K.-L. Kratz

The Chemical Composition and Age of the Metal-poor Halo Star BD +17°3248*

We have combined new high-resolution spectra obtained with the Hubble Space Telescope (HST )a nd ground-based facilities to make a comprehensive new abundance analysis of the metal-poor, halo star BD +17 � 3248. We have detected the third r-process peak elements osmium, platinum, and (for the first time in a metal-poor star) gold, elements whose abundances can only be reliably determined using HST. Our observations illustrate a pattern seen in other similar halo stars with the abundances of the heavier neutron capture elements, including the third r-process peak elements, consistent with a scaled solar system r-process distribution. The abundances of the lighter neutron capture elements, including germanium and silver, fall below that same scaled solar r-process curve, a result similar to that seen in the ultra–metal-poor star CS 22892-052. A single site with two regimes or sets of conditions, or perhaps two different sites for the lighter and heavier neutron capture elements, might explain the abundance pattern seen in this star. In addition, we have derived a

Charged-particle and neutron-capture processes in the high-entropy wind of core-collapse supernovae

The astrophysical site of the r-process is still uncertain, and a full exploration of the systematics of this process in terms of its dependence on nuclear properties from stability to the neutron drip-line within realistic stellar environments has still to be undertaken. Sufficiently high neutron-to-seed ratios can only be obtained either in very neutron-rich low-entropy environments or moderately neutron-rich high-entropy environments, related to neutron star mergers (or jets of neutron star matter) and the high-entropy wind of core-collapse supernova explosions. As chemical evolution models seem to disfavor neutron star mergers, we focus here on high-entropy environments characterized by entropy S, electron abundance Y-e, and expansion velocity V-exp. We investigate the termination point of charged-particle reactions, and we define a maximum entropy S-final for a given V-exp and Y-e, beyond which the seed production of heavy elements fails due to the very small matter density. We then investigate whether an r-process subsequent to the charged-particle freeze-out can in principle be understood on the basis of the classical approach, which assumes a chemical equilibrium between neutron captures and photodisintegrations, possibly followed by a beta-flow equilibrium. In particular, we illustrate how long such a chemical equilibrium approximation holds, how the freeze-out from such conditions affects the abundance pattern, and which role the late capture of neutrons originating from beta-delayed neutron emission can play. Furthermore, we analyze the impact of nuclear properties from different theoretical mass models on the final abundances after these late freeze-out phases and beta-decays back to stability. As only a superposition of astrophysical conditions can provide a good fit to the solar r-abundances, the question remains how such superpositions are attained, resulting in the apparently robust r-process pattern observed in low metallicity stars.

Heating in the Accreted Neutron Star Ocean: Implications for Superburst Ignition

We perform a self-consistent calculation of the thermal structure in the crust of a superbursting neutron star. In particular, we follow the nucleosynthetic evolution of an accreted fluid element from its deposition into the atmosphere down to a depth where the electron Fermi energy is 20 MeV. We include temperature-dependent continuum electron capture rates and realistic sources of heat loss by thermal neutrino emission from the crust and core. We show that, in contrast to previous calculations, electron captures to excited states and subsequent γ-emission significantly reduce the local heat loss due to weak-interaction neutrinos. Depending on the initial composition, these reactions release up to a factor of 10 times more heat at densities < 1011 g cm-3 than obtained previously. This heating reduces the ignition depth of superbursts. In particular, it reduces the discrepancy noted by Cumming et al. between the temperatures needed for unstable 12C ignition on timescales consistent with observations and the reduction in crust temperature from Cooper pair neutrino emission.

Nucleosynthesis in the early galaxy

Recent observations of r-process-enriched metal-poor star abundances reveal a nonuniform abundance pattern for elements -->Z ≤ 47. Based on noncorrelation trends between elemental abundances as a function of Eu richness in a large sample of metal-poor stars, it is shown that the mixing of a consistent and robust light element primary process (LEPP) and the r-process pattern found in r-II metal-poor stars explains such apparent nonuniformity. Furthermore, we derive the abundance pattern of the LEPP from observation and show that it is consistent with a missing component in the solar abundances when using a recent s-process model. As the astrophysical site of the LEPP is not known, we explore the possibility of a neutron-capture process within a site-independent approach. It is suggested that scenarios with neutron densities -->nn ≤ 1013 cm−3 or in the range -->nn ≥ 1024 cm−3 best explain the observations.

Half-life of the doubly magic r-process nucleus 78Ni.

Nuclei with magic numbers serve as important benchmarks in nuclear theory. In addition, neutron-rich nuclei play an important role in the astrophysical rapid neutron-capture process (r process). 78Ni is the only doubly magic nucleus that is also an important waiting point in the r process, and serves as a major bottleneck in the synthesis of heavier elements. The half-life of 78Ni has been experimentally deduced for the first time at the Coupled Cyclotron Facility of the National Superconducting Cyclotron Laboratory at Michigan State University, and was found to be 110(+100)(-60) ms. In the same experiment, a first half-life was deduced for 77Ni of 128(+27)(-33) ms, and more precise half-lives were deduced for 75Ni and 76Ni of 344(+20)(-24) ms and 238(+15)(-18) ms, respectively.

Decay of Neutron-Rich Mn Nuclides and Deformation of Heavy Fe Isotopes

The use of chemically selective laser ionization combined with {beta} -delayed neutron counting at CERN/ISOLDE has permitted identification and half-life measurements for 623-ms {sup 61}Mn up through 14-ms {sup 69}Mn . The measured half-lives are found to be significantly longer near N=40 than the values calculated with a quasiparticle random-phase-approximation shell model. Gamma-ray singles and coincidence spectroscopy has been performed for {sup 64,66}Mn decays to levels of {sup 64,66}Fe , revealing a significant drop in the energy of the first 2{sup +} state in these nuclides that suggests an unanticipated increase in collectivity near N=40 . {copyright} {ital 1999} {ital The American Physical Society }

Influence of shell-quenching far from stability on the astrophysical r-process

Abstract Comparison of results from r-process calculations within the waiting-point assumption and the r-process component ( N r ,⊙ ) of the solar-system composition of heavy elements, permits to test nuclear structure far from stability. Previous investigations, making use of nuclear mass predictions from global macroscopic-microscopic models, showed abundance deficiencies around A ∼- 120 and 140, indicating an overly strong N = 82 strength(some models also showed problems around A ∼- 180 related to the N = 126 shell). In this paper we calculate masses based on Skyrme interactions locally around N = 82, within the HF+BCS method with the SIII interaction and the HFB theory with SkP interaction. The shell-quenching obtained in the latter approach results in a considerable improvement of the global N r ,⊙ fit, indicating a solution to a puzzle existing in r-process nucleosynthesis.

THE END OF NUCLEOSYNTHESIS: PRODUCTION OF LEAD AND THORIUM IN THE EARLY GALAXY

We examine the Pb and Th abundances in 27 metal-poor stars (−3.1 56) enrichment was produced only by the rapid (r-) nucleosynthesis process. New abundances are derived from HubbleSpace Telescope/Space Telescope Imaging Spectrograph, Keck/High Resolution Echelle Spectrograph, and Very Large Telescope/UV–Visual Echelle Spectrograph spectra and combined with other measurements from the literature to form a more complete picture of nucleosynthesis of the heaviest elements produced in the r-process. In all cases, the abundance ratios among the rare earth elements and the third r-process peak elements considered (La, Eu, Er, Hf, and Ir) are constant and equivalent to the scaled solar system r-process abundance distribution. We compare the stellar observations with r-process calculations within the classical “waiting-point” approximation. In these computations a superposition of 15 weighted neutron-density components in the range 23 log nn 30 is fit to the r-process abundance peaks to successfully reproduce both the stable solar system isotopic distribution and the stable heavy element abundance pattern between Ba and U in low-metallicity stars. Under these astrophysical conditions, which are typical of the “main” r-process, we find very good agreement between the stellar Pb r-process abundances and those predicted by our model. For stars with anomalously high Th/Eu ratios (the so-called actinide boost), our observations demonstrate that any nucleosynthetic deviations from the main r-process affect—at most—only the elements beyond the third r-process peak, namely Pb, Th, and U. Our theoretical calculations also indicate that possible r-process abundance “losses” by nuclear fission are negligible for isotopes along the r-process path between Pb and the long-lived radioactive isotopes of Th and U.

Calculations of fission rates for r-process nucleosynthesis

Abstract Fission plays an important role in the r-process which is responsible not only for the yields of transuranium isotopes, but may have a strong influence on the formation of the majority of heavy nuclei due to fission recycling. We present calculations of beta-delayed and neutron-induced fission rates, taking into account different fission barriers predictions and mass formulae. It is shown that an increase of fission barriers results naturally in a reduction of fission rates, but that nevertheless fission leads to the termination of the r-process. Furthermore, it is discussed that the probability of triple fission could be high for A > 260 and have an effect on the formation of the abundances of heavy nuclei. Fission after beta-delayed neutron emission is discussed as well as different aspects of the influence of fission upon r-process calculations.

Nucleosynthesis Modes in The High-Entropy Wind of Type II Supernovae: Comparison of Calculations With Halo-Star Observations

While the high-entropy wind (HEW) of Type II supernovae remains one of the more promising sites for the rapid neutron-capture (r-) process, hydrodynamic simulations have yet to reproduce the astrophysical conditions under which the latter occurs. We have performed large-scale network calculations within an extended parameter range of the HEW, seeking to identify or to constrain the necessary conditions for a full reproduction of all r-process residuals N r,☉ = N ☉–N s,☉ by comparing the results with recent astronomical observations. A superposition of weighted entropy trajectories results in an excellent reproduction of the overall N r,☉ pattern beyond Sn. For the lighter elements, from the Fe group via Sr-Y-Zr to Ag, our HEW calculations indicate a transition from the need for clearly different sources (conditions/sites) to a possible co-production with r-process elements, provided a range of entropies are contributing. This explains recent halo-star observations of a clear noncorrelation of Zn and Ge and a weak correlation of Sr-Zr with heavier r-process elements. Moreover, new observational data on Ru and Pd also seem to confirm a partial correlation with Sr as well as the main r-process elements (e.g., Eu).