T. Rauscher

rp-process nucleosynthesis at extreme temperature and density conditions

We present nuclear reaction network calculations to investigate the influence of nuclear structure on the rp-process between Ge and Sn in various scenarios. Due to the lack of experimental data for neutron-deficient nuclei in this region, we discuss currently available model predictions for nuclear masses and deformations as well as methods of calculating reaction rates (Hauser-Feshbach) and beta-decay rates (QRPA and shell model). In addition, we apply a valence nucleon (NpNn) correlation scheme for the prediction of masses and deformations. We also describe the calculations of 2p-capture reactions, which had not been considered before in this mass region. We find that in X-ray bursts 2p-capture reactions accelerate the reaction flow into the Z greater than or equal to 36 region considerably. Therefore, the rp-process in most X-ray bursts does not end in the Z = 32-36 region as previously assumed and overproduction factors of 10(7)-10(8) are reached for some light p-nuclei in the A = 80-100 region. This might be of interest in respect of the yet unexplained large observed solar system abundances of these nuclei. Nuclei in this region can also be produced via the rp-proces in accretion disks around low mass black holes. Our results indicate that the rp-process energy production in the Z < 32 region cannot be neglected in these scenarios. We discuss in detail the influence of the various nuclear structure input parameters and their current uncertainties on these results. It turns out that rp-process nucleosynthesis is mainly determined by nuclear masses and beta-decay rates of nuclei along the proton drip line. We present a detailed list of nuclei for which mass or beta-decay rate measurements would be crucial to further constrain the models

End Point of therpProcess on Accreting Neutron Stars

We calculate the rapid proton (rp) capture process of hydrogen burning on the surface of an accreting neutron star with an updated reaction network that extends up to Xe, far beyond previous work. In both steady-state nuclear burning appropriate for rapidly accreting neutron stars (such as the magnetic polar caps of accreting x-ray pulsars) and unstable burning of type I x-ray bursts, we find that the rp process ends in a closed SnSbTe cycle. This prevents the synthesis of elements heavier than Te and has important consequences for x-ray burst profiles, the composition of accreting neutron stars, and potentially galactic nucleosynthesis of light p nuclei.

THE JINA REACLIB DATABASE: ITS RECENT UPDATES AND IMPACT ON TYPE-I X-RAY BURSTS

We present results from the JINA REACLIB project, an ongoing effort to maintain a current and accurate library of thermonuclear reaction rates for astrophysical applications. Ongoing updates are transparently documented and version tracked, and any set of rates is publicly available and can be downloaded via a Web interface at http://groups.nscl.msu.edu/jina/reaclib/db/. We discuss here our library V1.0, a snapshot of recommended rates for stable and explosive hydrogen and helium burning. We show that the updated reaction rates lead to modest but significant changes in full network, one-dimensional X-ray burst model calculations, compared with calculations with previously used reaction rate sets. The late time behavior of X-ray burst light curves shows significant changes, suggesting that the previously found small discrepancies between model calculations and observations may be solved with a better understanding of the nuclear input. Our X-ray burst model calculations are intended to serve as a benchmark for future model comparisons and sensitivity studies, as the complete underlying nuclear physics is fully documented and publicly available.

The astrophysical r-process : A comparison of calculations following adiabatic expansion with classical calculations based on neutron densities and temperatures

The rapid neutron-capture process (r-process) encounters unstable nuclei far from β-stability. Therefore its observable features, like the abundances, witness (still uncertain) nuclear structure as well as the conditions in the appropriate astrophysical environment. With the remaining lack of a full understanding of its astrophysical origin, parameterized calculations are still needed. We consider two approaches: (1) the classical approach is based on (constant) neutron number densities nn and temperatures T over duration timescales τ; (2) recent investigations, motivated by the neutrino wind scenario from hot neutron stars after a supernova explosion, followed the expansion of matter with initial entropies S and electron fractions Ye over expansion timescales τ. In the latter case the freezeout of reactions with declining temperatures and densities can be taken into account explicitly. We compare the similarities and differences between the two approaches with respect to resulting abundance features and their relation to solar r-process abundances, applying for the first time different nuclear mass models in entropy-based calculations. Special emphasis is given to the questions of (a) whether the same nuclear properties far from stability lead to similar abundance patterns and possible deficiencies in (1) and (2), and (b) whether some features can also provide clear constraints on the astrophysical conditions in terms of permitted entropies, Ye values, and expansion timescales in (2). This relates mostly to the A < 110 mass range, where a fit to solar r-abundances in high-entropy supernova scenarios seems to be hard to attain. Possible low-entropy alternatives are presented.

Models for Type I X-Ray Bursts with Improved Nuclear Physics

Multizone models of Type I X-ray bursts are presented that use an adaptive nuclear reaction network of unprecedented size, up to 1300 isotopes, for energy generation and include the most recent measurements and estimates of critical nuclear physics. Convection and radiation transport are included in calculations that carefully follow the changing composition in the accreted layer, both during the bursts themselves and in their ashes. Sequences of bursts, up to 15 in one case, are followed for two choices of accretion rate and metallicity, up to the point at which a limit cycle equilibrium is established. For (M)over dot=1.75x10(-9) M-circle dot yr(-1) (and (M)over dot=3.5x10(-10) M-circle dot yr(-1), for low metallicity), combined hydrogen-helium flashes occur. These bursts have light curves with slow rise times (seconds) and long tails. The rise times, shapes, and tails of these light curves are sensitive to the efficiency of nuclear burning at various waiting points along the rp-process path, and these sensitivities are explored. Each displays ``compositional inertia`` in that its properties are sensitive to the fact that accretion occurs onto the ashes of previous bursts that contain leftover hydrogen, helium, and CNO nuclei. This acts to reduce the sensitivity of burst properties to metallicity. Only the first anomalous burst in one model produces nuclei as heavy as A=100. For the present choice of nuclear physics and accretion rates, other bursts and models make chiefly nuclei with Aapproximate to64. The amount of carbon remaining after hydrogen-helium bursts is typically less than or similar to1 and decreases further as the ashes are periodically heated by subsequent bursts. For (M)over dot=3.5x10(-10) M-circle dot yr(-1) and solar metallicity, bursts are ignited in a hydrogen-free helium layer. At the base of this layer, up to 90 to carbon prior to the unstable ignition of the helium shell. These helium-ignited bursts have (1) briefer, brighter light curves with shorter tails, (2) very rapid rise times (>0.1 s), and (3) ashes lighter than the iron group.

Nuclear level density and the determination of thermonuclear rates for astrophysics

The prediction of cross sections for nuclei far off stability is crucial in the field of nuclear astrophysics. In recent calculations the nuclear level density{emdash}as an important ingredient to the statistical model (Hauser-Feshbach){emdash}has shown the highest uncertainties. We present a global parametrization of nuclear level densities within the back-shifted Fermi-gas formalism. Employment of an energy-dependent level density parameter a, based on microscopic corrections from a recent finite range droplet model mass formula, and a backshift {delta}, based on pairing and shell corrections, leads to a highly improved fit of level densities at the neutron-separation energy in the mass range 20{le}A{le}245. The importance of using proper microscopic corrections from mass formulas is emphasized. The resulting level description is well suited for astrophysical applications. The level density can also provide clues to the applicability of the statistical model which is only correct for a high density of excited states. Using the above description, one can derive a {open_quotes}map{close_quotes} for the applicability of the model to reactions of stable and unstable nuclei with neutral and charged particles. {copyright} {ital 1997} {ital The American Physical Society}

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.

Constraining the astrophysical origin of the p-nuclei through nuclear physics and meteoritic data

A small number of naturally occurring, proton-rich nuclides (the p-nuclei) cannot be made in the s- and r-processes. Their origin is not well understood. Massive stars can produce p-nuclei through photodisintegration of pre-existing intermediate and heavy nuclei. This so-called γ-process requires high stellar plasma temperatures and occurs mainly in explosive O/Ne burning during a core-collapse supernova. Although the γ-process in massive stars has been successful in producing a large range of p-nuclei, significant deficiencies remain. An increasing number of processes and sites has been studied in recent years in search of viable alternatives replacing or supplementing the massive star models. A large number of unstable nuclei, however, with only theoretically predicted reaction rates are included in the reaction network and thus the nuclear input may also bear considerable uncertainties. The current status of astrophysical models, nuclear input and observational constraints is reviewed. After an overview of currently discussed models, the focus is on the possibility to better constrain those models through different means. Meteoritic data not only provide the actual isotopic abundances of the p-nuclei but can also put constraints on the possible contribution of proton-rich nucleosynthesis. The main part of the review focuses on the nuclear uncertainties involved in the determination of the astrophysical reaction rates required for the extended reaction networks used in nucleosynthesis studies. Experimental approaches are discussed together with their necessary connection to theory, which is especially pronounced for reactions with intermediate and heavy nuclei in explosive nuclear burning, even close to stability.

IRON 60 EVIDENCE FOR EARLY INJECTION AND EFFICIENT MIXING OF STELLAR DEBRIS IN THE PROTOSOLAR NEBULA

Among extinct radioactivities present in meteorites, 60 Fe (t1/2 = 1.49 Myr) plays a key role as a high-resolution chronometer, a heat source in planetesimals, and a fingerprint of the astrophysical setting of solar system formation. A critical issue with 60 Fe is that it could have been heterogeneously distributed in the protoplanetary disk, calling into question the efficiency of mixing in the solar nebula or the timing of 60 Fe injection relative to planetesimal formation. If this were the case, one would expect meteorites that did not incorporate 60 Fe (either because of late injection or incomplete mixing) to show 60 Ni deficits (from lack of 60 Fe decay) and collateral effects on other neutron-rich isotopes of Fe and Ni (coproduced with 60 Fe in core-collapse supernovae and AGB-stars). Here, we show that measured iron meteorites and chondrites have Fe and Ni isotopic compositions identical to Earth. This demonstrates that 60 Fe must have been injected into the protosolar nebula and mixed to less than 10 % heterogeneity before formation of planetary bodies. Subject headings: solar system: formation — nuclear reactions, nucleosynthesis, abundances — methods: analytical — supernovae: general

The endpoint of the rp-process on accreting neutron stars

We have studied the evolution of the shell structure of nuclei near the neutron drip line in the Relativistic Hartree-Bogoliubov (RHB) theory with the vector self-coupling of omega meson. The experimental data on the shell effects about the waiting-point nucleus