Oscar Straniero

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

The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters

f {1}{2}

The alpha-enhanced isochrones and their impact on the FITS to the Galactic globular cluster system

--> 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.

THE GIANT, HORIZONTAL, AND ASYMPTOTIC BRANCHES OF GALACTIC GLOBULAR CLUSTERS. I. THE CATALOG, PHOTOMETRIC OBSERVABLES, AND FEATURES

High dispersion spectra (R > 40 000) for a quite large number of stars at the main sequence turn-o and at the base of the giant branch in NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLT UT2). The (Fe/H) values we found are 2:03 0:02 0:04 and 1:42 0:02 0:04 for NGC 6397 and NGC 6752 respectively, where the rst error bars refer to internal and the second ones to systematic errors (within the abundance scale dened by our analysis of 25 subdwarfs with good Hipparcos parallaxes). In both clusters the (Fe/H)s obtained for TO-stars agree perfectly (within a few percent) with that obtained for stars at the base of the RGB. The (O=Fe) = 0:21 0:05 value we obtain for NGC 6397 is quite low, but it agrees with previous results obtained for giants in this cluster. Moreover, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other hand, our results show clearly and for the rst time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC 6752, a more massive cluster than NGC 6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very dicult to explain the observed Na-O, and Mg-Al anticorrelation in NGC 6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

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

We analyze in detail the effect produced by the enhancement of the α-elements O, Ne, Mg, Si, S, and Ca on the evolutionary properties of low-mass, low-metallicity stars. In particular we address the evolutionary phases which extend from the main sequence up to the H reignition on the asymptotic giant branch. We find that the α-elements other than O cannot be neglected

Galactic chemical evolution of heavy elements: from barium to europium

A catalog including a set of the most recent color-magnitude diagrams (CMDs) is presented for a sample of 61 Galactic globular clusters (GGCs). We used this database to perform a homogeneous systematic analysis of the evolved sequences (namely, the red giant branch [RGB], horizontal branch [HB], and asymptotic giant branch [AGB]). Based on this analysis, we present (1) a new procedure to measure the level of the zero-age horizontal branch (VZAHB) and a homogeneous set of distance moduli obtained by adopting the HB as standard candle; (2) an independent estimate for RGB metallicity indicators and new calibrations of these parameters in terms of both spectroscopic ([Fe/H]CG97) and global metallicity ([M/H], including also the ?-element enhancement), such that the set of equations presented can be used to simultaneously derive a photometric estimate of the metal abundance and the reddening from the morphology and the location of the RGB in the (V, B-V) CMD; and (3) the location of the RGB bump (in 47 GGCs) and the AGB bump (in nine GGCs). The dependence of these features on metallicity is discussed. We find that by using the latest theoretical models and the new metallicity scales, the earlier discrepancy between theory and observations (~0.4 mag) completely disappears.

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

New models of thermally pulsing asymptotic giant branch (TP-AGB) stars of low mass and solar chemical composition are presented, namely, 1 ≤ M/M☉ ≤ 3, Z = 0.02, and Y = 0.28. The influence of various parameters (such as the initial core mass, the envelope mass, the mass-loss rate, the opacity, and the mixing length) on the properties of the models is discussed in detail. Our main findings are the following: 1. The third dredge-up (TDU) operates self-consistently (using the Schwarzschild criterion for convection and without invoking any extra-mixing) for masses as low as 1.5 M☉. The minimum core mass for which TDU is found is MH ~ 0.61 M☉. This value is attained after about 10 thermal pulses, almost independently of the initial mass. 2. During the early TP-AGB evolution, the relation between the pulse strength (i.e., the luminosity peak of the 3α burning during the pulse) and the core mass is in good agreement with previous findings. However, when TDU is settled on, the strength of the pulse increases more rapidly as the penetration of the convective envelope into the He intershell increases. No asymptotic limit is found. 3. Furthermore, the 3α luminosity peak is independent of the previous history: the strength of the pulse in a model with mass loss is the same as in a model without mass loss but having the same core and envelope masses. 4. Unless extreme mass-loss rates are assumed, carbon stars are obtained in all the sequences of models with initial mass M ≥ 1.5 M☉ after about 24-26 thermal pulses and 15-17 TDU episodes. At C-star formation, the core mass is less than 0.7 M☉, and the luminosity is of the order of 104 L☉. The dredged-up mass increases up to a maximum and then decreases as mass loss and/or the advancement of the H-burning shell consume the envelope. When the envelope mass is reduced below approximately 0.5 M☉, TDU eventually vanishes. 5. If some amount of protons is diffused below the base of the H-rich envelope during TDU, in the interpulse a13C-pocket is formed and then burnt radiatively via the 13C(α, n)16O reaction, before the onset of a new pulse. Thus, s-process nucleosynthesis occurs in a radiative environment characterized by a fairly low neutron density. In advanced thermal pulses, when the temperature at the bottom of the convective shell approaches 3 × 108 K, a secondary source of neutrons comes from the marginal activation of the 22Ne(α, n)25Mg reaction.

Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars

We follow the chemical evolution of the Galaxy for elements from Ba to Eu, using an evolutionary model suitable for reproducing a large set of Galactic (local and nonlocal) and extragalactic constraints. Input stellar yields for neutron-rich nuclei have been separated into their s-process and r-process components. The production of s-process elements in thermally pulsing asymptotic giant branch stars of low mass proceeds from the combined operation of two neutron sources: the dominant reaction 13C(α, n)16O, which releases neutrons in radiative conditions during the interpulse phase, and the reaction 22Ne(α, n)25Mg, marginally activated during thermal instabilities. The resulting s-process distribution is strongly dependent on the stellar metallicity. For the standard model discussed in this paper, there is a sharp production of the Ba-peak elements around Z Z☉/4. Concerning the r-process yields, we assume that the production of r-nuclei is a primary process occurring in stars near the lowest mass limit for Type II supernova progenitors. The r-contribution to each nucleus is computed as the difference between its solar abundance and its s-contribution, given by the Galactic chemical evolution model at the epoch of the formation of the solar system. We compare our results with spectroscopic abundances of elements from Ba to Eu at various metallicities (mainly from F and G stars), showing that the observed trends can be understood in the light of present knowledge of neutron capture nucleosynthesis. Finally, we discuss a number of emerging features that deserve further scrutiny.

Constraints on the Progenitors of Type Ia Supernovae and Implications for the Cosmological Equation of State

We present abundances for 22 chemical elements in 10 red giant members of the massive Galactic globular cluster ω Centauri. The spectra are of relatively high spectral resolution and signal-to-noise. Using these abundances plus published literature values, abundance trends are defined as a function of the standard metallicity indicator iron. The lowest metallicity stars in ω Cen have [Fe/H] ~ -1.8, and the initial abundance distribution in the cluster is established at this metallicity. The stars in the cluster span a range of [Fe/H] ~ -1.8 to -0.8. At the lowest metallicity, the heavy-element abundance is found to be well characterized by a scaled solar system r-process distribution, as found in other stellar populations at this metallicity. As iron increases, the s-process heavy-element abundances increase dramatically. Comparisons of the s-process increases with recent stellar models finds that s-process nucleosynthesis in 1.5–3 M⊙ asymptotic giant branch stars (AGB) fits well the heavy-element abundance distributions. In these low-mass AGB stars, the dominant neutron source is 13C(α, n)16O. A comparison of the Rb/Zr abundance ratios in ω Cen finds that these ratios are consistent with the 13C source. The reason ω Cen displays such a large s-process component is possibly due to the fact that in such a relatively low-mass stellar system, AGB ejecta, because of their low velocity winds, are more efficiently retained in the cluster relative to the much faster moving Type II supernova ejecta. Significant s-process enrichment relative to Fe, from the lower mass AGB stars, would require that the cluster was active in star formation for quite a long interval of time, of the order of 2–3 Gyr. The AGB ejecta were mixed with the retained fraction of Type II supernova ejecta and with the residual gas of initial composition. The analysis of α-rich elements shows that no significant amounts of Type Ia supernova debris were retained by the cluster. In this context, interpretation of the low and constant observed [Cu/Fe] ~ -0.6 (derived here for the first time in this cluster) finds a plausible interpretation.