Showing papers on "Stellar nucleosynthesis published in 2008"

Neutron-Capture Elements in the Early Galaxy

Abstract: The content of neutron-capture (trans-iron-peak) elements in the lowmetallicity Galactic halo varies widely from star to star. The differences are both in bulk amount of the neutron-capture elements with respect to lighter ones and in element-to-element ratios among themselves. Several well-defined abundance distributions have emerged that reveal characteristic rapid and slow neutron-capture nucleosynthesis patterns. In this review we summarize these observed metal-poor star’s abundances, contrasting them with the Solar-system values, comparing them to theoretical predictions, using them to assess the types of stars responsible for their specific anomalies, and speculating on the timing and nature of early Galactic nucleosynthesis.

An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays.

Abstract: Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis, accreting compact objects, and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk ( approximately 10-50 degrees from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10(41) positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.

The Effects of Variations in Nuclear Processes on Type I X-Ray Burst Nucleosynthesis

Abstract: Type I X-ray bursts are violent stellar events that take place in the H/He-rich envelopes of accreting neutron stars. We have investigated the role played by uncertainties in nuclear processes on the nucleosynthesis accompanying these explosive phenomena. Two different approaches have been adopted, in the framework of postprocessing calculations. In the first one, nuclear rates are varied individually within uncertainties. Ten different models, covering the characteristic parameter space for these stellar events, have been considered. The second, somewhat complementary approach involves a Monte Carlo code in which all nuclear rates are randomly varied within uncertainty limits simultaneously. All in all, about 50,000 postprocessing calculations, with a network containing 606 nuclides (H to 113Xe) and more than 3500 nuclear processes, have been performed in this work. A brief comparison between both procedures is outlined together with an overall account of the key nuclear reactions whose uncertainties have the largest impact in our X-ray burst nucleosynthesis studies.

Galactic interstellar 18 O/{^17}O ratios - a radial gradient?

Abstract: Context. The determination of interstellar abundances is essential for a better understanding of stellar nucleosynthesis and the “chemical” evolution of the Galaxy. Aims. The aim is to determine 18 O/ 17 O abundance ratios across the entire Galaxy. These provide a measure of the amount of enrichment by high-mass versus intermediate-mass stars. Methods. Such ratios, derived from the C 18 O and C 17 O $J = 1$–0 lines alone, may be affected by systematic errors. Therefore, the C 18 O and C 17 O (1–0), (2–1), and (3–2), as well as the 13 CO(1–0) and (2–1) lines, were observed towards 18 prominent galactic targets (a total of 25 positions). The combined dataset was analysed with a large velocity gradient model, accounting for optical depth effects. Results. The data cover galactocentric radii between 0.1 and 16.9 kpc (solar circle at 8.5 kpc). Near the centre of the Galaxy, 18 O/ 17 O = 2.88 ± 0.11. For the galactic disc out to a galactocentric distance of ~10 kpc, 18 O/ 17 O = 4.16 ± 0.09. At ~16.5 kpc from the galactic centre, 18 O/ 17 O = 5.03 ± 0.46. Assuming that 18 O is synthesised predominantly in high-mass stars ($M >8$ $M_{\odot}$), while C 17 O is mainly a product of lower mass stars, the ratio from the inner Galaxy indicates a dominance of CNO-hydrogen burning products that is also apparent in the carbon and nitrogen isotope ratios. The high 18 O/ 17 O value of the solar system (5.5) relative to that of the ambient interstellar medium suggests contamination by nearby high-mass stars during its formation. The outer Galaxy poses a fundamental problem. High values in the metal-poor environment of the outer Galaxy are not matched by the low values observed towards the even more metal-poor Large Magellanic Cloud. Apparently, the outer Galaxy cannot be considered as an intermediate environment between the solar neighbourhood and the interstellar medium of small metal-poor galaxies. The apparent 18 O/ 17 O gradient along the galactic disc and the discrepancy between outer disc and LMC isotope ratios may be explained by different ages of the respective stellar populations. More data from the central and far outer parts of the Galaxy are, however, needed to improve the statistical significance of our results.

Injection of Short-Lived Radionuclides into the Early Solar System from a Faint Supernova with Mixing Fallback

Abstract: Several short-lived radionuclides (SLRs), some of which should have formed just prior to or soon after the solar system formation, were present in the early solar system. Stellar nucleosynthesis has been proposed as the mechanism for the production of SLRs in the solar system, but no appropriate stellar source has been found that explains the abundances of all solar system SLRs. In this study, we propose a faint supernova with mixing and fallback as a stellar source of SLRs with mean lives of <5 Myr (26Al,41Ca,53Mn, and 60Fe) in the solar system. In such a supernova, the inner region of the exploding star experiences mixing, a small fraction of mixed materials is ejected, and the rest undergoes fallback onto the core. The modeled SLR abundances agree well with their solar system abundances if mixing fallback occurs within the C/O-burning layer. In some cases, the initial solar system abundances of the SLRs can be reproduced within a factor of 2. The dilution factor of supernova ejecta to the solar system materials is ~10−4, and the time interval between the supernova explosion and the formation of the oldest solid materials in the solar system is ~1 Myr. If the dilution occurred due to spherically symmetric expansion, a faint supernova should have occurred near the solar system-forming region in a star cluster.

Nucleosynthesis and Chemical Evolution of Oxygen

Abstract: Of the elements strictly synthesized in stars, oxygen is by far the most abundantly produced. We review the nucleosynthesis and galactic chemical evolution of this important element. We then review its isotopic composition in presolar grains recovered from primitive meteorites and from interplanetary dust particles. As we describe, knowledge of these isotopic compositions provide important constraints on theories of nucleosynthesis, stellar evolution, and galactic chemical evolution.

Nucleosynthesis in O-Ne-Mg Supernovae

Abstract: We have studied detailed nucleosynthesis in the shocked surface layers of an oxygen-neon-magnesium core collapse supernova with an eye to determining whether the conditions are suitable for r-process nucleosynthesis. We find no such conditions in an unmodified model, but do find overproduction of N=50 nuclei (previously seen in early neutron-rich neutrino winds) in amounts that, if ejected, would pose serious problems for Galactic chemical evolution.

Chemical evolution of the Milky Way and its Satellites

Abstract: This paper contains the lectures I delivered during the 37th Saas-Fee Advanced Course in March 2007. It reviews all the main ingredients necessary to build galactic chemical evolution models with particular attention to the Milky Way and the dwarf spheroidals of the Local Group. Both analytical and numerical models are discussed. Model results are compared to observations in order to infer constraints on stellar nucleosynthesis and on the formation and evolution of galaxies.Particular attention is devoted to interpret abundance ratios in galaxies with different star formation histories. Finally, the cosmic chemical evolution is discussed

Nuclear physics aspects of supernovae evolution and nucleosynthesis

Abstract: This paper reviews several aspects of supernova evolution and nucleosynthesis where nuclear physics input is relevant. These include the role of neutrino–nucleus inelastic scattering in supernova dynamics and the emitted supernova neutrino spectrum, nucleosynthesis in proton-rich supernova ejecta and fission in r-process nucleosynthesis.

The measurement of the Pb-206(n,gamma) cross section and stellar implications

Abstract: The neutron capture cross section of 206Pb has been measured at the CERN n\TOF spectrometer using a setup of two C6D6 detectors. In the energy interval from 1 eV to 600 keV the cross section is dominated by resonances, which were analyzed via the R-matrix analysis code SAMMY. In the relevant energy ranges for stellar nucleosynthesis, i.e., at thermal energies of kT = 8 keV and kT = 23 keV, the present Maxwellian average cross section differs by 20% and 9% from the recommended values of Bao et al respectively. From the new cross section the s-abundance of 206Pb could be reliably determined as 70(4)%. This result is of importance in order to test and constrain r-process abundance calculations in the actinide region, because the r-process portion of 206Pb is dominated by α-back decays of short-lived transbismuth isotopes.

Neutrino-induced nucleosynthesis in supernovae: synthesis of light elements and neutrino-driven r-process

Abstract: We report the results of our neutrino nucleosynthesis project that concern the neutrino-induced production of light and heavy elements in the helium shell of poor metal core collapse supernovae The outer half of the helium shell is shown to be a good site to obtain the neutrino-driven weak component of the r-process up to A ≈ 130 and simultaneously to produce significant yields of the light elements such as 7Li and 11B Special attention is given to an old idea that by means of the activation by shock heating reaction 13C(α, n)16O an enhanced amount of 13C in the helium shell could be an additional source of neutrons for the r-process The main conclusion is that both the neutrino and 13C neutron sources can contribute to the r-process only for poor metal stars, the latter being active for rather strong explosions of energy 3 × 1051 erg Contrary to the light elements, the r-process yields are strongly sensitive to the admixture of the neutron poisons such as 12C, 14N and 16O

Nucleosynthesis in stellar flares

Abstract: Nuclear interactions of ions accelerated at the surface of flaring stars can produce fresh isotopes in stellar atmospheres. Although this nucleosynthesis is not significant for the chemical evolution of the Galaxy, it can be important for a number of measurements of "anomalously" high 6-Li and 7-Li abundances. We discuss the possible role of stellar flares to explain the recent report of high 6-Li abundances in metal-poor halo stars and the well-established correlation between Li abundance and stellar activity in young open clusters. We then study the possibility of observing directly Li production during flares of nearby and active dwarfs of spectral type M.

Galactic globular clusters: Ideal laboratories to test stellar nucleosynthesis?

Abstract: Galactic globular cluster (GC) stars exhibit abundance patterns which are not shared by their field counterparts, namely the well-documented C-N, O-Na and Mg-Al anticorrelations Recent observations provided compelling evidence that these abundance anomalies were already present in the intracluster gas from which the presently observed stars formed A widely held hypothesis is that the gas was polluted early in the history of the GC by material processed through H-burning at high temperature and then lost by stars more massive than the observed long-lived stars However the “polluters" have not been unambiguously identified yet Most studies have focused on AGB stars, but rotating massive stars present an interesting alternative Here we critically analyse the pros and cons of both potential stellar polluters We discuss the constraints that the observational data bring on stellar nucleosynthesis and hydrodynamics as well as on nuclear reaction rates and we try to answer to the following question: “Are GC ideal laboratories to test stellar nucleosynthesis and hydrodynamics?"

Injection of Short-Lived Radionuclides into the Early Solar System from a Faint Supernova with Mixing-Fallback

Abstract: Several short-lived radionuclides (SLRs) were present in the early solar system, some of which should have formed just prior to or soon after the solar system formation. Stellar nucleosynthesis has been proposed as the mechanism for production of SLRs in the solar system, but no appropriate stellar source has been found to explain the abundances of all solar system SLRs. In this study, we propose a faint supernova with mixing and fallback as a stellar source of SLRs with mean lives of <5 Myr (26Al, 41Ca, 53Mn, and 60Fe) in the solar system. In such a supernova, the inner region of the exploding star experiences mixing, a small fraction of mixed materials is ejected, and the rest undergoes fallback onto the core. The modeled SLR abundances agree well with their solar system abundances if mixing-fallback occurs within the C/O-burning layer. In some cases, the initial solar system abundances of the SLRs can be reproduced within a factor of 2. The dilution factor of supernova ejecta to the solar system materials is ~10E-4 and the time interval between the supernova explosion and the formation of oldest solid materials in the solar system is ~1 Myr. If the dilution occurred due to spherically symmetric expansion, a faint supernova should have occurred nearby the solar system forming region in a star cluster.

Neutrino oscillation in supernova and grb nucleosynthesis

Abstract: Neutrinos play the critical roles in nucleosyntheses of light-to-heavy mass elements in core-collapse supernovae (SNe). The light element synthesis is affected strongly by neutrino oscillations (MSW effect) through the ν-process in outer layers of supernova explosions. Specifically the 7Li and 11B yields increase by factors of 1.9 and 1.3 respectively in the case of large mixing angle solution, normal mass hierarchy, and sin2 2θ13 = 2 × 10−3 compared to those without the oscillations. In the case of inverted mass hierarchy or nonadiabatic 13-mixing resonance, the increment of their yields is much smaller. We thus propose that precise constraint on mass hierarchy and sin2 2θ13 is given by future observations of Li/B ratio or Li abundance in stars and presolar grains which are made from supernova ejecta. Gamma ray burst (GRB) nucleosynthesis in contrast is not affected strongly by thermal neutrinos from the central core which culminates in black hole (BH), although the effect of neutrinos from proto-neutron star prior to black hole formation is still unknown. We calculate GRB nucleosynthesis by turning off the thermal neutrinos and find that the abundance pattern is totally different from ordinary SN nucleosynthesis which satisfies the universality to the solar abundance pattern.

Massive-star evolution at high metallicity

Abstract: After a review of the many effects of metallicity on the evolution of rotating and non-rotating stars, we discuss the consequences of a high metallicity on massive star populations and on stellar nucleosynthesis. The most striking effect of high metallicity is to enhance the amount of mass lost by stellar winds. Typically at a metallicity $Z=0.001$ only 9% of the total mass returned by non-rotating massive stars is ejected by winds (91% by supernovae explosion), while at solar metallicity this fraction may amount to more than 40%. High metallicity favors the formation of Wolf-Rayet stars and of type Ib supernovae. It however disfavors the occurrence of type Ic supernovae. We estimate empirical yields of carbon based on the observed population of WC stars in the solar neighborhood, and obtain that WC stars eject between 0.2 and 0.4% of the mass initially locked into stars under the form of new synthesized carbon. Models give values well in agreement with these empirical yields. Chemical evolution models indicate that such carbon yields may have important impacts on the abundance of carbon at high metallicity.

Planetary Nebulae, Tracers of Stellar Nucleosynthesis

Abstract: We review the information that planetary nebulae and their immediate progenitors, the post-AGB objects, can provide to probe the nucleosynthesis and mixing in low- and intermediate-mass stars. We emphasize new approaches based on high signal-to-noise spectroscopy of planetary nebulae and of their central stars. We mention some of the problems still to overcome. We emphasize that, as found by several authors, planetary nebulae in low-metallicity environments cannot be used to probe the oxygen abundance in the interstellar medium out of which their progenitors were formed, because of abundance modification during stellar evolution.

Nucleosynthesis in core collapse supernovae

Abstract: Core-collapse supernovae (CCSNe) are one of the most important nucleosynthesis sites and they hold a key role in the evolution of galaxies. In the explosion, CCSNe eject freshly synthesized iron-group nuclei from explosive burning alongside of intermediate mass elements (from hydrostatic and explosive burning), and carbon and oxygen from the pre-explosion evolution. In the neutrino-driven wind, nuclei beyond the iron group can be synthesized under neutron-rich conditions (weak r-process) and proton-rich conditions (\( u \)p-process). The signature of CCSN nucleosynthesis can be observed in the atmospheres of the oldest stars. Here, we will compare the nucleosynthesis from different progenitor models exploded with the PUSH method in spherical symmetry.

Nucleosynthesis Yields from the Explosion of Massive Stars

Abstract: Abstract Despite the complexity and uncertainties of core collapse supernova simulations there is a need to provide correct nucleosynthesis abundances for the progressing field of galactic evolution and observations of low metallicity stars. Especially the innermost ejecta are directly affected by the explosion mechanism, i.e. most strongly the yields of Fe-group nuclei for which an induced piston or thermal bomb treatment will not provide the correct yields because the effect of neutrino interactions is not included. Recent observations of metal-poor halo stars support the suggested existence of a lighter element primary process (LEPP) which operates very early in the galaxy and is independent of the r-process. We present a candidate for the LEPP, the so-called νp-process.

Nucleosynthesis in slowly evolving Cosmologies

Abstract: We explore aspects of Cosmological Nucleosynthesis in an FRW universe in which the scale factor evolves linearly with time: $a(t) \sim t$. A high Lepton number density during the period when significant nucleosynthesis takes place would lead to a dominant screening of the Coulomb potential of colliding nucleii. This would lead to a significant enhancement of nucleosynthesis rates. We demonstrate how adequate amount of $^4He$ and a collataral metallicity, close to the lowest metallicity observed in metal poor Pop II stars and clouds, can be produced with such an evolution.

Observing the Signatures of the r-Process in the Oldest Galactic Stars

Abstract: The abundance patterns of metal-poor stars provide us a wealth of chemical information about various stages of the chemical evolution of the Galaxy. In particular, these stars allow us to study the formation and evolution of the elements and the involved nucleosynthesis processes. This knowledge is invaluable for our understanding of the cosmic chemical evolution and the onset of star- and galaxy formation. Metal-poor stars are the local equivalent of the high-redshift Universe, and thus offer crucial observational constraint s on a variety of issues regarding the early Universe. This review presents an introduction to metal-poor stars and their role as “cosmic lab” for the study of neutron-capture nucleosynthesis processes, particularly that of the r-process. The metal-poor star HE 1523−0901 serves as an example for this group of objects. It displays in its spectrum the strongest overabundance of neutron-capture elements associated with the r-process. Heavy neutron-capture elements such as Eu, Os, and Ir were measured, as well as the radioactive elements Th and U. Abundance of Th and U, in conjunction with those of stable elements make possible nucleo-chronometry, i.e., the determination of s tellar ages. HE 1523−0901 appears to be ∼ 13 Gyr old. Age uncertainties range from 2 to 5 Gyr for individual chronometers, and are largly due to theoretical uncertainties in the initial production ratio of the employed chronometers. The decay product of the radioactive elements, lead, can be used to constrain r-process calculations. Only few such stars are currently known with detected U. These objects, however, are crucial for the study of this nucleosynthesis process. Once more objects are discovered, and assuming an old age for them (infered from their low metallicity), stars wit h measured Th and U abundances can become stellar age calibrators. This way, ages of stars in which only Th is measured (many more stars are available with a Th detection only), can be derived indepedently of model calculations.

Cross sections and rates of the thermonuclear reactions 64Zn(p,γ)65Ga and 66Zn(p,γ)67Ga

Abstract: The excitation functions for the reactions of radiative proton capture, 64Zn(p,γ)65Ga and 66Zn(p,γ)67Ga, which are of interest for stellar nucleosynthesis, have been measured in the range of incidentproton energies from 1 to 2.8 MeV. The astrophysical S factor and reaction rates have been derived from the reaction cross sections. The experimental results are compared with the predictions of the Hauser-Feshbach statistical theory.

Presolar SiC Grains and Rare Earth Element Production in AGB Stars

Abstract: The relative abundances of many isotopes of the rare earth elements are strongly affected by s‐process nucleosynthesis in low‐mass asymptotic giant branch (AGB) stars. We briefly explore the nucleosynthesis of the rare earth elements in AGB stars and their implications for measurements of presolar SiC grains from those stars.

Interpretation of Extremely Metal‐Poor Stars as Candidates of First Generation Stars

Abstract: The evolution of extremely metal‐poor (EMP) stars of low‐/intermediate‐masses is distinct from those of metal‐rich stars in that the convection driven by the helium shell flash can extend outward into the hydrogen‐rich layer during TP‐AGB phase. In the circumstance of [Fe/H]<−2.5, protons are mixed and converted into neutrons in the convective zone to promote nucleosynthesis through neutron and α‐captures. We study the nucleosynthesis in the helium‐flash convective zone, induced by this hydrogen mixing. In the dearth of the pristine metals, the neutron‐recycling reactions, 12C(n,γ)13C(α,n)16O, and in some cases, the subsequent 16O(n,γ)17O(α,n)20Ne, play an important role and catalyze the syntheses of O through Mg and still heavier elements. In particular, it is demonstrated that such peculiar abundance patterns of light elements from C through Al and heavy elements of Sr as observed from the two most iron‐deficient stars, HE0107‐5240 and HE1327‐2326, can well be reproduced in terms of the nucleosynthesis ...

Presupernova evolution and explosive nucleosynthesis in massive stars

Abstract: We investigate the presupernova evolution and the explosive nucleosynthesis in massive stars from 13 to 70 M⊙. The explosion energy is assumed to be 1–1.5×1051 erg. We found that the amount of 56Ni produced ranges from 0.07 to 0.15 M⊙. The ratio between the produced and solar abundance agrees with each other within factor 2–3 for A≤32. For the 12C(α,γ)16O rate, our results indicate that the higher rate in 1985 will be better than the lower one.

Complete nucleosynthesis calculations for low−mass stars from NuGrid

Abstract: Many nucleosynthesis and mixing processes of low−mass stars as they evolve from the Main Sequence to the thermal-pulse Asymptotic Giant Branch phase (TP−AGB) are well understood (although of course important physics components, e.g. rotation, magnetic fields, gravity wave mixing, remain poorly known). Nevertheless, in the last years presolar grain measurements with high resolution have presented new puzzling problems and strong constraints on nucleosynthesis processes in stars. The goal of the NuGrid collaboration is to pr esent uniform yields for a large range of masses and metallicities, including low−mass stars and massive stars and their explosions. Here we present the first calculations of stellar evolution a nd high−resolution, post−processing simulations of an AGB star with an initial mass of 2 M⊙ and solar−like metallicity (Z=0.01), based on the post−processing code PPN. In particular, we analyze the formation and evolution of the radiative 13 C−pocket between the 17th TP and the 18th TP. The s-process nucleosynthesis profile of a sample of heavy isotopes is also discussed, befor e the next convective TP occurrence.