[Show abstract][Hide abstract] ABSTRACT: We here present calculations of dust condensation in core-collapse (Type
II) and thermonuclear (Type Ia) supernovae, to understand the production
of large dust grains in Type II supernovae and small dust grains in
Meteoritics and Planetary Science Supplement. 09/2013;
[Show abstract][Hide abstract] ABSTRACT: A common model of the explosion mechanism of Type Ia supernovae is based on a
delayed detonation of a white dwarf. A variety of models differ primarily in
the method by which the deflagration leads to a detonation. A common feature of
the models, however, is that all of them involve the propagation of the
detonation through a white dwarf that is either expanding or contracting, where
the stellar internal velocity profile depends on both time and space. In this
work, we investigate the effects of the pre-detonation stellar internal
velocity profile and the post-detonation velocity of expansion on the
production of alpha-particle nuclei, including Ni56, which are the primary
nuclei produced by the detonation wave. We perform one-dimensional hydrodynamic
simulations of the explosion phase of the white dwarf for center and off-center
detonations with five different stellar velocity profiles at the onset of the
detonation. We observe two distinct post-detonation expansion phases:
rarefaction and bulk expansion. Almost all the burning to Ni56 occurs only in
the rarefaction phase, and its expansion time scale is influenced by
pre-existing flow structure in the star, in particular by the pre-detonation
stellar velocity profile. We find that the mass fractions of the alpha-particle
nuclei, including Ni56, are tight functions of the empirical physical parameter
rho_up/v_down, where rho_up is the mass density immediately upstream of the
detonation wave front and v_down is the velocity of the flow immediately
downstream of the detonation wave front. We also find that v_down depends on
the pre-detonation flow velocity. We conclude that the properties of the
pre-existing flow, in particular the internal stellar velocity profile,
influence the final isotopic composition of burned matter produced by the
The Astrophysical Journal 06/2013; 771(1). · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We explore the sensitivity of helium burning production nitrogen-15 to
supernova energies and reaction rates and the importance for low-density
[Show abstract][Hide abstract] ABSTRACT: Production of nitrogen-15 in explosive helium burning occurs by sequences of alpha and neutron capture reactions. Shocks stronger than in current supernova models increase the rates for these reactions and may help explain the N isotopes in presolar-grain SiC-X grains.
[Show abstract][Hide abstract] ABSTRACT: 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.
The Astrophysical Journal 12/2008; 540(1):L49. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: According to standard models supernovae produce radioactive
44Ti, which should be visible in gamma-rays following decay
to 44Ca for a few centuries. 44Ti production is
believed to be the source of cosmic 44Ca, whose abundance is
well established. Yet, gamma-ray telescopes have not seen the expected
young remnants of core collapse events. The 44Ti mean life of
τ ≃ 89 y and the Galactic supernova rate of ≃3/100 y imply
≃several detectable 44Ti gamma-ray sources, but only one
is clearly seen, the 340-year-old Cas A SNR. Furthermore, supernovae
which produce much 44Ti are expected to occur primarily in
the inner part of the Galaxy, where young massive stars are most
abundant. Because the Galaxy is transparent to gamma-rays, this should
be the dominant location of expected gamma-ray sources. Yet the Cas A
SNR as the only one source is located far from the inner Galaxy (at
longitude 112°). We evaluate the surprising absence of detectable
supernovae from the past three centuries. We discuss whether our
understanding of SN explosions, their 44Ti yields, their
spatial distributions, and statistical arguments can be stretched so
that this apparent disagreement may be accommodated within reasonable
expectations, or if we have to revise some or all of the above aspects
to bring expectations in agreement with the observations. We conclude
that either core collapse supernovae have been improbably rare in the
Galaxy during the past few centuries, or 44Ti-producing
supernovae are atypical supernovae. We also present a new argument based
on 44Ca/40Ca ratios in mainstream SiC stardust
grains that may cast doubt on massive-He-cap type I supernovae as the
source of most galactic 44Ca.
Astronomy and Astrophysics 04/2006; 450:1037-1050. · 4.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have constructed a matrix solver for large-scale sparse matrices that arise from the treatment of nuclear burning in convective or advective environments in one dimension. We use this solver to compute nuclear abundances in a parameterized problem relevant for nucleosynthesis in supernova explosions. We discuss some details of our solutions, possible future extensions to higher dimensionality, and possible inclusion in supernova codes themselves.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that nucleosynthesis in rapid, high-entropy expansions of proton-rich matter from high temperature and density can result in a wider variety of abundance patterns than heretofore appreciated. In particular, such expansions can produce iron-group nuclides, p-process nuclei, or even heavy, neutron-rich isotopes. Such diversity arises because the nucleosynthesis enters a little explored regime in which the free nucleons are not in equilibrium with the abundant alpha particles. This allows nuclei significantly heavier than iron to form in t he presence of abundant free nucleons early in the expansion. As the temperature drops, nucleons increasingly assemble into alpha particles and heavier nuclei. If the assembly is efficient, the resulting depletion of free neutrons allows disintegrat ion flows to drive nuclei back down to iron and nickel. If this assembly is inefficient, then the large abundance of free nucleons prevents the disintegration flows and leaves a distribution of heavy nuclei after reaction freezeout. For cases in between, an intermediate abundance distribution, enriched in p-process isotopes, is frozen out. These last expansions may contribute to the solar system's supply of the p-process nuclides if mildly proton-rich, high-entropy matter is ejected from proto-neutron stars winds or other astrophysical sites. Also sign ificant is the fact that, because the nucleosynthesis is primary, the signature of this nucleosyn thesis may be evident in metal poor stars. Comment: 11 pages, 2 tables, 1 figure. Submitted to ApJ Letters
The Astrophysical Journal 06/2004; · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Although the exact site for the origin of the r-process isotopes remains mysterious, most thinking has centered on matter ejected from the cores of massive stars in core-collapse supernovae . In the 1970's and 1980's, however, difficulties in understanding the yields from such models led workers to consider the possibility of r-process nucleosynthesis farther out in the exploding star, in particular, in the helium burning shell [4,5]. The essential idea was that shock passage through this shell would heat and compress this material to the point that the reactions 13C(alpha; n)16O and, especially, 22Ne(alpha; n)25Mg would generate enough neutrons to capture on preexisting seed nuclei and drive an "n process" , which could reproduce the r-process abundances. Subsequent work showed that the required 13C and 22Ne abundances were too large compared to the amounts available in realistic models  and recent thinking has returned to supernova core material or matter ejected from neutron star-neutron star collisions as the more likely r-process sites.
[Show abstract][Hide abstract] ABSTRACT: The alpha-rich freezeout from equilibrium occurs during the core-collapse explosion of a massive star when the supernova shock wave passes through the Si-rich shell of the star. The nuclei are heated to high temperature and broken down into nucleons and alpha particles. These subsequently reassemble as the material expands and cools, thereby producing new heavy nuclei, including a number of important supernova observables. In this paper we introduce two web-based applications. The first displays the results of a reaction-rate sensitivity study of alpha-rich freezeout yields. The second allows the interested reader to run paramaterized explosive silicon burning calculations in which the user inputs his own parameters. These tools are intended to aid in the identification of nuclear reaction rates important for experimental study. We then analyze several iron-group isotopes (59Ni, 57Co, 56Co, and 55Fe) in terms of their roles as observables and examine the reaction rates that are important in their production. Comment: 20 pages, 2 figures
[Show abstract][Hide abstract] ABSTRACT: We study the abundance and isotopic composition of iron in a massive-star supernova to identify those isotopic characteristics that can identify the location of the condensing matter that is contained in the presolar supernova grains from meteorites. Additional information is contained in the original extended abstract.
[Show abstract][Hide abstract] ABSTRACT: r-Process yields can be extremely sensitive to expansion parameters when a persistent disequilibrium between free nucleons and alpha particles is present. This may provide a natural scenario for understanding the variation of heavy and light r-process isotopes in different r-process events. Additional information is contained in the original extended abstract.
[Show abstract][Hide abstract] ABSTRACT: The alpha-rich freezeout from nuclear statistical equilibrium occurs during type II (core-collapse) supernovae when the shock wave passes through the Si shell of the star. The nuclei are heated to high temperature and broken down into nucleons and alpha particles. These subsequently reassemble as the material expands and cools. The alpha-rich freezeout is responsible for a number of important supernova observables. In this paper we introduce a web-based tool that displays the results of a reaction-rate sensitivity study of alpha-rich freezeout yields. This tool permits the user to identify nuclear reactions that govern the synthesis of important observables from the alpha-rich freezeout. The tool is intended to aid in the identification of nuclear reaction rates important for measurement.
[Show abstract][Hide abstract] ABSTRACT: This paper evaluates the condensation of carbon solids in a gas of pure C and O atoms when these exist within the interior of an expanding young supernova. We calculate the abundances of large carbon molecules, which serve as nucleations for condensation of graphites. Additional information is contained in the original extended abstract.
[Show abstract][Hide abstract] ABSTRACT: We have constructed an interactive web site that may be of interest to cosmochemists seeking to understand the evolution of isotopes in the Galaxy. The URL is http://photon.phys.clemson.edu/gce.html. Additional information is contained in the original extended abstract.
[Show abstract][Hide abstract] ABSTRACT: No isotopic anomalies have yet been reported for K, but the relevant published literature is sparse and error limits for the scarce (0.01% abundance) isotope 40K are relatively large, 0.5 to 1%. We have developed thermal ionization mass spectrometric procedures by which error limits on 40K are an order of magnitude lower and applied them to analysis of a series of sequential dissolution fractions of the carbonaceous chondrites Orgueil (CI) and Murchison (CM), a sampling procedure known to reveal pervasive isotopic anomalies in Cr. Most of the fractions analyzed have 40K abundances that are normal (i.e., consistent with terrestrial composition) within analytical error limits. Whole–rock 40K abundances of Orgueil and Murchison are normal within about 1 permil or less. However, some dissolution fractions do exhibit evident isotopic anomalies, excesses of 40K up to about 35 ϵ. For K, as for Cr, the most plausible interpretation is that the anomalies reflect the presence of presolar grains that have not been thoroughly mixed with other solar system materials. In detail, the K anomalies do not correlate with the Cr anomalies, and thus probably represent different mineral carriers. Neither carrier phase is yet identified, but they differ from known and well-studied forms of presolar grains in that they are not acid-resistant. The isotopes of K are likely co-synthesized with some short-lived radionuclides, notably 26Al, the presence of which in the early solar system demands a “late” nucleosynthetic injection into the interstellar molecular cloud from which the solar system formed, no more than about 1 Ma before its collapse. It has been suggested that the distribution of 26Al (and other short-lived radionuclides) in early solar system materials was radically heterogeneous, perhaps because of the late injection. Because of its relatively short half-life (1.25 Ga), not in the “extinct radionuclides” range but still short compared to the age of the galaxy, 40K provides a usefully sensitive measure of the distribution of late nucleosynthetic additions to the solar system or its antecedent cloud. As a specific quantitative illustration, if a model 25-solar-mass supernova is invoked to account for observed levels of 26Al it will also provide about 1% of nebular 39K and about 3% of nebular 40K; the difference in the proportions of the K isotopes simply reflects the circumstance that by the time of solar system formation most of the 40K ever added to the sun’s precursor materials over the history of the galaxy had already decayed. There would be a 24‰ 40K anomaly between materials that did or did not incorporate such a contribution. The absence of so large a difference between the earth and the carbonaceous chondrites implies that they incorporate nearly the same amounts of any freshly synthesized K component of this magnitude. Unless some efficient mechanism for nebular scale chemical separation is postulated, the same should be true for co-synthesized nuclides such as 26Al.
Geochimica et Cosmochimica Acta 07/2000; · 4.25 Impact Factor