Luciano Piersanti

EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES

The envelope of thermally pulsing asymptotic giant branch (TP-AGB) stars undergoing periodic third dredge-up (TDU) episodes is enriched in both light and heavy elements, the ashes of a complex internal nucleosynthesis involving p, α, and n captures over hundreds of stable and unstable isotopes. In this paper, new models of low-mass AGB stars (2 M ☉), with metallicity ranging between Z = 0.0138 (the solar one) and Z = 0.0001, are presented. Main features are (1) a full nuclear network (from H to Bi) coupled to the stellar evolution code, (2) a mass loss-period-luminosity relation, based on available data for long-period variables, and (3) molecular and atomic opacities for C- and/or N-enhanced mixtures, appropriate for the chemical modifications of the envelope caused by the TDU. For each model, a detailed description of the physical and chemical evolutions is presented; moreover, we present a uniform set of yields, comprehensive of all chemical species (from hydrogen to bismuth). The main nucleosynthesis site is the thin 13C pocket, which forms in the core-envelope transition region after each TDU episode. The formation of this 13C pocket is the principal by-product of the introduction of a new algorithm, which shapes the velocity profile of convective elements at the inner border of the convective envelope: both the physical grounds and the calibration of the algorithm are discussed in detail. We find that the pockets shrink (in mass) as the star climbs the AGB, so that the first pockets, the largest ones, leave the major imprint on the overall nucleosynthesis. Neutrons are released by the 13C(α, n)16O reaction during the interpulse phase in radiative conditions, when temperatures within the pockets attain T ~ 1.0 × 108 K, with typical densities of (106-107) neutrons cm–3. Exceptions are found, as in the case of the first pocket of the metal-rich models (Z = 0.0138, Z = 0.006 and Z = 0.003), where the 13C is only partially burned during the interpulse: the surviving part is ingested in the convective zone generated by the subsequent thermal pulse (TP) and then burned at T ~ 1.5 × 108 K, thus producing larger neutron densities (up to 1011 neutrons cm–3). An additional neutron exposure, caused by the 22Ne(α, n)25Mg during the TPs, is marginally activated at large Z, but becomes an important nucleosynthesis source at low Z, when most of the 22Ne is primary. The final surface compositions of the various models reflect the differences in the initial iron-seed content and in the physical structure of AGB stars belonging to different stellar populations. Thus, at large metallicities the nucleosynthesis of light s-elements (Sr, Y, Zr) is favored, whilst, decreasing the iron content, the overproduction of heavy s-elements (Ba, La, Ce, Nd, Sm) and lead becomes progressively more important. At low metallicities (Z = 0.0001) the main product is lead. The agreement with the observed [hs/ls] index observed in intrinsic C stars at different [Fe/H] is generally good. For the solar metallicity model, we found an interesting overproduction of some radioactive isotopes, like 60Fe, as a consequence of the anomalous first 13C pocket. Finally, light elements (C, F, Ne, and Na) are enhanced at any metallicity.INAF-Osservatorio Astronomico di Collurania, 64100 Teram o, Italy and R. Gallino2,3 Dipartimento di Fisica Generale, Universitá di Torino, 10 125 Torino, Italy Center for Stellar and Planetary Astrophysics, School of Ma thematical Sciences, Monash University, P.O. Box 28, Victoria 3800, Australia and L. Piersanti 1 INAF-Osservatorio Astronomico di Collurania, 64100 Teram o, Italy and I. Domı́nguez4 Departamento de Fı́sica Teórica y del Cosmos , Universidad de Granada, 18071 Granada, Spain and M.T. Lederer 5 Institut für Astronomie, Türkenschanzstraße 17, A-1180 Wien, Austria Received/ Accepted

The Chemical Composition of White Dwarfs as a Test of Convective Efficiency during Core Helium Burning

Pulsating white dwarfs provide constraints to the evolution of progenitor stars. We revise He-burning stellar models, with particular attention to core convection and to its connection with the nuclear reactions powering energy generation and chemical evolution. Theoretical results are compared to the available measurements for the variable white dwarf GD 358, which indicate a rather large abundance of central oxygen (Metcalfe and coworkers). We show that the attempt to constrain the relevant nuclear reaction rate by means of the white dwarf composition is faced with a large degree of uncertainty related to evaluating the efficiency of convection-induced mixing. By combining the uncertainty of the convection theory with the error on the relevant reaction rate, we derive that the present theoretical prediction for the central oxygen mass fraction in white dwarfs varies between 0.3 and 0.9. Unlike previous claims, we find that models taking into account semiconvection and a moderate 12C(α,γ)16O reaction rate are able to account for a high central oxygen abundance. The rate of the 12C(α,γ)16O used in these models agrees with the one recently obtained in laboratory experiments by Kunz and coworkers. On the other hand, when semiconvection is inhibited, as in the case of classical models (bare Schwarzschild criterion) or in models with mechanical overshoot, an extremely high rate of the 12C(α,γ)16O reaction is needed to account for a large oxygen production. Finally, we show that the apparent discrepancy between our result and those reported in previous studies depends on the method used to avoid the convective runaways (the so-called breathing pulses) that are usually encountered in modeling late stage of core He-burning phase.

Carbon-Oxygen White Dwarfs Accreting CO-rich Matter. I. A Comparison between Rotating and Nonrotating Models

We investigate the lifting effect of rotation on the thermal evolution of CO white dwarfs accreting CO-rich matter. We find that rotation induces the cooling of the accreting structure so that the delivered gravitational energy causes a greater expansion with respect to the standard nonrotating case. The increase in the surface radius produces a decrease in the surface value of the critical angular velocity and, therefore, the accreting white dwarf becomes gravitationally unbound (Roche instability). This occurrence is due to an increase in the total angular momentum of the accreting white dwarf and depends critically on the amount of specific angular momentum deposited by the accreted matter. If the specific angular momentum of the accreted matter is equal to that of the outer layers of the accreting structure, the Roche instability occurs well before the accreting white dwarf can attain the physical conditions for carbon burning. If the values of both initial angular velocity and accretion rate are small, we find that the accreting white dwarf undergoes a secular instability when its total mass approaches 1.4 M☉. At this stage, the ratio between the rotational energy and the gravitational binding energy of the white dwarf becomes of the order of 0.1, so that the star must deform by adopting an elliptical shape. In this case, since the angular velocity of the white dwarf is as large as ~1 rad s-1, the anisotropic mass distribution induces the loss of rotational energy and angular momentum via gravitational wave radiation. We find that, independent of the braking efficiency, the white dwarf contracts and achieves the physical conditions suitable for explosive carbon burning at the center so that a Type Ia supernova event is produced.

EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF AGB STARS AT DIFFERENT METALLICITIES. III. INTERMEDIATE-MASS MODELS, REVISED LOW-MASS MODELS, AND THE pH-FRUITY INTERFACE

We present a new set of models for intermediate mass AGB stars (4.0, 5.0 and, 6.0 Msun) at different metallicities (-2.15<=Fe/H]<=+0.15). This integrates the existing set of models for low mass AGB stars (1.3<=M/M<=3.0) already included in the FRUITY database. We describe the physical and chemical evolution of the computed models from the Main Sequence up to the end of the AGB phase. Due to less efficient third dredge up episodes, models with large core masses show modest surface enhancements. The latter is due to the fact that the interpulse phases are short and, then, Thermal Pulses are weak. Moreover, the high temperature at the base of the convective envelope prevents it to deeply penetrate the radiative underlying layers. Depending on the initial stellar mass, the heavy elements nucleosynthesis is dominated by different neutron sources. In particular, the s-process distributions of the more massive models are dominated by the \nean~reaction, which is efficiently activated during Thermal Pulses. At low metallicities, our models undergo hot bottom burning and hot third dredge up. We compare our theoretical final core masses to available white dwarf observations. Moreover, we quantify the weight that intermediate mass models have on the carbon stars luminosity function. Finally, we present the upgrade of the FRUITY web interface, now also including the physical quantities of the TP-AGB phase of all the models included in the database (ph-FRUITY).

METALLICITY AS A SOURCE OF DISPERSION IN THE SNIa BOLOMETRIC LIGHT CURVE LUMINOSITY-WIDTH RELATIONSHIP

The recognition that the metallicity of Type Ia supernova (SNIa) progenitors might bias their use for cosmological applications has led to an increasing interest in its role in shaping SNIa light curves. We explore the sensitivity of the synthesized mass of 56Ni, M(56Ni), to the progenitor metallicity starting from pre-main-sequence models with masses M 0 = 2-7 M ☉ and metallicities Z = 10–5-0.10. The interplay between convective mixing and carbon burning during the simmering phase eventually raises the neutron excess, η, and leads to a smaller 56Ni yield, but does not change substantially the dependence of M(56Ni) on Z. Uncertain attributes of the progenitor white dwarf, like the central density, have a minor effect on M(56Ni). Our main results are: (1) a sizeable amount of 56Ni is synthesized during incomplete Si-burning, which leads to a stronger dependence of M(56Ni) on Z than obtained by assuming that 56Ni is produced in material that burns fully to nuclear statistical equilibrium; (2) in one-dimensional delayed detonation simulations a composition dependence of the deflagration-to-detonation transition (DDT) density gives a nonlinear relationship between M(56Ni) and Z and predicts a luminosity larger than previously thought at low metallicities (however, the progenitor metallicity alone cannot explain the whole observational scatter of SNIa luminosities); and (3) an accurate measurement of the slope of the Hubble residuals versus metallicity for a large enough data set of SNIa might give clues to the physics of DDT in thermonuclear explosions.

He-accreting white dwarfs: accretion regimes and final outcomes

The behaviour of carbon-oxygen white dwarfs (WDs) subject to direct helium accretion is extensively studied. We aim to analyze the thermal response of the accreting WD to mass deposition at different time scales. The analysis has been performed for initial WDs masses and accretion rates in the range (0.60 - 1.02) Msun and 1.e-9 - 1.e-5 Msun/yr, respectively. Thermal regimes in the parameters space M_{WD} - dot{M}_{He}, leading to formation of red-giant-like structure, steady burning of He, mild, strong and dynamical flashes have been identified and the transition between those regimes has been studied in detail. In particular, the physical properties of WDs experiencing the He-flash accretion regime have been investigated in order to determine the mass retention efficiency as a function of the accretor total mass and accretion rate. We also discuss to what extent the building-up of a He-rich layer via H-burning could be described according to the behaviour of models accreting He-rich matter directly. Polynomial fits to the obtained results are provided for use in binary population synthesis computations. Several applications for close binary systems with He-rich donors and CO WD accretors are considered and the relevance of the results for the interpretation of He-novae is discussed.

Asymptotic-Giant-Branch Models at Very Low Metallicity

In this paper we present the evolution of a low-mass model (initial mass M = 1.5 M⊙) with a very low metal content (Z = 5 × 10–5, equivalent to [Fe/H] = –2.44). We find that, at the beginning of the Asymptotic Giant Branch (AGB) phase, protons are ingested from the envelope in the underlying convective shell generated by the first fully developed thermal pulse. This peculiar phase is followed by a deep third dredge-up episode, which carries to the surface the freshly synthesized 13C, 14N and 7Li. A standard thermally pulsing AGB (TP-AGB) evolution then follows. During the proton-ingestion phase, a very high neutron density is attained and the s process is efficiently activated. We therefore adopt a nuclear network of about 700 isotopes, linked by more than 1200 reactions, and we couple it with the physical evolution of the model. We discuss in detail the evolution of the surface chemical composition, starting from the proton ingestion up to the end of the TP-AGB phase.

Rotating type Ia SN progenitors : Explosion and light curves

Based on the rigidly rotating progenitor models found to be able to grow up to the canonical Chandrasekhar mass limit and beyond, and undergo a thermonuclear explosion, we compute the explosions, detailed nucleosynthesis, and corresponding light curves by means of a one-dimensional hydrodynamic code. Our results show that the inclusion of rotation in the evolution of the progenitors determines, in a natural way, a variation in the explosive physical conditions, mainly different explosive ignition densities (2.08 × 109 to 3.34 × 109 g cm-3), total masses (1.39-1.48 M☉), and binding energies (-5.3 × 1050 to -6.6 × 1050 ergs). Such a spread is related to the rotational velocity at the explosive carbon ignition stage and to the efficiency of angular momentum loss during the last part of the progenitor evolution. We explore the final outcome in the framework of the delayed detonation explosion models by fixing the value of the transition density and by considering two different braking efficiencies. Within the explored parameter space, the bolometric light curves at maximum show differences of ~0.1 mag due to the different amount of 56Ni synthesized during the explosion. Although rigid rotation cannot be considered responsible for the diversities in the observational properties of SNe Ia, it could explain the dispersion in the magnitude at maximum of standardized events. We also find that those models with high ignition densities produce a central remnant in which most of the neutron-rich species synthesized during the explosion are trapped.

IMPACT OF A REVISED 25Mg(p, γ)26Al REACTION RATE ON THE OPERATION OF THE Mg-Al CYCLE

Proton captures on Mg isotopes play an important role in the Mg-Al cycle active in stellar H-burning regions. In particular, low-energy nuclear resonances in the

Hydrogen-Accreting Carbon-Oxygen White Dwarfs of Low Mass: Thermal and Chemical Behavior of Burning Shells

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