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

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 identifed 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 briefly address special issues relevant to the growth of dust within Type Ia interiors during their expansions

Growth of Carbon Grains in Supernova Ejecta

We present a chemical reaction network that describes the condensation chemistry of carbon dust grains in an expanding supernova shell. We assume that the region of interest consists solely of gaseous free carbon and oxygen atoms and that the buildup of CO is counteracted by the radioactive decay of 56Co, which breaks up the CO mol-ecule and allows C to condense into solids. Our chemical model takes C to first form linear chains, which, at some critical length, transition into ringed isomers. These isomers are more resistant to oxidation than linear chains. These ringed isomers form the nuclei for the growth of larger carbon solids. The effect of the disruption of CO on grain growth is displayed, leading to a rethinking of previous assumptions on the importance of CO disruption. How the abundance and size distribution of grains are affected by various parameters of the ejecta is also studied, providing insight into the possible sites of grain condensation

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 Cn molecules transition to ringed Cn molecules. We survey the dependence of the abundances of these molecules on the C/O ratio and on several other kinetic rate parameters. The concept of population control is significant 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

RyR1-targeted drug discovery pipeline integrating FRET-based high-throughput screening and human myofiber dynamic Ca2+ assays.

Elevated cytoplasmic [Ca2+] is characteristic in severe skeletal and cardiac myopathies, diabetes, and neurodegeneration, and partly results from increased Ca2+ leak from sarcoplasmic reticulum stores via dysregulated ryanodine receptor (RyR) channels. Consequently, RyR is recognized as a high-value target for drug discovery to treat such pathologies. Using a FRET-based high-throughput screening assay that we previously reported, we identified small-molecule compounds that modulate the skeletal muscle channel isoform (RyR1) interaction with calmodulin and FK506 binding protein 12.6. Two such compounds, chloroxine and myricetin, increase FRET and inhibit [3H]ryanodine binding to RyR1 at nanomolar Ca2+. Both compounds also decrease RyR1 Ca2+ leak in human skinned skeletal muscle fibers. Furthermore, we identified compound concentrations that reduced leak by > 50% but only slightly affected Ca2+ release in excitation-contraction coupling, which is essential for normal muscle contraction. This report demonstrates a pipeline that effectively filters small-molecule RyR1 modulators towards clinical relevance