R. D. Hoffman

The nu-process

As the core of a massive star collapses to form a neutron star, the flux of neutrinos in the overlying shells of heavy elements becomes so great that, despite the small cross section, substantial nuclear transmutation is induced. Neutrinos excite heavy elements and even helium to particle unbound levels. The evaporation of a single neutron or proton, and the back reaction of these nucleons on other species present, significantly alters the outcome of traditional nucleosynthesis calculations leading to a new process: nu-nucleosynthesis. Modifications to traditional hydrostatic and explosive varieties of helium, carbon, neon, oxygen, and silicon burning are considered. The results show that a large number of rare isotopes, including many of the odd-Z nuclei from boron through copper, owe much of their present abundance in nature to this process. 112 refs.

The alpha-process and the r-process

The paper explores the alpha-rich freeze-out for values of neutron excesses larger than previously treated, and reports the discovery of nuclear systematics that, for neutron excesses greater than about 0.05, allow the creation of heavier elements all the way up to A of about 100, even when most of the ejecta is in the form of heavy elements. It is found that a large part of the nuclear flow in the r-process may be carried by charged particle reactions up to about A of about 100. It is speculated that the site of these processes is the high-entropy wind of a young neutron star in which neutrino energy deposition is driving mass loss. The pass of r-process ejected per supernova is expected to be about 0.0001 solar mass per Type II or Ib supernova, but this is sensitive to details of the presupernova structure, the explosion mechanism, and the amount of material that falls back onto the neutron star when the reverse shock arrives there. 106 refs.

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.

r-process nucleosynthesis in the high-entropy supernova bubble

We show that the high-temperature, high-entropy evacuated region outside the recent neutron star in a core-collapse supernova may be an ideal r-process site. In this high-entropy environment it is possible that most nucleons are in the form of free neutrons or bound into alpha particles. Thus, there can be many neutrons per seed nucleus even though the material is not particularly neutron rich. The predicted amount of r-process material ejected per event from this environment agrees well with that required by simple galactic evolution arguments. When averaged over regions of different neutron excess in the supernova ejecta, the calculated r-process abundance curve can give a good representation of the solar-system r-process abundances as long as the entropy per baryon is sufficiently high. Neutrino irradiation may aid in smoothing the final abundance distribution. 60 refs.

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.

Nucleosynthesis in Early Supernova Winds II: The Role of Neutrinos

One of the outstanding unsolved riddles of nuclear astrophysics is the origin of the so-called p-process nuclei from A = 92 to 126. Both the lighter and heavier p-process nuclei are adequately produced in the neon and oxygen shells of ordinary Type II supernovae, but the origin of these intermediate isotopes, especially 92,94Mo and 96,98Ru, has long been mysterious. Here we explore the production of these nuclei in the neutrino-driven wind from a young neutron star. We consider such early times that the wind still contains a proton excess because the rates for νe and positron captures on neutrons are faster than those for the inverse captures on protons. Following a suggestion by Frohlich and coworkers, we also include the possibility that—in addition to the protons, α-particles, and heavy seed—a small flux of neutrons is maintained by the reaction p(e, e+)n. This flux of neutrons is critical in bridging the long waiting points along the path of the rp-process by (n, p) reactions. Using the unmodified ejecta histories from a recent two-dimensional supernova model by Janka and coworkers, we find synthesis of p-rich nuclei up to 102Pd, although our calculations do not show efficient production of 92Mo. If the entropy of these ejecta is increased by a factor of 2, the synthesis extends to 120Te. Still larger increases in entropy, which might reflect the role of magnetic fields or vibrational energy input neglected in the hydrodynamical model, result in the production of nuclei up to A ≈ 170. Elements synthesized in these more extreme outflows include numerous s- and p-process nuclei, and even some r-process nuclei can be synthesized in these proton-rich conditions.

Integrated Nucleosynthesis in Neutrino Driven Winds

Although they are but a small fraction of the mass ejected in core-collapse supernovae, neutrino-driven winds (NDWs) from nascent proto-neutron stars (PNSs) have the potential to contribute significantly to supernova nucleosynthesis. In previous works, the NDW has been implicated as a possible source of r-process and light p-process isotopes. In this paper we present time-dependent hydrodynamic calculations of nucleosynthesis in the NDW which include accurate weak interaction physics coupled to a full nuclear reaction network. Using two published models of PNS neutrino luminosities, we predict the contribution of the NDW to the integrated nucleosynthetic yield of the entire supernova. For the neutrino luminosity histories considered, no true r-process occurs in the most basic scenario. The wind driven from an older 1.4M{sub {circle_dot}} model for a PNS is moderately neutron-rich at late times however, and produces {sup 87}Rb, {sup 88}Sr, {sup 89}Y, and {sup 90}Zr in near solar proportions relative to oxygen. The wind from a more recently studied 1.27M{sub {circle_dot}} PNS is proton-rich throughout its entire evolution and does not contribute significantly to the abundance of any element. It thus seems very unlikely that the simplest model of the NDW can produce the r-process. At most, it contributes to the production of the N = 50 closed shell elements and some light p-nuclei. In doing so, it may have left a distinctive signature on the abundances in metal poor stars, but the results are sensitive to both uncertain models for the explosion and the masses of the neutron stars involved.

NUCLEOSYNTHESIS IN GAMMA-RAY BURST ACCRETION DISKS

We follow the nuclear reactions that occur in the accretion disks of stellar-mass black holes that are accret- ing at a very high rate, 0.01-1 Ms � 1 , as is realized in many current models for gamma-ray bursts (GRBs). The degree of neutronization in the disk is a sensitive function of the accretion rate, black hole mass, Kerr parameter, and disk viscosity. For high accretion rates and low viscosity, material arriving at the black hole will consist predominantly of neutrons. This degree of neutronization will have important implications for the dynamics of the GRB-producing jet and perhaps for the synthesis of the r-process. For lower accretion rates and high viscosity, as might be appropriate for the outer disk in the collapsar model, neutron-proton equality persists, allowing the possible synthesis of 56 Ni in the disk wind. 56 Ni must be present to make any optically bright Type I supernova and, in particular, those associated with GRBs. Subject headings: accretion, accretion disks — gamma rays: bursts — nuclear reactions, nucleosynthesis, abundances

An Inexpensive Nuclear Energy Generation Network for Stellar Hydrodynamics

We compare the nuclear energy generation rate and abundance levels given by an α-chain nuclear reaction network that contains only seven isotopes with a standard 13 isotope α-chain reaction network. The energy generation rate of these two small networks are also compared to the energy generation rate given by a 489 isotope reaction network with weak reactions turned on and off. The comparison between the seven isotope and α-chain reaction networks indicate the extent to which one can be replaced by the other, and the comparison with the 489 isotope reaction network roughly indicates under what physical conditions it is safe to use the seven isotope and α-chain reaction networks. The seven isotope reaction network reproduces the nuclear energy generation rate of the standard α-chain reaction network to within 30%, but often much better, during hydrostatic and explosive helium, carbon, and oxygen burning. It will also provide energy generation rates within 30% of an α-chain reaction network for silicon burning at densities less than 107 g cm-3. Provided there remains an equal number of protons and neutrons (Ye = 0.5) over the course of the evolution, and that flows through α-particle channels dominate, then both of the small reaction networks return energy generation rates that are compatible with the energy generation rate returned by the 489 reaction network. If Ye is significantly different from 0.5, or if there are substantial flows through neutron and protons channels, then it is not generally safe to employ any α-chain based reaction network. The relative accuracy of the seven isotope reaction network, combined with its reduction in the computational cost, suggest that it is a suitable replacement for α-chain reaction networks for parameter space surveys of a wide class of multidimensional stellar models.

Nucleosynthesis in Outflows from the Inner Regions of Collapsars

We consider nucleosynthesis in outflows originating from the inner regions of viscous accretion disks formed after the collapse of a rotating massive star. We show that windlike outflows driven by viscous and neutrino heating can efficiently synthesize Fe group elements moving at near-relativistic velocities. The mass of 56Ni synthesized and the asymptotic velocities attained in our calculations are in accord with those inferred from observations of SN 1998bw and SN 2003dh. These steady windlike outflows are generally proton-rich, characterized by only modest entropies, and consequently synthesize essentially nothing heavier than the Fe group elements. We also discuss bubble-like outflows resulting from rapid energy deposition in localized regions near or in the accretion disk. These intermittent ejecta emerge with low electron fraction and are a promising site for the synthesis of the A = 130 r-process peak elements.