Bradley S. Meyer

The Physics of protoneutron star winds: implications for r-process nucleosynthesis

We solve the general-relativistic steady-state eigenvalue problem of neutrino-driven proto-neutron star winds, which immediately follow core-collapse supernova explosions. We provide velocity, density, temperature, and composition profiles and explore the systematics and structures generic to such a wind for a variety of proto-neutron star characteristics. Furthermore, we derive the entropy, dynamical timescale, and neutron-to-seed ratio in the general relativistic framework essential in assessing this site as a candidate for r-process nucleosynthesis. Generally, we find that for a given mass outflow rate (), the dynamical timescale of the wind is significantly shorter than previously thought. We argue against the existence or viability of a high entropy (300 per kB per baryon), long dynamical timescale r-process epoch. In support of this conclusion, we model the proto-neutron star cooling phase, calculate nucleosynthetic yields in our steady-state profiles, and estimate the integrated mass loss. We find that transonic winds enter a high-entropy phase only with very low (1 × 10-9 M☉ s-1) and extremely long dynamical timescale (τρ 0.5 s). Our results support the possible existence of an early r-process epoch at modest entropy (~150) and very short dynamical timescale, consistent in our calculations with a very massive or very compact proto-neutron star that contracts rapidly after the preceding supernova. We explore possible modifications to our models, which might yield significant r-process nucleosynthesis generically. Finally, we speculate on the effect of fallback and shocks on both the wind physics and nucleosynthesis. We find that a termination or reverse shock in the wind, but exterior to the wind sonic point, may have important nucleosynthetic consequences. The potential for the r-process in proto-neutron star winds remains an open question.

Evidence for a Late Supernova Injection of 60Fe into the Protoplanetary Disk

High-precision 60Fe-60Ni isotope data show that most meteorites originating from differentiated planetesimals that accreted within 1 million years of the solar systems formation have 60Ni/58Ni ratios that are ∼25 parts per million lower than samples from Earth, Mars, and chondrite parent bodies. This difference indicates that the oldest solar system planetesimals formed in the absence of 60Fe. Evidence for live 60Fe in younger objects suggests that 60Fe was injected into the protoplanetary disk ∼1 million years after solar system formation, when 26Al was already homogeneously distributed. Decoupling the first appearance of 26Al and 60Fe constrains the environment where the Suns formation could have taken place, indicating that it occurred in a dense stellar cluster in association with numerous massive stars.

Molybdenum and Zirconium Isotopes from a Supernova Neutron Burst

We analyze the nucleosynthesis implications of the recent discovery by M. J. Pellin and collaborators that two odd isotopes of molybdenum, 95Mo and 97Mo, are overabundant in type X SiC grains: X grains condensed within expanding supernova interiors. We find that a rapid release of neutrons (on a timescale of seconds) with fluence τ = 0.07-0.08 neutrons mbarn-1 produces the observed pattern by way of abundant production of progenitor radioactive Zr isotopes. This suggests that the condensing matter was in a supernova shell in which rapid burning was occurring at the time of ejection, probably owing to the passage of the shock wave from the core. Which shell, and the exact source of the neutrons, is still unknown, but we present a model based on the shock of an He shell.

Neutrino Capture and Supernova Nucleosynthesis

Author(s): Fuller, GM; Meyer, BS | Abstract: We investigate charged current neutrino and antineutrino capture on nuclei in the post-core bounce supernova environment. We point out that these processes may play an important, and heretofore overlooked, role in determining the nucleosynthesis in models of neutrino-heated supernova ejecta. In particular, we suggest that inclusion of these rates may help solve the problem in these models of overproduction of nuclides with neutron numbers near 50 and, in addition, enhance the production of the some of the light p-process nuclei in the α-process, particularly 92Mo. The neutrino capture rates on neutron-rich nuclei are found to be dominated by transitions to the Fermi (isobaric analog state) and Gamow-Teller resonances. In these cases, the neutrino capture thresholds are approximately just the nuclear Coulomb energy differences between nuclear parents and daughters, and the neutrino capture rates therefore exhibit only weak dependence on neutron and proton numbers compared to that of β--decay rates. We exploit this property to constrain the location of the r-process region in the post-core bounce supernova environment. We present analytic estimates for the rates of electron neutino and antineutrino capture on nuclei and nucleons.

S-process nucleosynthesis in advanced burning phases of massive stars

We present a detailed study of s-process nucleosynthesis in massive stars of solar-like initial composition and masses 15, 20, 25, and 30 M☉. We update our previous results of s-process nucleosynthesis during the core He burning of these stars and then focus on an analysis of the s-process under the physical conditions encountered during the shell carbon burning. We show that the recent compilation of the 22Ne(α,n)25Mg rate leads to a remarkable reduction of the efficiency of the s-process during core He burning. In particular, this rate leads to the lowest overproduction factor of 80Kr found to date during core He burning in massive stars. The s-process yields resulting from shell carbon burning turn out to be very sensitive to the structural evolution of the carbon shell. This structure is influenced by the mass fraction of 12C attained at the end of core helium burning, which in turn is mainly determined by the 12C(α,γ)16O reaction. The still-present uncertainty in the rate for this reaction implies that the s-process in massive stars is also subject to this uncertainty. We identify some isotopes like 70Zn and 87Rb as the signatures of the s-process during shell carbon burning in massive stars. In determining the relative contribution of our s-only stellar yields to the solar abundances, we find it is important to take into account the neutron exposure of shell carbon burning. When we analyze our yields with a Salpeter initial mass function, we find that massive stars contribute at least 40% to s-only nuclei with mass A ≤ 87. For s-only nuclei with mass A > 90, massive stars contribute on average ~7%, except for 152Gd, 187Os, and 198Hg, which contribute ~14%, ~13%, and ~11%, respectively.

Oxygen Isotopic Composition of the Sun and Mean Oxygen Isotopic Composition of the Protosolar Silicate Dust: Evidence from Refractory Inclusions

Preliminary analysis of the oxygen isotopic composition of the solar wind recorded by the Genesis spacecraft suggests that the Sun is 16O-rich compared to most chondrules, fine-grained chondrite matrices, and bulk compositions of chondrites, achondrites, and terrestrial planets (Δ17O = –26.5‰ ± 5.6‰ and –33‰ ± 8‰ (2σ) versus Δ17O ~ ±5‰). The inferred 16O-rich composition of the Sun is similar or slightly lighter than the 16O-rich compositions of amoeboid olivine aggregates and typical calcium-aluminum-rich inclusions (CAIs) from primitive (unmetamorphosed) chondrites (Δ17O = –24‰ ± 2‰), which are believed to have condensed from and been melted in a gas of approximately solar composition (dust/gas ratio ~ 0.01 by weight) within the first 0.1 Myr of the solar system formation. Based on solar system abundances, 26% of the solar system oxygen must be initially contained in dust and 74% in gas. Because solar oxygen is dominated by the gas component, these observations suggest that oxygen isotopic composition of the solar nebula gas was initially 16O-rich. Due to significant thermal processing of the protosolar molecular cloud silicate dust (primordial dust) in the solar nebula and its possible isotope exchange with the isotopically evolved solar nebula gas, the mean oxygen isotopic composition of the primordial dust is not known. In CO self-shielding models, it is assumed that primordial dust and solar nebula gas had initially identical, 16O-rich compositions, similar to that of the Sun (Δ17O ~ –25‰ or –35‰), and solids subsequently evolved toward the terrestrial value (Δ17O = 0). However, there is no clear evidence that the oxygen isotopic compositions of the solar system solids evolved in the direction of increasing Δ17O with time and no 16O-rich primordial dust have yet been discovered. Here we argue that the assumption of the CO self-shielding models that primordial dust and solar nebula gas had initially identical 16O-rich compositions is incorrect. We show that igneous CAIs with highly fractionated oxygen isotopic compositions, fractionation and unidentified nuclear effects (FUN), and fractionation (F) CAIs, have Δ17O ranging from –0.5‰ to –24.8‰. Within an individual FUN or F CAI, oxygen isotopic compositions of spinel, forsterite, and pyroxene define a mass-dependent fractionation trend with a constant Δ17O value. The degree of mass-dependent fractionation of these minerals correlates with the sequence of their crystallization from the host CAI melt. These observations and evaporation experiments on CAI-like melts indicate that FUN and F CAIs formed by melting of solid precursors with diverse Δ17O values in vacuum (total pressure 50☉) ejecta. The 16O-depleted compositions of chondrules, fine-grained matrices, chondrites, and achondrites compared to the Suns value reflect their formation in the protoplanetary disk regions with enhanced dust/gas ratio (up to 105× solar).

Condensation of Carbon in Radioactive Supernova Gas

The chemistry of carbon molecules leading to the formation of large carbon-bearing molecules and dust in the interior of an expanding supernova is explored and the equations governing their abundances are solved. A steady state between production and destruction is set up early and evolves adiabatically as the supernova evolves. Simple solutions for that steady state limit yield the abundance of each linear carbon molecule and its dependence on the C/O atomic ratio in the gas. Carbon dust condenses from initially gaseous C and O atoms because Compton electrons produced by the radioactivity cause dissociation of the CO molecules, which would otherwise form and limit the supply of C atoms. The resulting free C atoms enable carbon dust to grow faster by C association than its destruction by oxidation for various C/O ratios. Nucleation for graphite growth occurs when linear molecules transition to ringed C n molecules. We survey the dependence of the abundances of these molecules on the C/O ratio and on C n several other kinetic rate parameters. The concept of ii population control ˇˇ is signi—cant for the maximum sizes of carbon particles grown during supernova expansion. Interpretation of presolar micrometer-sized carbon solids found in meteorites and of infrared emission from supernova is relaxed to allow O to be more abundant than C, but the maximum grain size depends upon that ratio.

Nuclear Reactions Governing the Nucleosynthesis of 44Ti

Large excesses of 44Ca in certain presolar graphite and silicon carbide grains give strong evidence for 44Ti production in supernovae. Furthermore, recent detection of the 44Ti γ line from the Cas A supernova remnant by the Compton Gamma Ray Observatory Compton Telescope shows that radioactive 44Ti is produced in supernovae. These make the 44Ti abundance an observable diagnostic of supernovae. Through use of a nuclear reaction network, we have systematically varied reaction rates and groups of reaction rates to experimentally identify those that govern 44Ti abundance in core-collapse supernova nucleosynthesis. We survey the nuclear-rate dependence by repeated calculations of the identical adiabatic expansion, with peak temperature and density chosen to be 5.5 × 109 K and 107 g cm-3, respectively, to approximate the conditions in detailed supernova models. We find that, for equal total numbers of neutrons and protons (η = 0),44Ti production is most sensitive to the following reaction rates:44Ti(α, p)47V, α(2α, γ)12C,44Ti(α, γ)48Cr, and 45V(p, γ)46Cr. We tabulate the most sensitive reactions in order of their importance to the 44Ti production near the standard values of currently accepted reaction rates, at both a reduced reaction rate (times 0.01) and an increased reaction rate (times 100) relative to their standard values. Although most reactions retain their importance for η > 0, that of 45V(p, γ)46Cr drops rapidly for η ≥ 0.0004. Other reactions assume greater significance at greater neutron excess:12C(α, γ)16O,40Ca(α, γ)44Ti,27Al(α, n)30P,30Si(α, n)33S. Because many of these rates are unknown experimentally, our results suggest the most important targets for future cross section measurements governing the value of this observable abundance.

A New Study of s-Process Nucleosynthesis in Massive Stars

We present a comprehensive study of s-process nucleosynthesis in 15, 20, 25, and 30 M☉ stellar models having solar-like initial composition. The stars are evolved up to ignition of central neon with a 659 species network coupled to the stellar models. In this way, the initial composition from one burning phase to another is consistently determined, especially with respect to neutron capture reactions. The aim of our calculations is to gain a full account of the s-process yield from massive stars. In the present work, we describe our code in detail and then focus primarily on results from the s-process during central helium burning. We also illuminate some major uncertainties affecting the calculations. In order to motivate future work on burning in advanced stellar phases, we briefly present some results on the s-process in carbon shell burning; however, we leave a complete analysis of the s-process during the advanced evolutionary phases to a subsequent paper. Our results can help to constrain the yield of the s-process material from massive stars during their presupernova evolution.

Type X Silicon Carbide Presolar Grains: Type Ia Supernova Condensates?

In terms of nucleosynthesis issues alone, we demonstrate that the type X silicon carbide particles have chemical and isotopic compositions resembling those from explosive helium burning in 14N-rich matter. These particles are extracted chemically from meteorites and were once interstellar particles. They have already been identi-ed by their discoverers as supernova particles on the basis of their isotopic composi- tions, but we argue that they are from supernovae of Type Ia that explode with a cap of helium atop their CO structure. The relative abundances of the isotopes of C and Si and trace N, Mg, and Ca match those in the X particles without need of complicated and arbitrary mixing postulates. Furthermore, both C and Si abundances are enhanced and more abundant than O, which suggests that SiC is in fact the natural condensate of such matter. We also brieNy address special issues relevant to the growth of dust within Type Ia interiors during their expansions. Subject headings: dust, extinction E ISM: abundances E ISM: molecules E nuclear reactions, nucleosynthesis, abundances E supernovae: general