Nuclei in the Cosmos - IX

25 Jun 2006, 16:00 → 30 Jun 2006, 18:00 Europe/Zurich

CERN, Geneva

International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX

Sunday 25 June

Registration

Registration will commence at 17.00h and end at 20.00h. This
will take place in the Main Restaurant No 1 at CERN. We will
be offering one refreshment and you will be able to purchase
drinks from the bar. You will be given your conference bag
and envelope with your detailed information pack.

The CERN Restaurant will be open until 19.30h this evening and will be serving items from their grilled menu.

Monday 26 June

Introduction

Convener: Alberto Mengoni (IAEA, Vienna/CERN n_TOF)

slides welcome.ppt

1 Welcome

Speaker: Jos Engelen (CERN CSO)

2 In memory of Al Cameron

Speaker: John Cowan

Slides cowan1_nic.pdf cowan1_nic.ppt

3 In memory of John Bahcall and Ray Davis

Speaker: Peter Parker

Slides bahcall+davis.ppt

1 Stars: observations, evolution & nucleosynthesis

Convener: Michael Wiescher (U Notre Dame)

4 Nuclear astrophysics with gamma-ray line observations

Gamma-ray spectrometers with high spectral resolution are operating in space since 2002: RHESSI and SPI on INTEGRAL. Understanding the instrumental response and backgrounds is major effort but a prerequisite of detailed science interpretations. While 44Ti from core collapse supernovae could not be detected, this still adds constraints to Cas A 44Ti ejection. Diffuse nucleosynthesis is studied through 26Al, 60Fe, and positron annihilation gamma-ray measurements. With SPI on INTEGRAL, the gamma-ray line from decay of radioactive 26Al could be measured at unpredecented spectroscopic precision. This made possible a new determination of the total mass of 26Al produced by stellar sources throughout the Galaxy, and an analysis of the properties of the interstellar medium around 26Al sources. 60Fe is clearly detected with SPI, its intensity ratio to 26Al is confirmed to be on the lower side of theoretical predictions. Nucleosynthesis sources are probably minor contributors to Galactic positrons, as deduced from the bulge-centered spatial distribution of the annihilation gamma-ray emission.

Speaker: Roland Diehl (MPE Garching)

Slides NICIX_presentation_Diehl_06.pdf

5 From massive stars to supernovae

I will give a review on the evolution of massive stars and their paths to supernovae. Depending on their initial parameters, e.g., initial mass, metalicity, mass loss, and rotation, different kinds of supernovae are the result: normal core collapse supernovae, collapsar-powered supernovae like gamma-ray bursts, or even pair-instability supernovae. Similarly varied are the possible remnants of these supernovae and their nucleosynthesis. While the lowest mass stars may have only very little ejecta, the most massive pair-instability supernovae may eject as much as hundred solar masses in metals, out of which up to half can be radioactive nickel 56. I will present recent results of extended grids of nucleosynthesis of stars of low metallicity.

Speaker: Alexander Heger (Los Alamos/UC St Cruz)

6 The rp-process and X-ray bursts

Accreting neutron stars in X-ray binaries provide a unique laboratory for thermonuclear burning at extreme temperature and density conditions. A range of newly discovered, and largely puzzling, observables needs to be understood and interpreted. A key in this endeavor is the understanding of the underlying nuclear physics of unstable nuclei that span the entire range from the proton drip line to the neutron drip line. X-ray bursts and the rp-process play a central role in this context. I will discuss recent advances in our unserstanding of these systems from the observational, theoretical, and the experimental side. Concerning the latter, particular attention is payed to advances in the capabilities of radioactive beam facilities such as the National Superconducting Cyclotron laboratory at Michigan State University, where indirect methods have been developed to better constrain thermonuclear reaction rates at X-ray burst conditions.

Speaker: Hendrik Schatz (National Superconducting Cyclotron Laboratory, MSU)

11:00 break

2 Experiments in nuclear astrophysics I

Convener: Alan Shotter (TRIUMF)

7 Underground Nuclear Astrophysics

Cross section measurements for quiescent stellar H and He burning are hampered mainly by extremely low counting rate and cosmic background. Some of the main reactions of H-burning phase have been measured at the LUNA facility (Laboratory for Underground Nuclear Astrophysics) taking advantage of the very low background environment of the Underground Gran Sasso National Laboratory in Italy. An overview of the adopted experimental techniques will be given together with the latest results on the 14N(p,)15O reaction and the status of the ongoing experiments. Furthermore a brief summary of possible future experimental methods coupling low background environment and detector techniques will be presented.

Speaker: Heide Costantini (U Genova/Notre Dame U)

Slides NIC2006_6.pdf NIC2006_6.ppt

8 The 26gAl(p,g)27Si reaction in Novae

The strength of the 188 keV resonance in the 26gAl(p,g)27Si reaction has been measured directly in inverse kinematics using the DRAGON recoil separator at TRIUMF-ISAC. Radioactive 26Al beams with peak intensities of 5 x 10^9 ions/sec were utilised in conjunction with a windowless, recirculating hydrogen gas target. Recoil 27Si ions were separated and detected with a double-sided silicon strip detector in coincidence with capture gamma-rays at the target position in a highly efficient BGO detector array. Background from random coincidence was separated using time-of-flight through the length of the separator. Measured silicon charge state distributions using a 28Si beam, combined with stopping power information measured in the gas target allowed determination of the strength of this resonance at the level of <20% error. In addition, the resonance energy was measured via the distribution of gamma-ray hits in the BGO array, leading to the conclusion that it is lower than previously thought. The 188 keV resonance dominates the reaction rate at typical Oxygen-Neon Nova temperatures and the formation of 26Al in Novae depends sensitively on the value of this rate. We have found a value of resonance strength differing from the only existing unpublished measurement that has been used in the reaction networks so far in Nova nucleosynthesis models. The results of the experiment will help determine the importance of Novae as originators of Galactic 26Al compared to other sources, and we discuss the implications of the measurement in this context.

Speaker: C Ruiz (TRIUMF, Vancouver, BC V6T 2A3, Canada)

Slides Ruiz_NICIX_2006.pdf Ruiz_NICIX_2006.ppt

9 Direct measurement of the 18F(p,a)15O reaction for application to nova gamma-ray emission.

The 18F nucleus is one of the radioactive isotope produced during nova explosions. It is of particular interest since it is the main responsible for the 511 keV gamma-ray emission of novae that could be detected with the INTEGRAL satellite or future gamma-rays telescope. The amount of 18F synthesised still suffers from large uncertainties coming from missing nuclear information concerning the destruction reaction of 18F: 18F(p,alpha)15O. In particular, the interference sign between three 3/2+ resonances in 19Ne, situated slightly above the proton threshold (8 keV and/or 38 keV) and at higher energy (665 keV), is unknown. The maximum effect of these interferences is lying in the energy range corresponding to the Gamow peak region, having a strong impact on the 18F(p,alpha)15O reaction rate. We report here on the direct measurement at low energy (down to 400 keV in the center of mass) of the 18F(p,alpha)15O total cross section that we performed at the Louvain-la-Neuve CRC-RIB facility with the high intensity and purity 18F radioactive beam (T1/2 = 110 min). Total cross-section for the different incident energies will be presented and compared to previous experimental data, followed by a R-matrix analysis aiming at the determination of the interference sign of the relevant resonances.

Speaker: Nicolas de Sereville (Universite catholique de Louvain)

Slides NIC9a.pdf NIC9a.ppt

10 Measuring Difficult Reaction Rates Involving Radioactive Beams: A New Approach

Rates of sub-barrier, radiative capture reactions involving radioactive reactants, needed for understanding various astrophysics explosive scenarios, are often quite difficult to measure directly at relevant stellar temperatures. In general relatively intense radioactive beams (>1011/s) are needed for these inverse kinematic studies, as cross sections are very low. A new production approach is proposed herein that would supply such required intensities in a relatively straightforward fashion. While this system may have many applications, one area could be increasing our understanding of classical novae and X-ray bursts. 25Al (p,g), 30P(p,g), and 15O(a,g)19Ne are key reactions in our understanding of nova outbursts and/or X-ray bursts and their nucleosynthetic imprints in the Galactic abundances. The first one is important for the synthesis of 26Al, since it determines the amount of nuclear flow that bypasses 26Al synthesis through the isomeric state, 25Al(p,g)26Si(b+)26mAl. Hence, reliable predictions of the contribution of novae to the Galactic 26Al content depend critically on this rate. In turn, 30P(p,g) determines the path through the Si-Ca region in nova outbursts, and therefore, is crucial in the synthesis of these intermediate-mass elements, and for the location of the nucleosynthesis endpoint in such explosions. Moreover, competition between 30P(p,g) and 30P(b+) determines the final amount of 30Si, an important signature that helps to identify presolar meteoritic grains of a likely nova paternity. The 15O(a,g)19Ne reaction is believed to be the breakout path of the hot CNO cycle in an X-ray burst leading to the rp process. It is important to our understanding of ignition temperatures for the rp process to know the absolute value of this rate, rather than an upper limit. In this presentation a new approach to the production of required radioactive beam intensities is described which may lead to measurements of the rates of these key reaction.

Speaker: Mats Lindroos (CERN)

Slides Talk-2-4.pdf Talk-2-4.ppt

13:15 lunch

3 Nuclei far from stability

Convener: Karlheinz Langanke (GSI, Darmstadt)

11 Nuclear-physics data for modeling of the r-process

Nucleosynthesis theory predicts that about half of the chemical elements above iron are formed in explosive stellar scenarios by the r-process, i.e. a combination of rapid neutron captures, inverse photodisintegrations, and slower beta-decays, beta-delayed processes, as well as fission and possibly interactions with neutrinos. A correct modelling of this process, therefore, requires the knowledge of nuclear properties very far from stability and a detailed description of the astrophysical environments. With respect to nuclear data, after an initial period of measuring ''waiting-point'' nuclei with magic neutron numbers, recent investigations have paid special attention to shape transitions and the erosion of classical shell gaps with possible occurrence of new magic numbers. The status of experimental and theoretical nuclear data on masses and beta-decay properties will be briefly reviewed, and consequences on the overall r-process matter flow up to the cosmochronometers Th-232 and U-238 will be discussed.

Speaker: Karl-Ludwig Kratz (Institut für Kernchemie, Universität Mainz)

12 Progress in the investigation of nuclei approaching the r-process waiting point A=195

The complete understanding of the r-process still remains a challenge not only because of the identification of the possible astrophysical sites but also because of the interpretation of the observed abundances. With respect to this latter point, one of the main problems we have to overcome to fully understand the observed r-process abundances is the lack of information on the nuclei participating in this process, in particular for the heaviest ones. The main reason for this is that the heavy nuclei involved in the r-process are so neutron-rich that until now they have been far from any experimental access. During the last years promising results have been obtained investigating the properties of medium-mass neutron-rich nuclei close to the waiting point A=130 [1] while the waiting point around A=195 remains a completely unexplored territory. Nevertheless, the possibility to accelerate heavy ions at relativistic energies has allowed the investigation of reactions mechanisms leading to the production of heavy neutron-rich nuclei such as cold-fragmentation reactions [2]. In this work we report on an experiment performed with the FRS at GSI to explore the production of heavy neutron-rich nuclei close to the neutron shell N=126 and to measure their beta half-lives. We used cold-fragmentation reactions induced by a 208Pb beam at 1 AGeV impinging a Be target to produce heavy neutron-rich nuclei south of lead. The isotopic identification of the projectile residues was achieved by determining both the atomic number and the mass-over-charge ratio of each nucleus by measuring their magnetic rigidity, time-of-flight and energy loss. The identified nuclei were implanted in an active catcher made of four 5x5 cm2 Double- Side Silicon Strip Detectors 1 mm thick. The position and time correlations between the implanted nuclei and the subsequent beta decay allowed the determination of the beta half-lives. In this measurement we were able to identify for the first time around 30 new neutron-rich nuclei approaching the neutron shell N=126. In addition, the half-life of some of them has been determined. These half-lives have been compared with model calculations [3,4] which in general do not reproduce the measured values. This work opens new perspectives for further detailed spectroscopic investigations coupling with gammas that will allow us better understand the structure and decay properties of the A=195 waiting point nuclei. [1] Dillmann I., Kratz K.-L., et at Phys. Rev. Lett. 91, 162503 (2003) [2] Benlliure J., et al. Nucl. Phys. A 660, 87, (1999) [3] Tachibana T. et al. Proc. Int. Conf. on Exotic Nuclei and Masses, A 660, Arles, 763 (1995) [4] P. Möller et al. Atomic Data and Nuclear Data Tables, 66, 131 (1997)

Speaker: Teresa Kurtukian-Nieto (Universidad de Santiago de Compostela,Spain)

13 Building nuclei from the ground up

Investigations of rare isotopes in the laboratory are opening the way to understand and clarify the properties of all nuclei and bulk nuclear matter. In this talk I will assess where we stand today in solving the nuclear problem and how future rare isotope facilities will impact our understanding of nuclei and our ability to predict nuclear properties in stellar and other environments. The first part of the nuclear problem concerns our ability to describe complex nuclei from the ground up using as input the basic interactions among protons and neutrons. Indeed, our community is on the verge of discovering how light nuclear systems are built from bare nuclear interactions that have their roots in QCD. I will describe this exciting frontier of research through illustrating recent progress in the nuclear implementation of coupled-cluster methods, a quantum many-body technique that enjoys great success in quantum chemistry. After describing the basic coupled-cluster ideas, I will illustrate their power by reporting on results of ground- and excited state calculations for Oxygen and Calcium nuclei. This research is supported by the Office of Nuclear Physics, Office of Science of the U.S. Department of Energy under Contract Number DE-AC05-00OR22725 with UT-Battelle, LLC (Oak Ridge National Laboratory).

Speaker: Gaute Hagen (ORNL)

Paper hagen_nic9.pdf

14 Mass measurements of exotic nuclei and their role in stellar nucleosynthesis

The mass of nuclides far from stability provides information on decay and reaction energies that is crucial for modeling stellar nucleosynthesis. Low production rates, short half-lives, and the inherent precision required make masses perhaps the most difficult nuclear quantity to measure. The minuteness of the binding energy has also contributed to confounding attempts at reliable theoretical mass predictions. This talk will introduce the role played by masses in astrophysics and then quickly review and compare the (growing) multitude of mass-measurement programs now active worldwide. The evaluation process that links reaction, decay and binding energies is also described with its production of a benchmark for the development of the the mass models required by nucleosynthesis networks.

Speaker: David Lunney (Université de Paris Sud)

Slides lunney-nic9.pdf lunney-nic9.ppt

break + poster session 18

Poster sessions will be held outside the Main Auditorium, in the CERN Main building.

No oral presentation of poster contributions is schduled.

Poster session 18 List of contributions to poster session 18

4 Big-bang nucleosynthesis

Convener: Carmen Angulo (Université catholique de Louvain)

15 Recent results in Big-Bang Nucleosynthesis

Primordial nucleosynthesis (BBN) has been used for the determination of the baryonic density of the universe. It has now been superseded, for this purpose, by the more precise determination provided by the analysis of the CMB anisotropies by WMAP. Nevertheless, BBN is still very interesting as when we look back into the history of the universe, this is the last era for which, in principle, we know all the physics. Deviation from BBN predictions can hence give hints on non-standard Big Bang models. It is thus important that nuclear reactions involved in BBN be known with a good accuracy. Two recent nuclear physics experiments have improved the reliability of lithium calculated yields in standard big-bang nucleosynthesis. The cross section for the 7Be(d,p)2alpha reaction has been directly measured at BBN energies at Louvain-la-Neuve. A coulomb break-up experiment has provided a better determination of the D(alpha,gamma)6Li cross section over a wide energy range. Nevertheless, the discrepancy between the primordial 7Li abundance deduced from halo stars observations and BBN remains and the BBN 6Li production is still orders of magnitudes below the reported 6Li observations in some halo stars. Now that the baryonic density is accurately provided by the analysis of the CMB anisotropies, BBN can be used to constrain non-standard models: scalar-tensor theories of gravity for instance.

Speaker: Alain Coc (CSNSM,CNRS/IN2P3 and Universite Paris Sud)

16 Is Deuterium Cosmological?

All the astronomical observations of deuterium are consistent with a cosmological origin of D. Deuterium has been extensively studied because it is not produced via stellar nucleosynthesis and is thought to be primarily produced via the big-bang so its abundance will only decrease with time unless there are additional sources of D. The D/H ratio is an important prediction of standard and non-homogeneous big-bang models because the abundance of D depends critically on the temperature and baryonic density during the epoch of nucleosynthesis (first 1000 seconds). In homogeneous inflationary or other flat models, the D/H ratio gives the amount of dark matter and an upper limit to the number of neutrino families. Any Galactic source of D would undermine its use to estimate the baryonic density of the universe and place constraints on big-bang nucleosynthesis models. D nucleosynthesis models have included supernovae, supernovae shock-waves, cosmic-ray spallation reactions, accretion disks around neutron stars or black holes, gamma-ray photospallation reactions, stellar flares, and a large proton flux during an early active phase of the Galaxy as possible sources for deuterium. If D is produced via any stellar or Galactic nucleosynthesis process, then its abundance would be a maximum value in the Galactic Center (which is the most active and heavily processed region of the Galaxy). We review observations of deuterium in the Galaxy, external galaxies, active galaxies, and in quasar absorption systems, including our own observations. D has been detected in molecular clouds, diffuse clouds, HI regions, and HII region from observations of deuterated molecules, Lyman lines, Balmer lines, QSO absorption lines, and the DI 92-cm hyperfine-structure line. The Galactic D/H ratios range from 2 ppm in the Galactic Center to 23 ppm towards the anticenter (12 kpc from the GC). The QSO D/H ratios range from 20 – 30 ppm. Deuterium has not been detected in planetary nebulae, SNRs, or AGN. Because the D/H ratio is lowest value in the Galactic Center yet increases with distance from the Galactic Center, D is not produced via stellar or galactic activity (massive stars and star formation, cosmic rays, or stellar flares). Thus the observed D is cosmological with the observed D abundances reduced by astration, infall, mixing, and depletion onto grains.

Speaker: Donald Lubowich (Hofstra University, Hempstead, New York)

17 New measurement of the cross section of the Big Bang nucleosynthesis reaction D(a,g)6Li and its astrophysical impact

The recent observations of non-negligible amounts of 6Li in old halo stars [1] have renewed interest in the Big-Bang Nucleosynthesis (BBN) of 6Li. The deduced primordial 6Li abundance was found to be unexpectedly large compared to the BBN predictions. One important ingredient in the BBN predictions is the low-energy D(a,g)6Li cross section. Up to now, the only available experimental result [2] for this cross section introduced an error of about a factor of 20 in the 6Li abundance at the energies of astrophysical interest (Ecm<300 keV). This uncertainty arises from the discrepancy between the theoretical low energy dependence of the S-factor and the experimental data. Accordingly, new measurements of the cross section of the D(a,g)6Li reaction using Coulomb dissociation (CD) of 6Li at 150 A MeV have been performed recently at GSI. The preliminary GSI results, which indicate a drop of the S-factor as predicted by theory [3] will be presented as well their impact on the calculated 6Li abundance as a function of the baryon-to-photon ratio eta. [1] M. Asplund et al., astro-ph/0510636, Astrophys J. in press [2] J. Kiener et al., Phys. Rev. C 44, 2195 (1991) [3] A. Kharbach et al., Phys. Rev. C 58, 1066 (1998)

Speaker: Fairouz Hammache (IPN-Orsay)

Slides Hammache.pdf Hammache.ppt

Reception

Convener: Mats Lindroos (CERN)

Tuesday 27 June

5 Element production and stellar evolution: MP/UMP and Novae

Convener: André Maeder (Geneva Observatory)

18 r-Process Enhanced Metal-Poor Stars

Abundance observations indicate the presence of rapid-neutron capture (i.e., r-process) elements in old Galactic halo and globular cluster stars. These observations provide insight into the nature of the earliest generations of stars in the Galaxy -- the progenitors of the halo stars -- responsible for neutron-capture synthesis of the heavy elements. The large star-to-star scatter observed in the abundances of neutron-capture element/iron ratios at low metallicities -- which disappears with increasing metallicity or [Fe/H] -- suggests the formation of these heavy elements (presumably from certain types of supernovae) was rare in the early Galaxy. The stellar abundances also indicate a change from the r-process to the slow neutron capture ( i.e., s-) process at higher metallicities in the Galaxy and provide insight into Galactic chemical evolution. Finally, the detection of thorium and uranium in halo and globular cluster stars offers an independent age-dating technique that can put lower limits on the age of the Galaxy, and hence the Universe.

Speaker: John Cowan (University of Oklahoma)

Slides cowan2_nic.ppt

19 The First Nova Explosions

Classical nova outbursts are powered by thermonuclear runaways (hereafter, TNRs) that take place in the hydrogen-rich accreted envelopes of white dwarfs in close binary systems. Extensive numerical simulations of nova outbursts have shown that the accreted envelopes attain peak temperatures ranging between 100 and 400 MK for about several hundred seconds, and therefore, their ejecta is expected to show signatures of a significant nuclear activity, Indeed, it has been claimed that novae can play a certain role in the enrichment of the interstellar medium through a number of intermediate-mass elements. This includes 17O, 15N and 13C, systematically overproduced in huge amounts with respect to solar abundances, with a lower contribution in a number of other species with A < 40, such as 7Li, 19F, or 26Al. Estimates of the contribution of novae to the Galactic abundances usually rely on poorly known quantities, and implicitly assume that novae have been the same sort of objects during the whole Galaxy's history: that is, an explosion on a particular white dwarf, of a given mass and luminosity, is today similar to those contaminating the interstellar medium in the early epochs of the Galaxy. In this presentation, we analyse the first nova explosions and demonstrate that these objects were more important contributors to the Galactic abundances in the past.

Speaker: Jordi Jose (Institut d'Estudis Espacials de Catalunya/UPC)

Slides jjose.pdf jjose.ppt

20 Mass loss at very low metallicity: impacts on nucleosynthesis and GRB progenitors

Massive stars with no or very little amount of metals are generally considered as having very weak stellar winds. We reconsider here this question through stellar models accounting for the effects of axial rotation and show that rotating models of massive stars might lose a significant fraction of their initial mass through mass loss. The physical mechanisms triggering these mass losses and the chemical composition of the stellar winds will be discussed. Consequences of these models for nucleosynthesis and the nature of the GRB progenitors will be presented.

Speaker: Georges Meynet (Geneva Observatory)

Slides NICjuin06.ppt

21 Chemical Compositions Derived from Near Ultra-Violet Observations of Low-Metallicity Stars: New Insights into the Sites of Neutron-Capture Nucleosynthesis Processes

Numerous signatures of r- and s-process nucleosynthesis can be observed in the photospheres of stars. To date, however, the stellar abundance determinations of many of the elements have been scarce because a large portion of the dominant atomic transitions reside in the UV. To be reported in the context of other recent results of neutron-capture studies, are the abundances (or upper limits) of Nb, Pd, Ag, Yb, Hf, Pt, Pb, and other neutron-capture elements for a sample of stars observed with the near-UV sensitive detector on the High Resolution Echelle Spectrometer of the Keck I telescope.

Speaker: Inese Ivans (Carnegie Observatories & princeton university)

22 The Frequency of Carbon-Enhanced Stars in HERES and SDSS

Recent large surveys of metal-poor stars in the Galaxy have revealed that a surprising fraction of them are enhanced in their carbon-to-iron ratios by factors of from 10-10,000 relative to the solar ratio. Although most of the stars in the metallicity interval -2.7 < [Fe/H] < -2.0 are likely to have arisen from Asymptotic Giant Branch processing (and subsequent dumping via mass transfer to a surviving companion), there exist many stars with [Fe/H] < -3.0 (including the two lowest [Fe/H] stars known, with [Fe/H] < -5.0) that cannot be accounted for by this process. Rather, primordial (or nearly primordial) progenitors are implicated. We report on the existing information from present surveys, including cool giants from the recently completed HERES (Hamburg/ESO R-process Enhanced Star) survey, and from warm turnoff stars from SDSS-I. We will also describe the results that will come from the recently-funded extension of the SDSS, which includes the program SEGUE = Sloan Extension for Galactic Understanding and Exploration. SEGUE will identify some 20,000 stars with [Fe/H] < -2.0, several thousand of which are expected to be carbon enhanced. New carbon-enhanced models have been used in the analyses of these spectra, and we consider the impact of these models on the derived [Fe/H], [C/Fe], and in some cases, [N/Fe], that are derived.

Speaker: Timothy Beers (Michigan State University and JINA)

Paper beers.pdf

Slides beers_nic.ppt

break + poster session 19

Poster sessions will be held outside the Main Auditorium, in the CERN Main building.

No oral presentation of poster contributions is schduled.

Poster session 19 List of contributions to poster session 19

6 Evidence of nucleosynthesis in stars and presolar grains

Convener: Roberto Gallino (Universita' di Torino/INFN Torino)

23 Heavy elements in presolar grains: constraints on conditions in asymptotic giant branch stars

Presolar SiC grains come from a variety of kinds of stars, but the most common type, the mainstream grains, are believed to have formed in the outflows of low mass, carbon-rich asymptotic giant branch (AGB) stars. Measurements of the isotopic composition of the s-process elements Sr, Zr, Mo, Ru and Ba in individual mainstream SiC grains allow constraints of the range of conditions used in stellar models of AGB stars.

Speaker: Andrew Davis (University of Chicago)

Slides DavisNICIX.pdf DavisNICIX.ppt

24 On the stellar sources of presolar graphite in primitive meteorites

Primitive meteorites contain graphite spherules whose anomalous isotopic compositions indicate a stellar origin [1]. Because the isolation of presolar graphite grains is difficult, they have been less well studied than presolar SiC and presolar oxide grains. It has been known that the isotopic compositions of presolar graphite grains depends on their density, but detailed isotopic measurements have been made only on low-density (<2 g.cm-3) individual grains [2]. The NanoSIMS ion microprobe has enabled us to measure O and Si isotopic ratios in graphite grains with a range of densities from the carbonaceous chondrites Murchison and Orgueil [3-5]. These measurements confirm that low-density grains originated from supernovae and indicate that most high-density (>2 g.cm-3) grains come from C-rich AGB stars of low metallicity. Low-density grains and a few grains of higher density are characterized by large 18O and 28Si excesses that are signatures of Type II supernovae. Many high-density grains have high 12C/13C ratios (>300) and large excesses in 30Si and smaller ones in 29Si. These are best explained by low-metallicity AGB stars. In these stars the enrichments of the envelope in the heavy Si isotopes and in 12C, products of nucleosynthesis in the He shell, are much larger than those expected for solar-metallicity parent stars. The high 12C/13C ratios imply also high C/O ratios, which cause the preferential condensation of graphite grains over SiC grains. The Ne-E(L) component, almost pure 22Ne, which is characteristic of presolar graphite and led to its discovery [6], apparently has two sources [7]. In SN grains it is due to the decay of short-lived (T1/2 = 2.6 yr) 22Na, which condenses into the grains, whereas in AGB grains it is due to the abundant 22Ne in the He shell, produced by α-captures on 14N, the result of previous H burning in the CNO cycle. [1] Hoppe P. et al. (1995) Geochim. Cosmochim. Acta 59, 4029-4056. [2] Travaglio C. et al. (1999) Astrophys. J. 510, 325-354. [3] Amari S. et al. (2004) Meteorit. Planet. Sci. 39, A13. [4] Amari S. et al. (2005) Meteorit. Planet. Sci. 40, A15. [5] Jadhav M. et al. (2006) New Astron. Rev. submitted. [6] Amari S. et al. (1990) Nature 345, 238-240. [7] Amari S. (2006) New Astron. Rev. submitted.

Speaker: Ernst Zinner (Washington University)

Slides EZinner._NIC_IX.ppt

25 Isotopic Composition of Presolar Spinel Grain OC2: Constraining Intermediate-Mass Asymptotic Giant Branch Models

Presolar spinel (MgAl2O4) grains have been recently discovered in meteorites and represent the most abundant type of presolar oxides. The O, Mg and Al isotopic compositions of the vast majority of presolar oxide grains indicate that these grains originated in red giant and asymptotic giant branch (AGB) stars of masses lower than approximately 3 solar masses. Grain OC2 has a unique composition, showing most extreme O and Mg isotopic ratios among presolar oxide grains: O17/O16 three times higher than solar, O18/O16 26 times lower than solar, and excesses in Mg25 and Mg26 of (43+/-1)% and (117+/-1)%, respectively, with respect to solar. Its origin has thus been tentatively attributed to an AGB star of intermediate mass, around 5 solar masses. In intermediate-mass AGB stars the heavy Mg isotopes are produced in the He intershell by alpha-capture reactions on Ne22, while the O and Al compositions are mostly determined by proton captures at the base of the convective envelope (hot bottom burning). Using detailed models of AGB stars of different masses and metallicities that include Vassiliadis & Wood mass-loss rates and time-dependent convective mixing during the nucleosynthesis postprocessing, we analyse the O, Mg and Al compositions in AGB stars and discuss them in the light of the extremely precise measurements of the composition of grain OC2.

Speaker: Maria Lugaro (University of Utrecht)

Slides

• LugaroNIC-IX.pdf
• LugaroNIC-IX.pdf
• LugaroNIC-IX.ppt

26 Ne22 a primary source of neutron for the s-process and a major neutron poison in CEMP AGB stars

In AGB stars of low mass and very low metallicity, [Fe/H]<-2, a large abundance of C12 is mixed with the envelope by each third dredge up episode. The further activation of the H burning shell at the bottom of the envelope converts almost all CNO nuclei into N14. Thus the H-burning ashes contain N14 from the original CNO nuclei, plus an increasing amount of primary N14. During the subsequent convective thermal instability in the He shell, all N14 nuclides present in the He intershell are converted to Ne22 by double alpha capture on N14 during the early development of the thermal instability. At the peak temperature reached at the base of the thermal pulse, the Ne22(alpha,n)Mg25 reaction is partly activated, giving rise to an efficient neutron exposure feeding the s-process. At the same time, although the neutron capture cross section of Ne22 is very small (MACS(Ne22,30Kev)=0.059+-0.0057 mbarn, Beer et al. 1991), Ne22 acts as a major poison against the s-process. This poison effect is substantial also in case of addition of a C13-pocket with a range of neutron exposure efficiencies. Some fraction of primary O16 is also made in the thermal pulse by alpha capture on C12 (with mass fraction X(O16) = 0.04, while X(C12) = 0.20). Besides C12 and Ne22, a number of light isotopes are largely produced in a primary way, among which F19 (from neutron capture on O18, and other channels as well), Ne21, Na23, some Mg24, Mg25, Mg26. An effort should be devoted to the measurement of the MACS of all the light isotopes involved with improved accuracy, in order to better constrain both the s-process efficiency and the production of light isotopes in these stars.

Speaker: Roberto Gallino (University of Torino)

Slides gallinoneonnic9.ppt

27 Accelerator Mass Spectrometry and Nuclear Astrophysics

Accelerator Mass Spectrometry (AMS) is an extremely sensitive method for determination of rare ions. Besides ongoing applications in quite different scientific fields it is still a rather new tool in nuclear astrophysics. In this presentation I will show the reasons for this unique sensitivity and where are the limits: from the table top machine too a large facility. Applications in nuclear astrophysics range from direct determination of nuclear synthesis in the past until measurements of relevant cross sections. Examples of recent and also ongoing measurements will be discussed.

Speaker: Gunther Korschinek (Fachbereich Physik, Technische Universität München)

Slides NIC2006.ppt

13:30 lunch

7 Experiments in nuclear astrophysics: indirect methods

Convener: Claudio Spitaleri (Universita' di Catania/INFN Catania)

28 Indirect techniques in nuclear astrophysics - ANCs and THM

A puzzle in gamma ray astronomy has been the lack of a signal from the decay of 22Na in novae sites. The isotope should be produced in the Ne-Na cycle following the proton capture reaction 21Na(p,gamm)22Mg and then the beta decay of 22Mg to 22Na. A reaction that could play a role in understanding the lack of a signal gamma-ray astronomy is 22Mg(p,gamma)23Al. Depending on the stellar conditions, this reaction could reduce the amount of 22Na that occurs in a novae. Little is known about the reaction rate for proton capture on 22Mg. Furthermore there is a question about the spin-parity of the ground state of 23Al which can change the reaction rate by more than an order of magnitude depending on whether the ground state is 1/2+ or 5/2+. Two separate experiments have been carried out at the Texas A&M University Cyclotron Institute to better understand this reaction rate. We have obtained the asymptotic normalization coefficient for 22Mg+p to 23Al using the 13C(22Ne,23Ne)12C reaction and mirror symmetry. We also have determined the spin-parity for the ground state of 23Al by observing its beta decay to 23Mg. With this new information, we can now determine the direct capture rate for this reaction which fixes the stellar reaction rate and provides a basis for evaluating the importance of the capture reaction in depleting 22Na in novae.

Speaker: Robert Tribble (Texas A&M University)

Slides tribble.pdf tribble.ppt

29 Reaction rate of 15O(alpha,gamma)19Ne via indirect measurements

15O(alpha,gamma) is one of the main breakout reactions from the hot CNO cycles, which triggers the thermonuclear runaways or X-ray bursts occurring in accreting neutron stars. A recent study has shown that this reaction is critical for the burst amplitude and periodicity of X-ray bursters. However, a direct measurement of this reaction rate at astrophysically relevant temperatures is not feasible yet due to the lack of very high intensity radioactive 15O beams. There has been considerable effort in the past to investigate this reaction rate indirectly by obtaining gamma and alpha decay widths of the alpha-unbound states in 19Ne. While this approach has been successful for investigating higher energy resonances, the critical level at 4.03 MeV remains unknown. This leaves the reaction rate largely uncertain since previous attempts have only provided limits on its gamma width and its alpha decay branching ratio. In this talk we present new experimental work conducted at the University of Notre Dame. Lifetimes of the 4.03 MeV state and other relevant states in 19Ne have been measured successfully using the 17O(3He,n-gamma) reaction. We will also present the results of our recent measurement of the alpha-decay branching ratios. Alpha-unbound states in 19Ne were populated via the reaction 19F(3He,3H-alpha) and triton-alpha coincidences were observed using a low energy particle detection Silicon array and the TWINSOL facility. The experimental results and the astrophysical implications will be discussed in the presentation.

Speaker: Wanpeng Tan (University of Notre Dame)

Slides NIC-Wiescher.pdf NIC-Wiescher.ppt

30 Study of astrophysically important resonant states in 26Si by the 28Si(4He,6He)26Si reaction

The emission of 1.809 MeV gamma-ray from the first excited state of 26Mg followed by beta-decay of 26Al in its ground state (26Alg.s.) has been identified by gamma-ray telescopes such the Compton Gamma-Ray Observatory (CGRO). To resolve controversy over the possible sources of the observational 1.809 MeV gamma-rays, one needs accurate knowledge of the production rate of 26Al. The 25Al(p,gamma)26Si reaction which is the competition reaction for production of 26Alg.s. is one of the important subjects to be investigated. Illiadis et al. suggested that the 25Al(p,gamma)26Si reaction is dominated by the 3+ unnatural parity state under explosive Hydrogen burning conditions. Recent studies of 28Si(p,t)26Si, 24Si(3He,ngamma)26Si, and 29Si(3He,6He)26Si reduced the uncertainties in the 26Si levels above the proton threshold and identified new states as a candidate for the unnatural parity state. However, for the candidate, they could not make any spin assignment directly using the angular distribution measurement. In this work, the astrophysically important 26Si states were studied via the 28Si(4He,6He)26Si reaction which can excite unnatural parity state directly, which is contradictory to the (p,t) reaction that can not excite unnatural parity state. We have preformed an angular distribution measurement using the high resolution QDD spectrograph (PA) at Center for Nuclear Study (CNS), University of Tokyo. The experimental results and data analysis will be presented.

Speaker: Young Kwan KWON (Department of Physics, Chung-Ang University, Seoul 156-756, Republic of Korea)

Slides nic-9_kwon-1.ppt

31 Influences on the triple alpha process beyond the Hoyle state

The triple alpha process, the fusion of three alpha particles, is responsible for the main stellar production of 12C. It is known that the rate of this process is dominated by the 7.65 MeV resonance predicted by Hoyle and identified in 1953 [1]. At present two reaction rates are widely employed [2,3]. Work is ongoing to improve these rates both at the most important temperature range [4], and for temperatures where other natural parity resonances in 12C play a role [5] which is where the two existing rates actually differ. To clarify the latter, we present a detailed experimental analysis of the 0+ and 2+ strength in the triple alpha continuum above the Hoyle state and investigate their influence on the triple alpha process. Since the 0+ and 2+ states in question show a strong coupling to the triple alpha continuum, they are broad and not easily distinguished among the other states in this 12C energy region. Our approach is to selectively fed the states through the beta decay of 12N and 12B, and we detect the subsequent triple alpha breakup to identify the states. A new experiment (IG301) with improved detector setup has been performed at the IGISOL separator, Finland, to allow a detailed analysis of the properties of the states. We use a setup of segmented silicon detectors to measure triple alpha coincidences with a sufficient detection efficiency in most regions of three particle phase space. This has for example permitted us to observe previously unknown breakup channels involving the 8Be 2+ exited state. Since the 12C states overlap in energy and some have the same spin and parity they interfere. This is evident from the data and our analysis takes this into account. In this way we determine: properties of the states; their interference; and their coupling to the possible breakup channels 8Be(0+)+alpha and 8Be(2+)+alpha. With this knowledge we investigate the influence from these states and breakup channels on the reaction rate of the triple alpha process, and thus explore the influences on the triple alpha process beyond the Hoyle state. References: 1. F. Hoyle, Astrophysical Journal Supplement Series 1, 121 (1954) 2. G.R. Caughlan and W.A. Fowler, Atomic Data and Nuclear Data Tables 40, 283 (1988) 3. C. Angulo et al., Nuclear Physics A 656, 3 (1999) 4. S.M. Austin, Nuclear Physics A 758, 375c (2005) 5. H.O.U. Fynbo et al., Nature 433, 136 (2005)

Speaker: Christian Aa. Diget (Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark)

Slides Diget-NIC-IX-2006.pdf

32 Experimental determination of reaction rates via Coulomb dissociation

Intermediate-energy Coulomb dissociation using fast radioactive-isotope (RI) beams has been employed to study astrophysical (p,gamma) reactions. This method has an advantage of large cross sections and high experimental efficiency. Several experiments have been performed so far at fragmentation-based RI-beam facilities like, GSI, MSU and RIKEN. The most studied case is for the 7Be(p,gamma)8B reaction, which is important in estimating high-energy neutrino flux from the sun. Coulomb dissociation experiments provide an opportunity to determine the cross section independent to direct capture measurements. To extract accurate results, possible contributions of nuclear breakup, higher order process, possible mixture of different multiplicity contribution mixed into the dominant E1 contribution are being investigated both experimentally and theoretically.

Speaker: Tohru Motobayashi (RIKEN Nishina Center for Accelerator-Based Science)

Slides motobayashi_NIC9.pdf

16:30 break

8 Experiments in nuclear astrophysics II

Convener: Yasuki Nagai (Osaka University)

33 Beta decay of highly charged ions

Ion storage rings and ion traps have provided for the first time the opportunity to investigate beta decay of highly charged atoms, i.e. with only a few or even none bound electrons. The impact of this new field of research for nuclear astrophysics and, in particular, for s-process nucleosynthesis in hot stellar plasmas is obvious. In this talk an overview is given on the activities in this field during the last decade at the ion storage-cooler ring ESR of GSI, with the emphasis on bound-state beta decay and its astrophysical implications. Moreover, first results of a new technique, single-ion decay spectroscopy, will be presented and analyzed, i.e. the direct observation of two-body beta decays (bound-state beta decay and orbital electron capture) of single, stored and cooled ions at well-defined atomic charge states.

Speaker: Fritz Bosch (GSI Darmstadt, Germany)

34 Alpha-induced reactions in stellar burning

Alpha-induced reactions play an important role in a variety of astrophysical environments. They provide the neutron sources for the main s-process which takes place in highly convective AGB stars and for the weak process during core Helium burning in massive stars. In addition, alpha induced reactions on 15O and 18Ne provide a break-out from the CNO cycle which is important for the dynamics of explosive Hydrogen burning. To illuminate experimental difficulties in determinating reaction rates results of recent and ongoing experiments will be presented and future developments at the Nuclear Structure Laboratory at Notre Dame will be discussed.

Speaker: Joachim Goerres (University of Notre Dame and Joint Institute for Nuclear Astrophysics)

Slides NIC9_JG.pdf NIC9_JG.ppt

35 Measuring 12C(alpha,gamma)16O with ERNA

The fusion of carbon and helium via $^{12}$C$(\alpha,\gamma)^{16}$O in the helium burning phase of red giant stars is generally accepted to be a key reaction of nuclear astrophysics. Although there exist several direct and indirect measurements, the cross section in the Gamow peak is still not known sufficiently well. A new measurement of the $^{12}$C$(\alpha,\gamma)^{16}$O reaction cross section has been done using the European Recoil separator for Nuclear Astrophysics ERNA. \par In ERNA the reaction is performed in inverse kinematics, i.e. guiding a $^{12}$C beam on a $^{4}$He gas target. In the recoil separator the high intensity beam is filtered from the oxygen recoil nuclei using velocity and momentum filters. ERNA also needs to provide the necessary acceptances in angle and energy needed to cover the kinematics governed by the gamma ray emission. At the end of the separator the recoil nuclei are freely (i.e. without coincidence conditions) detected in a $\Delta$E-E ionisation chamber telescope. Assuming full acceptances for the choosen charge state one therefore measures the total cross section. Additional measurement of the coincident gamma rays will provide information on the different capture amplitudes.\par The key parameters of the recoil separator like acceptance of the recoil nuclei and suppression of the incoming beam are discussed. The results of the measurements of the total cross section of $^{12}$C$(\alpha,\gamma)^{16}$O in an energy range of 1.9-5 MeV are presented and compared to R-Matrix calculations.

Speaker: Daniel Schuermann (Ruhr-Universitaet Bochum)

Slides ERNA_nic2006.ppt

36 Measurement of the cascade cross section to the 6.049-MeV state in 16O in 12C(a,g)16O

The cascade through the 6.049-MeV J(pi)=0+ state 16O of has rarely been discussed as contributing to the 12C(a,g)16O cross section at low energies largely due to experimental difficulties in observing this transition. We report here first measurements of this transition in 12C(a,g)16O using the DRAGON recoil separator facility at TRIUMF. The experiment was performed in inverse kinematics with an incident 12C beam on a windowless 4He gas target, covering center of mass energies between 2.2 MeV and 5.42 MeV. The coincidence setup included a BGO array around the gas target and a DSSS Detector for the detection of 16O recoil particles at the focal plane of DRAGON. To derive actual cross sections, the acceptance of DRAGON including the BGO array has been simulated in GEANT. The transition strength has been derived and analyzed in the R-matrix formalism. Information on the 6.92-MeV cascade transition and the ground state transition were also obtained from the same data set. We derived the 12C(a,g)16O total cross section and found it in good agreement with a recently reported measurement

Speaker: C. Matei (Ohio University, Athens, OH)

37 The supernova-nucleosynthesis 40Ca(alpha,gamma)44Ti reaction

The 44Ti(t1/2= 59 y) nuclide is considered an important signature of core-collapse supernova (SN) nucleosynthesis and has recently been observed as live radioactivity by gamma-ray astronomy from the Cas A SN remnant. We investigated in the laboratory the major 44Ti production reaction, 40Ca(alpha,gamma)44Ti (Ecm ~0.6-1.2 MeV/u), by off-line counting of 44Ti nuclei using accelerator mass spectrometry [1]. The observed yield is significantly higher than inferred from previous prompt-gamma spectroscopy experiments. The present data strongly support the BRUSLIB statistical model [2] which incorporates a microscopic model of nuclear level densities and of the gamma-ray strength function, and a global alpha-nucleus optical-model potential. Comparison of the data with the statistical model confirms the strong suppression in yield expected for (alpha,gamma) reactions on self-conjugate (N=Z) nuclei. The derived astrophysical rate of the 40Ca(alpha,gamma)44Ti reaction is a factor 5-10 higher than calculated in current models. We will present results of stellar calculations in spherical hydrodynamics, as those described in [3] but using this reaction rate, showing an increase of the calculated SN 44Ti yield by a factor ~2 over current estimates. An increase by a factor of ~2 in 44Ti is found also in the calculated fall back material. The yields calculated by multi-dimensional SN explosion calculations proposed to explain the observed 44Ti yield of Cas A, in which parts of deeper layers can be ejected while some of the outer layers fall back, are expected to be enhanced in 44Ti as well. This work was supported in part by the US- DOE, Office of Nuclear Physics, under Contract No. W-31-109-ENG-38, the DOE Program for Scientific Discovery through Advanced Computing (SciDAC; DE-FC02-01ER41176), by DOE contract W-7405-ENG-36 to the Los Alamos National Laboratory, and by the USA-Israel Binational Science Foundation (BSF). [1] H. Nassar et al., Phys. Rev. Lett., to be published. [2] M.Arnould and S.Goriely, Nucl. Phys. A, to be published. [3] T.Rauscher et al., Astrophys. J., 576, 323 (2002).

Speaker: Hisham Nassar (Hebrew University, Jerusalem, Israel 91904)

38 Study of the 40Ca(alpha,gamma)44Ti reaction at stellar temperatures with DRAGON

44Ti (60.0 yr half-life) is one of the few short-lived radionuclides which has been detected in space by gamma-ray astronomy and thus confirm ongoing nucleosynthesis. Since it is produced predominantly in supernovae during the alpha-rich freezeout, its measured abundance can be used to constrain supernova models. The 40Ca(alpha,gamma)44Ti reaction plays a key role in 44Ti production. It has been studied partly in the past by prompt gamma-ray measurements. A recent integral measurement over a larger temperature regime by off-line counting of 44Ti nuclei with AMS showed a significantly larger 44Ti yield compared to previous results from prompt gamma-ray measurements. We have measured this reaction in inverse kinematics at the recoil mass spectrometer DRAGON, located at the ISAC facility at TRIUMF (Vancouver, Canada). High-purity 40Ca beam (less than 0.5% 40Ar contamination) was accelerated to energies of 0.8 – 1.2 MeV/amu impinging on a windowless He gas target surrounded by a high-efficiency BGO gamma-ray detector array. 44Ti recoils are then separated from the 40Ca beam by the recoil mass spectrometer and identified in an ionization chamber. The advantage of direct detection of 44Ti recoils and prompt gamma rays allows a detailed study of this reaction over a large energy range with sufficient resolution to resolve individual resonances. In this presentation, we report on the status of our investigations, which begins at the strong isospin triplet around E_x = 9.2 MeV and continues from here to lower energies covering a temperature regime of T_9 ~ 1.5 – 2.5 relevant for supernova nucleosynthesis.

Speaker: Christof Vockenhuber (TRIUMF, Vancouver, BC, Canada)

Slides 40CaDRAGON_3.ppt

Big Poster-Session [all posters]

All posters List of all poster contributions

poster 210_Takibay_NIC-IX.pdf

poster ID: 215 Prof. TAKIBAYEV, Nurgali "Neutrons and features of primordial nucleosynthesis"

Wednesday 28 June

9 Element production, stellar evolution, and stellar explosions

Convener: Verne V Smith (NOAO, Tucson)

39 New Ideas in the Theory of Core-Collapse Supernova Explosions

Core-collapse supernova explosions are fundamentally aspherical and require multi- dimensional radiation/hydrodynamical tools to address them. Recent simulations have hinted that the inner core of the protoneutron star executes vigorous g-mode oscillations that damp by the emission of acoustic power. I will present results from our recent 2D simulations that explore such core pulsations, the generation of sound, neutrino emissions, and explosion. The sound pulses radiated from the core steepen into shock waves that merge as they propagate into the outer mantle and deposit their energy and momentum with high efficiency. All models we address explode with the aid of such acoustic power, but what the ultimate role of sound may be in the supernova phenomenon remains to be seen. I will address the implications of the new simulations for the mechanism of supernova explosions, the r-process, pulsar kicks, supernova blast morphology, and the gravitational radiation signatures of the deaths of massive stars and I will provide a roadmap for future theoretical explorations to test, verify, or refute the new ideas that are emerging.

Speaker: Adam Burrows (University of Arizona)

40 The Role of Neutrinos in Explosive Nucleosynthesis

A new nucleosynthesis process, that we denote $\nu p$-process, will be presented. It occurs in supernovae (and possibly gamma-ray bursts) when strong neutrino fluxes create proton-rich ejecta. In this process, antineutrino absorptions in the proton-rich environment produce neutrons that are immediately captured by neutron-deficient nuclei. This allows for the nucleosynthesis of nuclei with mass numbers $A>64$. Making this process a possible candidate to explain the origin of the solar abundances of the light p-nulcei (such as $^{92,94}$Mo and $^{96,98}$Ru). This process also offers a natural explanation for the large abundance of Sr seen in an hyper-metal-poor star.

Speaker: Carla Frohlich (University of Basel)

41 Neutrinos and Nucleosynthesis in Gamma Ray Bursts

Gamma-ray bursts, while rare, may be important contributors to galactic nucleosynthesis. Here we consider the types of nucleosynthesis that can occur as material is ejected from a gamma-ray burst accretion disk. We calculate the composition of material within the disk as it dissociates into protons and neutrons and then use a parameterized outflow model to follow nuclear recombination in the wind. From the resulting nucleosynthesis we delineate the disk and outflow conditions in which iron peak, r-process, or light p-process nuclei may form. In all cases the neutrinos have an important impact on the final abundance distributions.

Speaker: Rebecca Surman (Union College)

Paper nic.pdf

Slides

• surman.pdf
• surman.pdf
• surman.pdf
• surman.ppt

42 Presupernova Evolution and Explosive Nucleosynthesis of Massive Stars

I will review the presupernova evolution and the associated explosive nucleosynthesis of massive stars with mass loss in the mass interval between 11 and 120 Msun. For the solar metallicity models i will address the following topics: 1) the evolutionary properties of Wolf-Rayet stars and their comparison with observations; 2) the chemical yields provided by a generation of core collapse supernovae; 3) the contribution of these stars to the production of Al26 and Fe60 in the Galaxy. For the zero metallicity models i will mainly disucuss their chemical yields and their connection with "normal" and C-rich extremely metal poor stars.

Speaker: Alessandro Chieffi (INAF-Osservatorio Astronomico di Roma)

Slides chieffi_NIC_IX.pdf chieffi_NIC_IX.ppt

break + poster session 20

Poster sessions will be held outside the Main Auditorium, in the CERN Main building.

No oral presentation of poster contributions is schduled.

Poster session 20 List of contributions to Poster session 20.

10 Element production & stellar evolution II

Convener: Robert Hoffman (LLNL, Livermore)

43 Globular clusters : Ideal laboratories to test hydrogen-burning nucleosynthesis and hydrodynamics in stars?

Galactic globular clusters (GC) stars exhibit abundance patterns which are not shared by their field counterparts, e.g. the well-documented 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 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 presently 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. In this talk we try to answer to the following question : "Are GC ideal laboratories to test hydrogen-burning nucleosynthesis and hydrodynamics in stars?" We critically analyse the pros and cons of both potential stellar polluters. We discuss the constraints that the observational data bring on the stellar nucleosynthesis and hydrodynamics as well as on nuclear reaction rates.

Speaker: Corinne Charbonnel (Geneva Observatory & CNRS)

44 Neutron-capture elements in globular cluster M15

We report on observations of six giants in the globular cluster M15 (NGC 7078). The Subaru/HDS was used to measure neutron-capture elemental abundances. Previous studies have reported a significant star-to-star variation in the neutron-capture elemental abundances of M15, and deduced that their origin was from the r-process. Our abundance analyses based on high-quality blue spectra confirm the scatter in the abundances of heavy neutron-capture elements (e.g., Eu). Observed [La/Eu] ratios indicate there are no significant s-process contributions. We have found, for the first time, that there are anti-correlations between the abundance ratios of light to heavy neutron-capture elements ([Y/Eu] and [Zr/Eu]) and heavy ones (e.g., Eu). This indicates that light neutron-capture elements in these stars cannot be explained by only a single r-process, but another process that has significantly contributed to the light neutron-capture elements is required to have occurred in M15. We will also discuss possible r-process enrichment model to explain our results.

Speaker: Kaori Otsuki (University of Chicago)

45 Chemical evolution of C-Zn and r-process elements produced by the first generation stars

Metal-poor stars record enrichment history of the Galaxy at the early epoch. Abundance analysis of these stars reveals large star-to-star scatters in r-process elements. This may be interpreted as a result of incomplete mixing of the interstellar medium (ISM) at the beginning of the Galaxy. However, recent studies also show considerable small dispersions for abundance ratios of C-Zn (Cayrel et al. 2004). We construct an inhomogeneous chemical evolution model, assuming supernova induced star formation. Then we discuss whether inhomogeneity of the ISM consistently accounts for observed differences between r-process and lighter elements, using several latest sets of supernova yields for metal-poor stars. If metal-poor stars are enriched by only one or a few supernovae, huge dispersions in r-process elements possibly imply that their yields are highly dependent on supernova progenitor masses. However, the site of r-process is still uncertain even from nucleosynthesis studies. We, then, attempt to determine the site of r-process, using an inhomogeneous chemical evolution model. In our previous study, we have shown that values of [Eu/Fe] of three metal poor stars given by Subaru observation strongly support the model where the r-process site is assumed as the low mass-end of supernova progenitors, such as 8-10 solar mass stars. On the other hand, a large dispersion has been found in [Sr/Ba] at lower metallicity (e.g., Ryan et al. 1996; Honda et al. 2004), suggesting that lighter elements such as Sr does not come from a universal process, which produces Ba and Eu, but from `weak' r-process. We show that this scenario well explains observations, when weak r-process produces ~60% of Sr but only ~1% of Ba in metal- poor stars. Intermediate mass elements between Sr and Ba must provide clues to understand the nucleosynthesis of weak r-process. We estimate Pd abundances of very metal-poor stars, using Subaru and also show that weak r-process pattern gradually decreases with atomic mass from Sr to Ba.

Speaker: Yuhri Ishimaru (Academic Support Center, Kogakuin University)

46 Reaction rate uncertainties and the operation of the NeNa and MgAl chains during HBB in intermediate-mass AGB stars

We study the effect of uncertainties in the proton-capture reaction rates on nucleosynthesis due to the operation of the NeNa and MgAl chains during hot bottom burning (HBB) in intermediate-mass asymptotic giant branch (AGB) stars. This kind of nucleosynthesis is associated with the production of sodium, of the radioactive nucleus Al26 and of the heavy magnesium isotopes, and it is possibly responsible for the O, Na, Mg and Al abundance anomalies observed in globular cluster stars. We model HBB with an analytic code based on full stellar evolution models. In this way we can calculate a very large number of stars in a relatively quick time (e.g. 10^6 stars in 12 hours). We have computed stellar models at two different metallicities (0.02 and 0.004) and two masses (5 and 6 solar masses). We vary in turn each of the p-capture rates involved in the NeNa and MgAl chains of factors corresponding to their current uncertainties in the range of temperatures relevant for HBB (T_9 ~ 0.06 - 0.1 K). We find large uncertainties, variations of up to one order of magnitude, in the final yields of Na23 and Ne22, Al26, Al27, and Mg24, because of uncertanties in the Ne22(p,g)Na23, Mg25(p,g)Al26, Al26(p,g)Si27, Mg26(p,g)Al27 and Na23(p,g)Mg24 reaction rates. The uncertainty ranges increase with decreasing metallicity and increasing mass of the star, because the temperature at the base of the convective envelope increases in these cases, making HBB more efficient. We are in the process of modelling stars of lower metallicity (0.0001), where the effect of HBB is even greater. Current work also involves the computation of models in which we vary all the reaction rates in the same model. We then calculate the variation of each isotopic yield due to the uncertainties associated with all the reaction rates involved in the NeNa and MgAl chains.

Speaker: Robert Izzard (University of Utrecht)

Slides Izzard_NIC_geneva_2006.pdf

47 The new solar chemical composition: Does the Sun have a sub-solar metallicity?

In the Sun, the convection zone reaches up to the solar atmosphere and can thus directly influence the emergent spectrum. Traditionally, the effects of convection has been modelled with the local mixing length theory in 1D hydrostatic model atmospheres for stars like the Sun. In a different approach, we have performed realistic time-dependent, 3D, radiative-hydrodynamical simulations of the outer layers of the solar convection zone, including the atmosphere. Both the different mean stratification and the presence of atmospheric inhomogeneities in 3D impact the spectral line formation. We have applied this 3D solar model atmosphere to the problem of the solar chemical composition while adopting the best possible atomic and molecular line data and taking into account departures from LTE in the line formation when necessary. The inferred C, N, O and Ne abundances are all significantly lower than estimated from previous 1D modelling by 0.2-0.3 dex. These results have significant implications for a range of topics in contemporary astrophysics, including causing a severe headache for helioseismology. In this review talk I will present an overview of our analysis, give arguments why our results are trustworthy and discuss some of their ramifications.

Speaker: Martin Asplund (Australian National University)

13:00 lunch

Excursions

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