537 research outputs found
Fremdlinge in Leoville and Allende CAI â Clues to Post-Formation Cooling and Alteration
Fremdlinge are perhaps the most exotic and least understood objects in CAI and their very
existence places severe constraints regarding formation and cooling histories of the host CAI.
Following the discovery and description of Willy, which appears to be an "Ur-Fremdling" or
prototype for smaller Fremdlinge in CAI, we have begun a systematic study of CAI of different
petrographic types to see if the Fremdlinge are consistent with a common mode of formation
and differing degrees of reprocessing. Fifteen type B I, B2, and compact A CAI from Allende
and Leoville were selected. The relative abundance of Fremdlinge varies dramatically among
CAI, however some CAI of each type and from each meteorite contain abundant Fremdlinge.
The range of phase assemblages and mineral chemistries of Fremdlinge from Allende and Leoville
is very similar suggesting that the parent sources for Fremdlinge were quite similar. Chemically
and texturally, Fremdlinge grade continuously from complex, heterogeneous Willy-like objects
to altered, homogeneous metal nuggets. Some complex Fremdlinge like Willy were found in B1
CAI; none were observed in B2 CAI
Mg Isotopic studies of Leoville "compact" type a CAI
One long-standing problem in the application of Al-Mg isotopic systematics to the chronology of CAI is the enigmatic Mg isotopic record of hibonite. Hibonite (ideally, CaAI_(12)O_(19)) is one of the first major element bearing phases to appear in the condensation sequence [1] and occurs as a major constituent only in CAI whose bulk composition is considerably more refractory
than Allende Type Bl CAI
The Cr Isotopic Composition of Phosphates in IIIB Iron Meteorites: A Search for ^(53)Mn
The metal phases of iron meteorites contain excess ^(107) Ag most plausibly derived from the in situ decay of ^(107)Pd (T_(1/2) = 6.5 x 10^6y) (Kaiser and Wasserburg, 1983; Chen and Wasserburg, 1983). The Ag isotopic data suggest that diverse types of iron meteorites, including the Type IIIABs, usually associated with fractional crystallization and core formation in planetesimals, formed and cooled to - 600°C within - 10^7y of ^(107)Pd production
Absolute Determination of the 22Na(p,g) Reaction Rate in Novae
Gamma-ray telescopes in orbit around the Earth are searching for evidence of
the elusive radionuclide 22Na produced in novae. Previously published
uncertainties in the dominant destructive reaction, 22Na(p,g)23Mg, indicated
new measurements in the proton energy range of 150 to 300 keV were needed to
constrain predictions. We have measured the resonance strengths, energies, and
branches directly and absolutely by using protons from the University of
Washington accelerator with a specially designed beamline, which included beam
rastering and cold vacuum protection of the 22Na implanted targets. The
targets, fabricated at TRIUMF-ISAC, displayed minimal degradation over a ~ 20 C
bombardment as a result of protective layers. We avoided the need to know the
stopping power, and hence the target composition, by extracting resonance
strengths from excitation functions integrated over proton energy. Our
measurements revealed that resonance strengths for E_p = 213, 288, 454, and 610
keV are stronger by factors of 2.4 to 3.2 than previously reported. Upper
limits have been placed on proposed resonances at 198-, 209-, and 232-keV. We
have re-evaluated the 22Na(p,g) reaction rate, and our measurements indicate
the resonance at 213 keV makes the most significant contribution to 22Na
destruction in novae. Hydrodynamic simulations including our rate indicate that
the expected abundance of 22Na ejecta from a classical nova is reduced by
factors between 1.5 and 2, depending on the mass of the white-dwarf star
hosting the nova explosion.Comment: 20 pages, 18 figures; shortened paper, accepted in Phys. Rev.
On the close to threshold meson production in neutron-neutron collisions
A method of measuring the close to threshold meson production in
neutron-neutron collisions is described where the momenta of the colliding
neutrons can be determined with the accuracy obtainable for the proton-proton
reaction. The technique is based on the double quasi-free nn --> nn X^0
reaction, where deuterons are used as a source of neutronsComment: 6 pages, 2 figures, to be published in Phys. Lett.
Firing frequency response to current and conductance periodic inputs in a Ih/INap biophysical neuron model
A perspective from extinct radionuclides on a Young Stellar Object: The Sun and its accretion disk
Meteorites, which are remnants of solar system formation, provide a direct
glimpse into the dynamics and evolution of a young stellar object (YSO), namely
our Sun. Much of our knowledge about the astrophysical context of the birth of
the Sun, the chronology of planetary growth from micrometer-sized dust to
terrestrial planets, and the activity of the young Sun comes from the study of
extinct radionuclides such as 26Al (t1/2 = 0.717 Myr). Here we review how the
signatures of extinct radionuclides (short-lived isotopes that were present
when the solar system formed and that have now decayed below detection level)
in planetary materials influence the current paradigm of solar system
formation. Particular attention is given to tying meteorite measurements to
remote astronomical observations of YSOs and modeling efforts. Some extinct
radionuclides were inherited from the long-term chemical evolution of the
Galaxy, others were injected into the solar system by a nearby supernova, and
some were produced by particle irradiation from the T-Tauri Sun. The chronology
inferred from extinct radionuclides reveals that dust agglomeration to form
centimeter-sized particles in the inner part of the disk was very rapid (<50
kyr), planetesimal formation started early and spanned several million years,
planetary embryos (possibly like Mars) were formed in a few million years, and
terrestrial planets (like Earth) completed their growths several tens of
million years after the birth of the Sun.Comment: 49 pages, 9 figures, 1 table. Uncorrected preprin
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