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Clayton, D. D.

Publications and source records attributed to Clayton, D. D..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Sudden grain nucleation and growth in supernova and nova ejecta

The thermal conditions leading to the rapid nucleation and growth of dust in astronomical explosions are examined. The contribution to nucleation physics lies only in the clarification of the ambient conditions where it apparently occurs. In both nova and interior shells of supernovae, dust precipitates in gas densities of the order of 10 to the -14th g/cu cm a few months after the explosion. The ambient conditions differ widely, however. Supernova condensation occurs in a thermal equilibrium, with photons, ions, electrons, and grains having equal temperatures. In novae huge disequilibria exist, with photon and electron temperatures near 50,000 K, photon energy density near 750 K, and forming refractory grains near 1800 K. In neither type of explosion can the condensed matter easily maintain chemical equilibrium with the total system. Interesting isotopic anomalies are trapped in both types of refractory condensate in the interstellar medium. The nova provides the best astronomical laboratory for observing the condensation.

Clayton, D. D.↗
NASA NTRS1979

On isotopic anomalies in samarium

The solar isotopic composition of Sm is decomposed into s, r, and p components. The anomaly pattern discovered by Lugmair et al. (1978) in EK1-04 Allende inclusion can be presented as a fractionation of the average s-pattern from the average r-pattern. This representation requires a fractionation of 0.029%/(amu) and either (1) a 0.42% deficiency of s relative to r and a 0.15% deficiency of p relative to r, or (2) a 0.42% excess of r relative to s and a 0.27% excess of p relative to s. The nature of this anomaly suggest a systematic physical fractionation of r, s, and p nuclei from each other in the initial condition leading to EK1-04. A neighboring supernova injection would not be expected to produce this anomaly.

Clayton, D. D.↗
NASA NTRS1978

Precondensed matter - Key to the early solar system

Explicit astrophysical details are developed for the hypothesis that chemical and isotopic anomalies in primitive solar-system samples reflect routine initial chemical conditions within precondensed matter. The central feature of this theory concerns the chemical state of presolar dust, which is regarded as never having been vaporized in the region where the most chemically primitive samples (carbonaceous meteorites) accumulated. It is suggested that the initial chemical state of heavy atoms during meteorite and planetary accumulation was distributed between a refractory-mineral component from high-temperature condensation and a volatile component resulting from cold matter adhering to preexisting grains. Thermal conditions in the solar nebula are considered along with the existence of supernova condensates and other thermal condensates in the interstellar dust. Fractionation into volatile and refractory elements is idealized in terms of four distinct interstellar components, and the fractionated precondensed matter is described.

Clayton, D. D.↗
NASA NTRS1978

s-process studies - Xenon and krypton isotopic abundances

We propose an analysis of the s-process contributions to the isotopes of xenon and krypton. The object is to aid studies of the possibility that meteorites may contain gas that was carried in presolar grains that were grown in stellar ejecta and that were not degassed prior to incorporation into parent bodies. That model suggests routine interstellar fractionation of s-isotopes from r-isotopes owing to differential incorporation into dust. We show that a deficiency of s-process nuclei cannot yield details of Xe-X, but the gross similarities are strong enough to lead one to think that such a deficiency may play a role in a more complicated explanation. We predict the existence of an s-rich complement somewhere if fractional separation of this type has played a role in Xe-X. We show that the analogous decomposition of krypton is more uncertain, and we call for measurements of neutron-capture cross sections to alleviate these uncertainties.

Clayton, D. D.↗
NASA NTRS1978

An interpretation of special and general isotopic anomalies in r-process nuclei

My analysis of new discoveries by McCulloch and Wasserburg of Ba and Nd isotopic anomalies in inclusions of the Allende meteorite argues that (1) these anomalies contain special extinct radioactivities resulting from radioactive decay within grains formed in and ejected from the supernova interior, (2) the inclusions studied are fused assemblies of interstellar grains that were never totally vaporized, and (3) theoretical separation into r and s abundances suggests that fluctuations between r and s components has occurred during the accumulation processes. These points lend support to a new chemical picture of the early solar system that I have developed, although many interpretations remain possible. Measurements of the neutron-capture cross sections of Nd isotopes are urgently needed to experimentally validate these conclusions.

Clayton, D. D.↗
NASA NTRS1978

On strontium isotopic anomalies and odd-A p-process abundances

Several aspects of the nucleosynthesis of Sr isotopes are considered in an attempt to shed light on the problem of the Sr isotopic anomalies discovered in an inclusion of the Allende meteorite. Decomposition of the Sr isotopes into average r-, s-, and p-process nucleosynthetic classes is performed. It is suggested that the Allende inclusion most likely has an excess of s-process Sr and that the initial Sr-87/Sr-86 isotopic ratio is probably slightly more primitive than basaltic achondrites. The results also show that Sn-115 is mostly due to the r-process and that odd-A yields are very small. It is concluded that if the Sr anomaly in the inclusion is an average s enhancement, it argues somewhat in favor of a model of gas/dust fractionation of s and r isotopes during accumulation of the inclusion parent in the protosolar cloud.

Clayton, D. D.↗
NASA NTRS1978

The cloudy state of interstellar matter

The expression 'cloudy state' is used to describe the state of the diffuse interstellar matter, with emphasis on its denser and more opaque regions. Questions of morphology with respect to the Galaxy are examined, taking into account neutral hydrogen, molecular regions, H II regions, infrared sources and masers, coronal gas in the Galaxy, and the major components of the interstellar medium. Aspects of dynamics are also considered, giving attention to the two-phase interstellar medium, the three-phase interstellar medium, the density-wave compression of clouds, and problems related to the concept of collapsing clouds. Developments concerning chemistry are explored. Radioactive chronologies are discussed along with isotopic anomalies and aspects of interstellar chemistry.

Clayton, D. D.↗
NASA NTRS1977

Interstellar potassium and argon

Isotopic anomalies are calculated in K and Ar that will be expected to be carried in interstellar grains. Special attention is given to the supernova condensates that precipitate during the expansion of the explosive oxygen-burning zone of the interior, because that is where K-39, K-41 and Ar-36, Ar-38 find their natural origins. Expectations are given for K-Ar ages in excess of the solar age in accumulations of presolar grains, and for K anomalies in Ca-rich mineral within Allende inclusions, because experiments searching for such effects are currently underway. Attention is also given to predictions of excess Ar-36 owing to extinct Cl-36 within presolar grains, and to descriptions of new correlations that can confirm its existence.

Clayton, D. D.↗
NASA NTRS1977

Isotopic anomalies and proton irradiation in the early solar system

Nuclear cross sections relevant to the various isotopic-abundance anomalies found in solar-system objects are evaluated in an attempt to set constraints on the hypothesized mechanism of irradiation of forming planetesimals by energetic protons from the young sun. A power-law proton spectrum is adopted, attention is restricted to proton energies less than about 20 MeV, and average cross sections are calculated for several reactions that might be expected to lead to the observed anomalies. The following specific anomalies are examined in detail: Al-26, Na-22, Xe-126, I-129, Kr-80, V-50, Nb-92, La-138, Ta-180, Hg-196, K-40, Ar-36, O-17, O-18, N-15, C-13, Li, Be, and B. It is suggested that the picture of presolar-grain carriers accounts for the facts more naturally than do irradiation models.

Clayton, D. D.↗
NASA NTRS1976

Gamma-ray emission and nucleosynthesis of lithium by young pulsars

It is proposed that Li-7 is produced in the Galaxy primarily by alpha-alpha collisions surrounding newly born pulsars. About 10 percent of the pulsar energy losses are converted to medium-energy alpha-particles which collide in a dominantly He nebula. The problem of the origin of lithium would be solved by the scenario, and clear-cut tests by nuclear gamma-ray astronomy are described.

Clayton, D. D.↗
NASA NTRS1976

Spectrum of carbonaceous-chondrite fission xenon

Estimations of the fission spectrum in xenon isotopes from the progenitor of the strange carbonaceous-chondrite xenon must take account of p-process nucleosynthesis if the latter is the source of anomalous Xe-124, 126. Sample calculations of the p-process yields illustrate the magnitude of the effect, which can greatly increase the estimated Xe-132 fission yield.

Clayton, D. D.↗
NASA NTRS1976

Grains of anomalous isotopic composition from novae

The effects of grain formation in nova ejecta are studied in order to model the optical and infrared luminosities schematically and to identify the anomalies of isotopic composition that should be present in large abundance in these grains. The large carbon concentration makes rapid and efficient grain formation possible and accounts for the peculiar luminosities observed in Nova Serpentis 1970. In the anticipated range of nova conditions, rapid addition of hot protons during the outburst produces large overabundances of C-13, O-18, Na-22, Al-26, Si-30, and perhaps others. Anomalous C-14 is expected subsequent to the C-13(alpha, n)O-16 reaction, and will be trapped in grains formed by subsequent atmospheric loss. Each of these anomalies may have been detected on the moon (due to accretion of interstellar dust) or in carbonaceous chondrites. Perhaps nova grains are responsible.

Clayton, D. D.↗
NASA NTRS1975

Na-22, Ne-E, extinct radioactive anomalies and unsupported Ar-40

The possibility that the origin of extinct radioactivities depended on their living long enough for grains to form in the expanding nucleosynthetic envelope, rather than on their living long enough for meteorites to form, is examined. As an example, the interpretation of Na-22 as a detectable extinct radioactivity, with a half life of only 2.6 years, is explored and related to Ne-22 occurrence. Similar arguments involving He-4, Ar-40, K-40, K-41, and calcium, titanium, chromium, manganese, iron, nickel, and cobalt isotopes are briefly presented.

Clayton, D. D.↗
NASA NTRS1975

Extinct radioactivities - Trapped residuals of presolar grains

The xenon anomalies trapped in meteorites and the moon may have first been trapped in circumstellar grains formed in or outside of postexplosive stars. In that case, the initial solar nebula need not have contained most of their radioactive progenitors, and this would necessitate major revision of the history of solar system formation.

Clayton, D. D.↗
NASA NTRS1975

Solar models of low neutrino-counting rate - The depleted Maxwellian tail

Evolutionary sequences for the sun are presented which confirm that the Cl-37 neutrino counting rate will be greatly reduced if the high-energy tail of the Maxwellian distribution of relative energies is progressively depleted. Thermonuclear reaction rates and pressure are reevaluated for a distribution function modified by the correction factor suggested by Clayton (1974), and the effect of the results on solar models calculated with a simple Henyey code is discussed. It is shown that if the depletion is characterized by a certain exponential dependence on the distribution function, the counting rate will fall below 1 SNU for a distribution function of not less than 0.01. Suggestions are made for measuring the distribution function in the sun by means of neutrino spectroscopy and photography.

Clayton, D. D.↗
NASA NTRS1975

On emission lines in the cosmic gamma-ray background

The composite spectrum of gamma-rays is calculated which results from the decay of Ni-56 to Co-56 to Fe-56 throughout the history of the universe. The results for several cosmological models are presented and compared with the Apollo 15 measurements at low resolution of the cosmic background. The radioactivity background is a significant fraction of the total, and several of its features may be detectable.-

Clayton, D. D.↗
NASA NTRS1974

On the e-process - Its components and their neutron excesses

The pattern of abundances within the iron-abundance peak of the solar system is analyzed for various Cr, Fe, and Ni abundances, and a method is developed for finding the best fit to a given set of abundances with a chosen number of zones, i.e., mass contributions characterized by differing values of eta. This material can be synthesized by a superposition of e-process compositions in a low-eta region (eta = 0.003) and a high-eta region (eta = 0.065 -0.080) with at least 85% coming from the low-eta region. Addition of a third eta zone is unproductive. The applicability of the particle-poor freeze out is discussed in the light of these abundances, and the results of employing different numbers and types of zones are interpreted as an indication of the relative abundances themselves. Ejection of the low-eta zones is of great interest in gamma-ray astronomy and for empirical testing of theories of nucleosynthesis. The distribution of high zones should give important information about the formation of collapsed remnants.

Hainebach, K. L.↗