amounts. Observational material is provided by the large and homogeneous photometric database of OGLE 2 project for the Magellanic clouds. Attention is paid to the stud…
The understanding of transport processes inside the stars is one of the main goals of asteroseismology. Chemical turbulent mixing can affect the internal distribution of μnear the energy generating core, having an effect on the evolutionary tracks similar to that of overshooting. This mixing leads to a smoother chemical composition profile near the edge of the convective core, which is reflected in the behavior of the buoyancy frequency and, therefore, in the frequencies of gravity modes. We describe the effects of convective overshooting and turbulent mixing on the frequencies of gravity mode…
The concomitant overabundances of C, N and s-process elements are commonly ascribed to the complex interplay of nucleosynthesis, mixing and mass loss taking place in Asymptotic Giant Branch stars. At low metallicity, the enhancement of C and/or N may be up to 1000 times larger than the original iron content and significantly affects the stellar structure and its evolution. For this reason, the interpretation of the already available and still growing amount of data concerning C-rich metal-poor stars belonging to our Galaxy as well as to dwarf spheroidal galaxies would require reliable AGB stel…
