R. Gallino

Neutron Capture in Low-Mass Asymptotic Giant Branch Stars: Cross Sections and Abundance Signatures

Recently improved information on the stellar (n, γ) cross sections of neutron magic nuclei at N = 82, and in particular of 142Nd, turn out to represent a sensitive test for models of s-process nucleosynthesis. While these data were found to be incompatible with the classical approach based on an exponential distribution of neutron exposures, they provide significantly better agreement between the solar abundance distribution of s nuclei and the predictions of models for low-mass asymptotic giant branch (AGB) stars. The origin of this phenomenon is identified as lying in the high neutron exposures at low neutron density obtained between thermal pulses when 13C burns radiatively in a narrow layer of a few 10-4 M☉. This effect is studied in some detail, and the influence of the currently available nuclear physics data is discussed with respect to specific further questions. In this context, particular attention is paid to a consistent description of s-process branchings in the region of the rare earth elements. It is shown that, in certain cases, the nuclear data are sufficiently accurate that the resulting abundance uncertainties can be completely attributed to stellar modeling. Thus, the s-process becomes important for testing the role of different stellar masses and metallicities as well as for constraining the assumptions used in describing the low neutron density provided by the 13C source.

Evolution and Nucleosynthesis in Low-Mass Asymptotic Giant Branch Stars. II. Neutron Capture and the s-Process

We present a new analysis of neutron capture occurring in low-mass asymptotic giant branch (AGB) stars suffering recurrent thermal pulses. We use dedicated evolutionary models for stars of initial mass in the range 1 to 3 M? and metallicity from solar to half solar. Mass loss is taken into account with the Reimers parameterization. The third dredge-up mechanism is self-consistently found to occur after a limited number of pulses, mixing with the envelope freshly synthesized 12C and s-processed material from the He intershell. During thermal pulses, the temperature at the base of the convective region barely reaches T8 ~ 3 (T8 being the temperature in units of 108 K), leading to a marginal activation of the 22Ne(?, n)25Mg neutron source. The alternative and much faster reaction 13C(?, n)16O must then play the major role. However, the 13C abundance left behind by the H shell is far too low to drive the synthesis of the s-elements. We assume instead that at any third dredge-up episode, hydrogen downflows from the envelope penetrate into a tiny region placed at the top of the 12C-rich intershell, of the order of a few 10-4 M?. At H reignition, a13C-rich (and 14N-rich) zone is formed. Neutrons by the major 13C source are then released in radiative conditions at T8 ~ 0.9 during the interpulse period, giving rise to an efficient s-processing that depends on the 13C profile in the pocket. A second small neutron burst from the 22Ne source operates during convective pulses over previously s-processed material diluted with fresh Fe seeds and H-burning ashes. The main features of the final s-process abundance distribution in the material cumulatively mixed with the envelope through the various third dredge-up episodes are discussed. Contrary to current expectations, the distribution cannot be approximated by a simple exponential law of neutron irradiations. The s-process nucleosynthesis mostly occurs inside the 13C pocket; the form of the distribution is built through the interplay of the s-processing occurring in the intershell zones and the geometrical overlap of different pulses. The 13C pocket is of primary origin, resulting from proton captures on newly synthesized 12C. Consequently, the s-process nucleosynthesis also depends on Fe seeds, a lower metallicity favoring the production of the heaviest elements. This allows a wide range of s-element abundance distributions to be produced in AGB stars of different metallicities, in agreement with spectroscopic evidence and with the Galactic enrichment of the heavy s-elements at the time of formation of the solar system. AGB stars of metallicity Z

Galactic evolution of Sr, Y, and Zr : a multiplicity of nucleosynthetic processes

f {1}{2}

Nucleosynthesis and Mixing on the Asymptotic Giant Branch. III. Predicted and Observed s-Process Abundances

--> Z? are the best candidates for the buildup of the main component, i.e., for the s-distribution of the heavy elements from the Sr-Y-Zr peak up to the Pb peak, as deduced by meteoritic and solar spectroscopic analyses. A number of AGB stars may actually show in their envelopes an s-process abundance distribution almost identical to that of the main component. Eventually, the astrophysical origin of mainstream circumstellar SiC grains recovered from pristine meteorites, showing a nonsolar s-signatures in a number of trace heavy elements, is likely identified in the circumstellar envelopes of AGB stars of about solar metallicity, locally polluting the interstellar medium from which the solar system condensed.

Abundances of Actinides and Short-lived Nonactinides in the Interstellar Medium: Diverse Supernova Sources for the r-Processes

In this paper we follow the Galactic enrichment of three easily observed light n-capture elements: Sr, Y, and Zr. Input stellar yields have been first separated into their respective main and weak s-process components and r-process component. The s-process yields from asymptotic giant branch (AGB) stars of low to intermediate mass are computed, exploring a wide range of efficiencies of the major neutron source, 13 C, and covering both disk and halo metallicities. AGB stars have been shown to reproduce the mains-component in the solar system, i.e., the s-process isotopic distribution of allheavy isotopes with atomic mass number A > 90, with a minor contribution to the light s-process isotopes up to A � 90. The concurrent weak s-process, which accounts for the major fraction of the light s-process isotopes in the solar system and occurs in massive stars by the operation of the 22 Ne neutron source, is discussed in detail. Neither the main s -n or the weaks-components are shown to contribute significantly to the neutron-capture element abundances observed in unevolved halo stars. Knowing the s-process distribution at the epoch of the solar system formation, we first employed the r-process residuals method to infer the isotopic distribution of the r-process. We assumed a primary r-process production in the Galaxy from moderately massive Type II supernovae that best reproduces the observational Galactic trend of metallicity versus Eu, an almost pure r-process element. We present a detailed analysis of a large published database of spectroscopic observations of Sr, Y, Zr, Ba, and Eu for Galactic stars at various metallicities, showing that the observed trends versus metallicity can be understood in light of a multiplicity of stellar neutron-capture components. Spectroscopic observations of the Sr, Y, and Zr to Ba and Eu abundance ratios versus metallicity provide useful diagnostics of the types of neutron-capture processes forming Sr, Y, and Zr. In particular, the observed [Sr, Y, Zr/Ba, Eu] ratio is clearly not flat at low metallicities, as we would expect if Ba, Eu and Sr, Y, Zr all had the same r-process nucleosynthetic origin. We discuss our chemical evolution predictions, taking into account the interplay between different processes to produce Sr-Y-Zr. Making use of the very r-process‐rich and very metal-poor stars like CS 22892� 052 and CS 31082� 001, we find hints and discuss the possibility of a primary process in low-metallicity massive stars, different from the ‘‘classical s-process’’ and from the ‘‘classical r-process’’ that we tentatively define LEPP (lighter element primary process). This allows us to revise the estimates of the r-process contributions to the solar Sr, Y, and Zr abundances, as well as of the contribution to the s-only isotopes 86 Sr, 87 Sr, and 96 Mo. Subject headings: Galaxy: abundances — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: AGB and post-AGB

The s Process: Nuclear Physics, Stellar Models, Observations

We present the results of s-process nucleosynthesis calculations for asymptotic giant branch (AGB) stars of different metallicities and different initial stellar masses (1.5 and 3 M☉), and we present comparisons of them with observational constraints from high-resolution spectroscopy of evolved stars over a wide metallicity range. The computations were based on previously published stellar evolutionary models that account for the third dredge-up phenomenon occurring late on the AGB. Neutron production is driven by the 13C(α, 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 neutron source 13C is manufactured locally by proton captures on the abundant 12C; a few protons are assumed to penetrate from the convective envelope into the radiative layer at any third dredge-up episode, when a chemical discontinuity is established between the convective envelope and the He- and C-rich zones. A weaker neutron release is also guaranteed by the marginal activation of the reaction 22Ne(α, n)25Mg during the convective thermal pulses. Owing to the lack of a consistent model for 13C formation, the abundance of 13C burnt per cycle is allowed to vary as a free parameter over a wide interval (a factor of 50). The s-enriched material is subsequently mixed with the envelope by the third dredge-up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s [hs]) and that of the Zr-peak ones (light s [ls]), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency (e.g., of the neutron exposure). Our model predictions for this parameter show a complex trend versus metallicity. Especially noteworthy is the prediction that the flow along the s-path at low metallicities drains the Zr and Ba peaks and builds an excess at the doubly magic 208Pb, which is at the termination of the s-path. We then discuss the effects on the models of variations in the crucial parameters of the 13C pocket, finding that they are not critical for interpreting the results. The theoretical predictions are compared with published abundances of s-elements for AGB giants of classes MS, S, SC, post-AGB supergiants, and for various classes of binary stars, which supposedly derive their composition by mass transfer from an AGB companion. This is done for objects belonging both to the Galactic disk and to the halo. The observations in general confirm the complex dependence of neutron captures on metallicity. They suggest that a moderate spread exists in the abundance of 13C that is burnt in different stars. Although additional observations are needed, it seems that a good understanding has been achieved of s-process operation in AGB stars. Finally, the detailed abundance distribution including the light elements (CNO) of a few s-enriched stars at different metallicities are examined and satisfactorily reproduced by model envelope compositions.

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

Abundances of ^(244)Pu, ^(235)U, ^(238)U, and ^(232)Th in the early solar system are about those expected for uniform production over most of galactic history. The inferred abundance of 182Hf is also compatible with this model. We here associate production of ^(182)Hf with the same r-process SN sources that produce actinides (SNACS). This requires that r-process nucleosynthesis in SNACS took place rather uniformly over the age of the galaxy until ~10^7 yr prior to solar system formation. The low abundance of ^(107)Pd and ^(129)I in the early solar system indicates that SNACS cannot produce these nuclei at the high yields expected from standard r-process models. We propose that there are distinctive SN sources for different r-process nuclei with a sharp distinction in different SN contributions below and above A ~ 140. Abundances in stars with very low metallicities will vary depending on the type of SN contributing to the local region of star formation. A time scale of ~10^7 yr is much shorter than the 108 yr time usually associated with processes in the galaxy and with the last time of r-process injection accounting for ^(129)I, but may be compatible with the rate of SN occurrence. The hypothesis of a nearby SN polluting the protosolar nebula is critically discussed.

Evolution and Nucleosynthesis in Low-Mass Asymptotic Giant Branch Stars. I. Formation of Population I Carbon Stars

Nucleosynthesis in the

Galactic chemical evolution of heavy elements: from barium to europium

s

The Chemical Evolution of the Globular Cluster ω Centauri (NGC 5139)

process takes place in the He-burning layers of low-mass asymptotic giant branch (AGB) stars and during the He- and C-burning phases of massive stars. The