Maria Lugaro

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

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

f {1}{2}

Reaction Rate Uncertainties and the Production of 19F in Asymptotic Giant Branch Stars

--> 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.

s-Process Nucleosynthesis in Asymptotic Giant Branch Stars: A Test for Stellar Evolution

The strontium, zirconium, molybdenum, and barium isotopic compositions predicted in the mass-losing envelopes of asymptotic giant branch (AGB) stars of solar metallicity and mass 1.5, 3, and 5 M☉ are discussed and compared with recent measurements in single presolar silicon carbide (SiC) grains from the Murchison meteorite. Heavy-element nucleosynthesis via the s-process occurs in the helium intershell, the region between the helium-burning and hydrogen-burning shells, producing heavy elements beyond iron. After a limited number of thermal runaways of the helium shell (thermal pulses), at the quenching of each instability, the convective envelope penetrates into the top layers of the helium intershell (third dredge-up), mixing newly synthesized 12C and s-process material to the stellar surface. Eventually, the envelope becomes carbon-rich (C ≥ O), a necessary condition for SiC grains to condense. In the helium intershell, neutrons are released by (α, n) reactions on 13C and 22Ne during interpulse phases and the thermal pulses, respectively. A 13C pocket is assumed to form in a tiny region in the top layers of the helium intershell by injection of a small amount of protons from the envelope during each third dredge-up episode. This 13C then burns radiately during the interpulse phase. The average neutron density produced is low, but of long duration, so the total neutron exposure is high. We have explored a large range of possible 13C abundances in the pocket. In low-mass AGB stars (1.5 M☉ ≤ M ≤ 4 M☉), a second small burst of neutrons is released by marginal 22Ne burning in the thermal pulse. The neutron density reaches quite a high peak value but is of short duration, so the neutron exposure is low. In intermediate-mass AGB stars (4 M☉ < M ≤ 8 M☉), the 22Ne neutron source is more efficiently activated. The neutron capture process has been followed with a postprocessing code that considers all relevant nuclei from 4He to 210Po. The predicted isotopic compositions of strontium, zirconium, molybdenum, and barium in the envelopes of low-mass AGB stars of solar metallicity are in agreement with the isotopic ratios measured in individual presolar SiC grains, whereas predictions for intermediate-mass stars exclude them as the sources of these grains. A multiplicity of low-mass AGB stars with metallicity around solar, having different masses and experiencing different neutron exposures, are required to account for the measured spread in heavy-element isotopic compositions among single presolar SiC grains. The range of neutron exposures corresponds, on average, to a lower mean neutron exposure than that required to reproduce the s-process main component in the solar system.

Silicon and Carbon Isotopic Ratios in AGB Stars: SiC Grain Data, Models, and the Galactic Evolution of the Si Isotopes

We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. During the asymptotic giant branch (AGB) phase, 19F is produced as a consequence of nucleosynthesis occurring during the convective thermal pulses and also during the interpulse periods if protons from the envelope are partially mixed in the top layers of the He intershell (partial mixing zone). 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(α, γ)18O and 19F(α, p)22Ne reaction rates. For these two reactions we present new estimates of the rates and their uncertainties. In both cases the revised rates are lower than previous estimates. The effect of the inclusion of the partial mixing zone on the production of fluorine strongly depends on the very uncertain 14C(α, γ)18O reaction rate. The importance of the partial mixing zone is reduced when using our estimate for this rate. Overall, rate uncertainties result in uncertainties in the fluorine production of about 50% in stellar models with mass 3 M☉ and of about a factor of 7 in stellar models of mass 5 M☉. This larger effect at high masses is due to the high uncertainties of the 19F(α, p)22Ne reaction rate. Taking into account both the uncertainties related to the partial mixing zone and those related to nuclear reactions, the highest values of 19F enhancements observed in AGB stars are not matched by the models. This is a problem that will have to be revised by providing a better understanding of the formation and nucleosynthesis in the partial mixing zone, as well as in relation to reducing the uncertainties of the 14C(α, γ)18O reaction rate. At the same time, the possible effect of cool bottom processing at the base of the convective envelope should be included in the computation of AGB nucleosynthesis. This process could, in principle, help to match the highest 19F abundances observed by decreasing the C/O ratio at the surface of the star, while leaving the 19F abundance unchanged.

Presolar SiC Grains of Type Y: Origin from Low-Metallicity Asymptotic Giant Branch Stars

We study the slow neutron capture process (s-process) in asymptotic giant branch (AGB) stars using three different stellar evolutionary models computed for a 3 M☉, solar metallicity star. First we investigate the formation and the efficiency of the main neutron source: the 13C(α, n)16O reaction that occurs in radiative conditions. A tiny region rich in 13C (the 13C pocket) is created by proton captures on the abundant 12C in the top layers of the He intershell, the zone between the H shell and the He shell. We parametrically vary the number of protons mixed from the envelope. For high local proton-to-12C number ratios, p/12C 0.3, most of the 13C nuclei produced are further converted by proton capture to 14N. Besides, 14N nuclei represent a major neutron poison. We find that a linear relationship exists between the amount of 12C in the He intershell and the maximum value of the time-integrated neutron flux. Then we generate detailed s-process calculations on the basis of stellar evolutionary models constructed with three different codes, all of them self-consistently finding the third dredge-up, although with different efficiency. One of the codes includes a mechanism at each convective boundary that simulates time-dependent hydrodynamic overshoot. This mechanism depends on a free parameter f and results in partial mixing beyond convective boundaries, the most efficient third dredge-up, and the formation of the 13C pocket. For the other two codes, an identical 13C pocket is introduced in the postprocessing nucleosynthesis calculations. The models typically produce enhancements of heavy elements of about 2 orders of magnitude in the He intershell and of up to 1 order of magnitude at the stellar surface, after dilution with the convective envelope, thus generally reproducing spectroscopic observations. The results of the cases without overshoot are remarkably similar, pointing out that the important uncertainty in s-process predictions is the 13C pocket and not the intrinsic differences among different codes when no overshoot mechanism is included. The code including hydrodynamic overshoot at each convective boundary finds that the He intershell convective zone driven by the recurrent thermal instabilities of the He shell (thermal pulses) penetrates the C-O core, producing a He intershell composition near that observed in H-deficient central stars of planetary nebulae. As a result of this intershell dredge-up, the neutron fluxes have a higher efficiency, both during the interpulse periods and within thermal pulses. The s-element distribution is pushed toward the heavier s-process elements, and large abundances of neutron-rich isotopes fed by branching points in the s-process path are produced. Several observational constraints are better matched by the models without overshoot. Our study needs to be extended to different masses and metallicities and in the space of the free overshoot parameter f.

THE s-PROCESS IN ROTATING ASYMPTOTIC GIANT BRANCH STARS

PresolarSiCgrainsofthemainstream,Y,andZtypearebelievedtocomefromcarbonstars.WecomparedtheirCand Si isotopicratios withtheoretical modelsfor theenvelopecompositions of AGB stars.Two setsof models (FRANEC and Monash) use a range of stellar masses (1.5–5M� ) and metallicities, different prescriptions for mass loss, and two sets of neutron-capture cross sections for the Si isotopes. They predict that the shifts in Si isotopic ratios and the increase of 12 C/ 13 C in the envelope during third dredge-up are higher for higher stellar mass, lower metallicity, and lower mass-loss rate. Because the 22 Ne neutron source dominates Si nucleosynthesis, the effect of the 13 C source is negligible. Comparison of the model predictions with grain data confirms an AGB origin for these grains, with Yand Z grains having originated in stars with lower than solar metallicity. The Si isotopic ratios of the Z grains favor the Si cross sections by Guber et al. over those by Bao et al. The 12 C/ 13 C ratios of low-metallicity models are much higher than those found in Z grains, and cool bottom processing must be invoked to explain the grains’ C isotopic ratios. By combining Z grain Si data with the models, we determined the evolution of the 29 Si/ 28 Si ratio in the Galaxy as function of metallicity Z .A tZ <0:01 this ratio rises much faster than current Galactic evolution models predict and suggests an early source of the heavy Si isotopes not considered in these models. Subject headingg dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — stars: carbon

Meteoritic Silicon Carbide Grains with Unusual Si-Isotopic Compositions: Evidence for an Origin in Low-Mass, Low-Metallicity Asymptotic Giant Branch Stars

We report isotopic data for 27 presolar SiC grains of the rare subtype Y in an acid-resistant residue of the Murchison (CM2) meteorite. Presolar SiC grains of type Y constitute only ~1% of Murchison SiC grains larger than ~2 μm and are defined as having 12C/13C > 100 (solar = 89) and 14N/15N > 272 (solar). In a Si 3-isotope plot, their Si isotopic compositions plot to the right of the correlation line defined by the majority of presolar SiC grains (the mainstream population), whose isotopic compositions indicate an origin in C-rich asymptotic giant branch (AGB) stars of near-solar metallicity. Because of their low abundance, the new Y grains were identified by automatic isotopic imaging of the 12C/13C ratio in the ion microprobe. We report C, N, and Si isotopic ratios of all 27 grains, inferred initial 26Al/27Al ratios of 18, and Ti isotopic ratios of 20 grains. Whereas 14N/15N and 26Al/27Al ratios exhibit the same range as mainstream grains, the C, Si, and Ti isotopic ratios are distinct. Carbon-12/carbon-13 ratios range up to 295 and 30Si/28Si excesses up to 183‰ relative to solar. The average 29Si/28Si ratio of Y grains is by 59‰ smaller than that of mainstream grains. Ti isotopic ratios relative to 48Ti are somewhat similar to those of mainstream grains, but extend to more extreme anomalous compositions. One grain has 46Ti/48Ti, 49Ti/48Ti, and 50Ti/48Ti excesses of 183‰, 365‰, and 990‰, respectively, relative to solar. These features exhibited by Y grains point to an origin in AGB stars of somewhat lower than solar metallicity. In the envelope of such stars the proportion of 12C and s-processed material dredged up from deep zones that experienced partial He burning and was mixed with original material is higher than in stars of solar metallicity. Their envelopes are therefore expected to have larger 12C/13C, 30Si/28Si, and 49Ti/48Ti and 50Ti/48Ti ratios than mainstream grains. We compare the C, Si, and Ti isotopic compositions of Y grains with the results of theoretical models of AGB stars with 1.5, 3, and 5 M☉ and Z = 0.006, 0.01, and 0.02. While solar-metallicity (Z = 0.02) AGB models cannot account for the Y grain data, the models with Z = 0.01 can reproduce the measured isotopic compositions reasonably well. A range of stellar masses (from 1.5 M☉ possibly up to 5 M☉) is indicated by the grain data. The present study together with additional data on SiC grains of type Z furthermore indicate that the rate of change of the ratios of the secondary Si isotopes (29Si and 30Si) relative to 28Si prior to solar system formation was lower than has been generally assumed, implying larger contributions of 28Si from Type Ia supernovae compared to those from Type II supernovae. The Si isotopic ratios of Galactic cosmic rays also suggest such an evolution.

Nucleosynthesis Predictions for Intermediate-Mass Asymptotic Giant Branch Stars: Comparison to Observations of Type I Planetary Nebulae

We model the nucleosynthesis during the thermal pulse phase of a rotating, solar metallicity, asymptotic giant branch (AGB) star of 3 M☉, which was evolved from a main-sequence model rotating with 250 km s-1 at the stellar equator. Rotationally induced mixing during the thermal pulses produces a layer (~2 × 10-5 M☉) on top of the CO core where large amounts of protons and 12C coexist. With a postprocessing nucleosynthesis and mixing code, we follow the abundance evolution in this layer, in particular that of the neutron source 13C and of the neutron poison 14N. In our AGB model mixing persists during the entire interpulse phase because of the steep angular velocity gradient at the core-envelope interface, thereby spreading 14N over the entire 13C-rich part of the layer. We follow the neutron production during the interpulse phase and find a resulting maximum neutron exposure of τmax = 0.04 mbarn-1, which is too small to produce any significant s-process. In parametric models, we then investigate the combined effects of diffusive overshooting from the convective envelope and rotationally induced mixing. Just adding the overshooting and leaving the rotational mixing unchanged results in a small maximum neutron exposure (0.03 mbarn-1). Models with overshoot and weaker interpulse mixing—as perhaps expected from more slowly rotating stars—yield larger neutron exposures. In a model with overshooting without any interpulse mixing a neutron exposure of up to 0.72 mbarn-1 is obtained, which is larger than required by observations. We conclude that the incorporation of rotationally induced mixing processes has important consequences for the production of heavy elements in AGB stars. While through a distribution of initial rotation rates, it may lead to a natural spread in the neutron exposures obtained in AGB stars of a given mass in general—as appears to be required by observations—it may moderate the large neutron exposures found in models with diffusive overshoot in particular. Our results suggest that both processes, diffusive overshoot and rotational mixing, may be required to obtain a consistent description of the s-process in AGB stars that fulfills all observational constraints. Finally, we find that mixing due to rotation within our current framework does increase the production of 15N in the partial mixing zone. However, this increase is not large enough to boost the production of fluorine to the level required by observations.