Lionel Siess

Evolution of massive AGB stars: II. Model properties at non-solar metallicity and the fate of Super-AGB stars

Context. Massive AGB (hereafter super-AGB or SAGB) stars ignite carbon off-center and have initial masses ranging between Mup, the minimum initial mass for carbon ignition, and Mmas the minimum mass for the formation of an iron core collapse supernova. In this mass interval, stars more massive than Mn 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 Mand calculated for 7d ifferent 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 Mup, Mn and Mmas present a nonlinear behavior with Z, characterized by a minimum around Z = 10 −4 ,d ) the values of Mup, Mn and Mmas are decreased by ∼2 Mwhen core overshooting is considered, e) our models predict a minimum oxygen- neon white dwarf mass of ∼1.05 M� , f) the determination of Mn 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 Mwidth 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 = u Mloss/ u Mcore is greater than about 70−90,

Evolution of massive AGB stars - III. the thermally pulsing super-AGB phase

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.

The accretion of brown dwarfs and planets by giant stars ‐‐ I. Asymptotic giant branch stars

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 ∼ 0.01 and ∼ 0.1 M⊙. The calculations show that for high accretion rates (˙Macc= 10-4 M⊙yr -1), the considerable release of accretion energy produces a substantial expansion of the star and gives rise to hot bottom burning at the base of the convective envelope. For somewhat lower accretion rates (˙ Macc= 10-5 M⊙ yr -1), the accretion luminosity represents only a small fraction of the stellar luminosity, and as a result of the increase in mass (and concomitantly of the gravitational force), the star contracts. Our simulations also indicate that the triggering of thermal pulses is delayed (accelerated) if mass is injected at a slower (faster) rate. We analyse the effects of this accretion process on the surface chemical abundances and show that chemical modifications are mainly the result of deposition of fresh material rather than of active nucleosynthesis. Finally, we suggest that the accretion of brown dwarfs and planets can induce the ejection of shells around giant stars, increase their surface lithium abundance and lead to significant spin-up. The combination of these features is frequently observed among G and K giant stars.

Hot Jupiters and Cool Stars

Close-in planets are in jeopardy, as their host stars evolve off the main sequence (MS) to the subgiant and red giant phases. In this paper, we explore the influences of the stellar mass (in the range 1.5-2M similar to), 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 along 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 masses and increasing planetary masses. Regarding the planets 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 MS 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.

Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars

Context. With the progress of observational constraints on stellar rotation and on the angular velocity profile in stars, it is necessary to understand how angular momentum is transported in stellar interiors during their whole evolution. In this context, more highly refined dynamical stellar evolution models have been built that take into account transport mechanisms.Aims. Internal gravity waves (IGWs) excited by convective regions constitute an efficient transport mechanism over long distances in stellar radiation zones. They are one of the mechanisms that are suspected of being responsible for the quasi-flat rotation profile of the solar radiative region up to 0.2 R⊙. Therefore, we include them in our detailed analysis started in Paper I of the main physical processes responsible for the transport of angular momentum and chemical species in stellar radiation zones. Here, we focus on the complete interaction between differential rotation, meridional circulation, shear-induced turbulence, and IGWs during the main sequence.Methods. We improved the diagnosis tools designed in Paper I to unravel angular momentum transport and chemical mixing in rotating stars by taking into account IGWs. The star’s secular hydrodynamics is treated using projection on axisymmetric spherical harmonics and appropriate horizontal averages that allow the problem to be reduced to one dimension while preserving the non-diffusive character of angular momentum transport by the meridional circulation and IGWs. Wave excitation by convective zones is computed at each time-step of the evolution track. We choose here to analyse the evolution of a 1.1 M⊙, Z⊙ star in which IGWs are known to be efficient.Results. We quantify the relative importance of the physical mechanisms that sustain meridional currents and that drive the transport of angular momentum, heat, and chemicals when IGWs are taken into account. First, angular momentum extraction, Reynolds stresses caused by IGWs, and viscous stresses sustain a large-scale multi-cellular meridional circulation. This circulation in turn advects entropy, which generates temperature fluctuations and a new rotation profile because of thermal wind.Conclusions. We have refined our diagnosis of secular transport processes in stellar interiors. We confirm that meridional circulation is sustained by applied torques, internal stresses, and structural readjustments, rather than by thermal imbalance, and we detail the impact of IGWs. These large-scale flows then modify the thermal structure of stars, their internal rotation profile, and their chemical stratification. The tools we developed in Paper I and generalised for the present analysis will be used in the near future to study secular hydrodynamics of rotating stars taking into account IGWs in the whole Hertzsprung-Russell diagram.

Super- and massive AGB stars - IV. Final fates initial-to-final mass relation

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-asymptotic giant branch (super-AGB) and massive AGB star models are in the mass range between 5.0 and 10.0 M circle dot 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 (EC-SN) 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 EC-SN rates for lower metallicities which are significantly lower than existing values from parametric studies in the literature. We conclude that 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 approximate to 0.2 M circle dot. This implies that between 2 and 5 per cent of all gravitational collapse supernova are EC-SNe in the metallicity range Z = 0.02-0.0001. 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.

Super and massive AGB stars – II. Nucleosynthesis and yields – Z = 0.02, 0.008 and 0.004

We have computed detailed evolution and nucleosynthesis models for super and massive AGB stars over the mass range 6.5-9.0 Msun in divisions of 0.5 Msun with metallicities Z=0.02, 0.008 and 0.004. These calculations, in which we find third dredge-up and hot bottom burning, fill the gap between existing low and intermediate-mass AGB star models and high mass star models that become supernovae. For the considered metallicities, the composition of the yields is largely dominated by the thermodynamic conditions at the base of the convective envelope rather than by the pollution arising from third dredge up. We investigate the effects of various uncertainties, related to the mass-loss rate, mixing length parameter, and the treatment of evolution after the envelope instability that develops near the end of the (Super)AGB phase. Varying these parameters alter the yields mainly because of their impact on the amount of third dredge up enrichment, and to a lesser extent on the hot bottom burning conditions. Our models produce significant amounts of He4, Li7 (depending on the mass-loss formulation) C13, N14, O17, Na23, Mg25, as well the radioactive isotope Al26 in agreement with previous investigation. In addition our results show enrichment of Ne22, Mg26 and Fe60, as well as a substantial increase in our proxy neutron capture species representing all species heavier than iron. These stars may provide important contributions to the Galaxys inventory of the heavier Mg isotopes, N14, Li7 and Al27.

Nucleosynthesis of s-elements in rotating AGB stars

We analyze the s-process nucleosynthesis in models of rotating AGB stars, using a complete nuclear network covering nuclei up to Polonium. During the stage of thermal pulses, the extreme shear field that develops at the base of the convective envelope leads to the injection of protons into the adjacent

Pulsating red giant stars in eccentric binary systems discovered from Kepler space-based photometry : A sample study and the analysis of KIC 5006817

\rm{}^{12}\kern-0.8ptC

CS 30322--023: An Ultra Metal-Poor TP-AGB Star?

-rich core. Subsequent proton captures lead to overlapping