The r-, s-, and p-Processes in Nucleosynthesis

A goal of this paper is to review the recent progress astrophysicists, astronomers, and physicists have made in the r-, s-, and p-processes in nucleosynthesis and to point out the problems that remain in our understanding of the formation of the heavy nuclei. Another, perhaps deeper, goal is to to seek some understanding of why there are three major processes available to nature for synthesis of heavy elements

Neutrino Capture and r-Process Nucleosynthesis

We explore neutrino capture during r-process nucleosynthesis in neutrino-driven ejecta from nascent neutron stars. We focus on the interplay between charged-current weak interactions and element synthesis, and we delineate the important role of equilibrium nuclear dynamics. During the period of coexistence of free nucleons and light and/or heavy nuclei, electron neutrino capture inhibits the r-process. At all stages, capture on free neutrons has a larger impact than capture on nuclei. However, neutrino capture on heavey nuclei by itself, if it is very strong, is also detrimental to the r-process until large nuclear equilibrium clusters break down and the classical neutron-capture phase of the r-process begins. The sensitivity of the r-process to neutrino irradiation means that neutrino-capture effects can strongly constrain the r-process site, neutrino physics, or both. These results apply also to r-process scenarios other than neutrino-heated winds.Comment: 20 pages, 17 figures, Submitted to Physical Review

Digraph Branchings and Matrix Determinants

We present a version of the matrix-tree theorem, which relates the determinant of a matrix to sums of weights of arborescences of its directed graph representation. Our treatment allows for non-zero column sums in the parent matrix by adding a root vertex to the usually considered matrix directed graph. We use our result to prove a version of the matrix-forest, or all-minors, theorem, which relates minors of the matrix to forests of arborescences of the matrix digraph. We then show that it is possible, when the source and target vertices of an arc are not strongly connected, to move the source of the arc in the matrix directed graph and leave the resulting matrix determinant unchanged, as long as the source and target vertices are not strongly connected after the move. This result enables graphical strategies for factoring matrix determinants

Iron Isotopic Diagnostics of Presolar Supernova Grains

The most thoroughly studied of the isotopi-cally anomalous presolar grains are silicon-carbide crystals (1). Both X-type SiC and low-density graphite grains condensed within the interiors of supernovae during their expansion and cooling (2). This paper concerns itself with iron in those su-pernova condensates (SUNOCONs), for which Fe anomalies was an early prediction (3). A major problem in SUNOCON interpretation has been that although the grains clearly rep-resent supernova interiors, it is not clear from which parcels of gas the grains condensed. The presence of 60-yr Ti in SUNOCONs as a major isotope of Ti demonstrates the prompt condensation of titanium, long before supernova ejecta have mixed with circumstellar matter, and even long before the reverse shock reheats the supernovae ejecta

Radioactive Chemical Kinetics of Large Supernova Dust

Supernovae are profoundly radioactive. Accord-ingly, we seek a new picture for radioactive regulation of the condensation of dust within their comoving, expanding, and cooling gaseous interiors. Such supernova condensates (SUNOCONs 1,2) from presolar galactic supernovae are recovered from meteorites (graphite, SiC) and are identified by predicted (3) excess 26Mg, 41K, and 44Ca from post-event decay of 26Al, 41Ca and 44Ti (4). Their isotopic compositions suggest mixed-shell material. This suggests two puzzles: 1.Why are the discovered SUNOCONs so large? 2.Where are the expected more numerous ones from unmixed super-nova shells? We propose that the large sizes and the miner-alized structures of SUNOCONs result from five controls: population control, cooling, admixed seed grains, catalysis, and entropic arrow. The constraints are driven by radioac-tivity and rapid decline of gas density, which maintains atomic abundances far from thermal equilibrium. Central to the carbide SUNOCONs is the CO molecule, whose disrup-tion by Compton electrons (5) maintains free carbon that enables graphite or SiC growth

Formation of Cn Molecules in Oxygen-Rich Interiors of Type II Supernovae

Two reaction-rate-based kinetic models for condensation of carbon dust via the growth of precursor linear carbon chains are currently under debate: the first involves the formation of C2 molecules via radiative association of free C atoms, and the second forms C2 molecules by the endoergic reaction CO + C → C2 + O.Bothare followedby C captures until the linear chain eventually makes an isomeric transition to ringed carbon on which rapid growth of graphite may occur. These two approaches give vastly different results. Because of the high importance of condensable carbon for current problems in astronomy, we study these competing claims with an intentionally limited reaction rate network which clearly shows that initiation by C + C → C2 + γ is the dominant pathway to carbon rings. We propose an explanation for why the second pathway is not nearly as effective as its proponents calculated it to be

Theory of Quasi-Equilibrium Nucelosynthesis and Applications to Matter Expanding from High Temperature and Density

Our first purpose is construction of a formal theory of quasi-equilibrium. We define quasi-equilibrium, in its simplest form, as statistical equilibrium in the face of an extra constraint on the nuclear populations. We show that the extra constraint introduces a uniform translation of the chemical potentials for the heavy nuclei and derive the abundances in terms of it. We then generalize this theory to accommodate any number of constraints. For nucleosynthesis, the most important constraint occurs when the total number of heavy nuclei Yh within a system of nuclei differs from the number that would exist in nuclear statistical equilibrium (NSE) under the same conditions of density and temperature. Three situations of high relevance are (1) silicon burning, wherein the total number of nuclei exceeds but asymptotically approaches the NSE number; (2) alpha-rich freezeout expansions of high entropy, wherein Yh is less than the NSE number; and (3) expansions from high temperature of low-entropy matter, in which Yh exceeds the NSE number. These are of importance, respectively, within (1) supernova shells, (2) Type II supernova cores modestly outside the mass cut, and (3) Type Ia supernova cores in near-Chandrasekhar-mass events

Heavy-Element Abundances from a Neutron Burst that Produces Xe-H

We examine quantitatively the suggestion that the heavy anomalous isotopes of Xe-HL found in meteoritic diamonds were produced by a short intense neutron burst and then implanted into the diamonds. Using a large nuclear reaction network we establish one (out of many) neutron irradiation histories that successfully reproduces the heavy isotopes of Xe-HL, and then evaluate what that same history would produce in every heavy element. This has become more relevant following recent measurement of anomalous Ba and Sr in those same diamond samples. Therefore we offer these calculations as a guide to the anomalies to be expected in all elements if this scenario is correct. We also discuss several other aspects of the problem, especially the established contradictions for Ba, the observed Kr pattern, the near normalcy of 129Xe, and some related astrophysical ideas. In particular we argue from p-process theory that the observed deficit of 78Kr in correlation with 124–126Xe excess implicates Type II supernovae as the diamond sources. However, our more complete astrophysical conclusions will be published elsewhere. This present work is offered as computational expectation for this class of models and as a guide to considerations that may accelerate the digestion of new experimental results in the diamonds

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 c line from the Cas A super-nova 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 adia-batic expansion, with peak temperature and density chosen to be 5.5]109 K and 107 gcm~3, respec-tively, to approximate the conditions in detailed supernova models. We Ðnd that, for equal total numbers of neutrons and protons (g\0), 44Ti production is most sensitive to the following reaction rates: 44Ti(a, p)47V, a(2a, c)12C, 44Ti(a, c)48Cr, and 45V(p, c)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 g[0, that of 45V(p, c)46Cr drops rapidly for gº0.0004. Other reactions assume greater significance at greater neutron excess: 12C(a, c)16O, 40Ca(a, c)44Ti, 27Al(a, n)30P, 30Si(a, n)33S. Because many of these rates are unknown experimentally, our results suggest the most important targets for future cross section measure-ments governing the value of this observable abundance