摘要:

Primitive meteorites contain interstellar grains that formed in stellar outflows or from supernova ejecta, survived interstellar travel and the formation of the solar system, and were incorporated into meteorites. To date diamond, silicon carbide, graphite, corundum, and possibly silicon nitride have been identified. Their isotopic compositions provide new information on nucleosynthesis and stellar evolution and on the numbers and types of stars that contributed material to the solar system. Examples are the C‐, N‐, and Si‐isotopic ratios of SiC grains from AGB stars, the C‐, N‐, O‐, Mg‐, and Si‐isotopic ratios of graphite grains from massive stars (WR or SN), and the O‐isotopic ratios of corundum grains from red giants. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47340

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Graphite grains from two density fractions isolated from the Murchison carbonaceous chondrite have been measured for their C, N, O, Mg, Si, Ca, and Ti isotopic compositions. Many graphite grains from the low density fraction (1.65–1.72 g/cm3) have enormous18O excesses (18O/16O ranges up to 100x solar).28Si excesses, and26Al/27Al ratios from 10−2to 10−1. Some of the grains also have42Ca,43Ca, and49Ti excesses. These isotopic ratios indicate that the low density grains originated from massive stars. Type II supernovae can qualitatively explain the correlations between several isotopic ratios. Graphite grains from the high density fraction (2.15–2.20 g/cm3), on the other hand, have no18O and28Si excesses, but have29Si and30Si excesses. None of them have26Mg excesses originating from the decay of26Al. Several grains also have42Ca,43Ca, and49Ti excesses. These and the Kr isotopic ratios suggest AGB stars as probable stellar sources. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47420

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Isotopic analyses of forty circumstellar oxide grains isolated from primitive meteorites have revealed a large range of O‐isotopic ratios and evidence for the prior presence of the short‐lived radionuclide26Al. One group of grains is characterized by slight18O depletions, and a large range of16O/17O and intial26Al/27Al ratios:16O/18O=502–850 (Solar ratio=499),16O/17O=385–5000 (Solar ratio=2510), and26Al/27Al=(0–8)×10−3. These grains probably formed around red giant stars. Another group had extreme18O depletions, as well as enrichments in26Al and17O:16O/18O=0.016–2.3)×105,16O/17O=735–1500, initial26Al/27Al=(2–8)×10−3. These grains may have formed in hot‐bottom‐burning TP‐AGB (thermally pulsing asymptotic giant branch) stars or in Wolf‐Rayet stars. Three grains have enrichments in18O(16O/18O=164–320); one of these grains also has25Mg and26Mg excesses, suggestive of He‐burning. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47341

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A reliable estimate of the abundance of60Fe in the early solar system is critically dependent on the absolute ages of the meteorites in which its decay product,60Ni, was first discovered. Because of their violent early history the ages of these meteorites are difficult to determine with the required resolution and accuracy. The long‐lived U–Pb chronometers reflect these disturbances and do not yield the primary formation ages of these meteorites. An indirect approach had to be taken using the short‐lived53Mn–53Cr chronometer. The results indicate that the60Fe abundance in the early solar system may have been lower than originally assumed. This lower estimate brings it closer to coincide with recent results obtained for stellar production rates. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47342

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

41K excess that can be attributed toin‐situdecay of41Ca has been found in several Ca–Al‐rich refractory inclusions (CAIs) of the Efremovka carbonaceous chondrite. Our observation shows that41Ca (&tgr;∼0.15 Myr) was present in the early solar system. We have inferred an initial41Ca/40Ca ratio of (1.6±0.3)×10−8at the time of formation of the Efremovka CAIs based on our data. Observation of excess41K in these objects constrains the time interval between the cessation of fresh nucleosynthetic input to the solar nebula and the formation of some of the first solar system solids (CAIs) to less than a million years. Several possibilities exist for the stellar nucleosynthesis of41Ca and its addition to the early solar system. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47343

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Abinitioabundances of extinct radio nuclides in the solar abundance distribution (SAD) provide clues sufficient to characterize the molecular cloud environment within which the solar system formed 4566±2 Ma ago. Key observations are: (i) the low abundance of the longer‐lived r‐process radionuclide129I indicates an ∼102Ma isolation time from freshly‐synthesized Type II supernova products; (ii) abundances of the shorter‐lived species (26Al,60Fe,53Mn) are consistent with late ‘‘injection’’ of freshly‐synthesized Type II supernova produces. This apparent contradiction is resolved in a simple two time scale molecular cloud self‐contamination model consistent with formation of the sun in an evolved star‐forming region in the vicinity of an OB association. Admixture of an ∼10−5to ∼10−6mass fraction of Type II supernova ejecta into the cloud dominates the shorter‐lived species and107Pd, but contributes only ∼1/2 of the129I budget.129I consequently preserves the longer time scale information constraining the mean isolation/condensation/accretion model age of the cool molecular mass reservoir with respect to129I/127I in the global ISM. All extinct radionuclide abundances except26Al are reproduced in this model. An alternate hypothesis involving late admixture of matter from a mass‐losing low mass AGB star can account for26Al,60Fe, and107Pd, but fails for53Mn, requires unusual s‐process conditions, and otherwise appearsadhocand unlikely. Cosmic ray spallation in an OB association environment may contribute significantly to53Mn in the protosolar reservoir, but is limited as a putative26Al source by over production of53Mn,92gNb, and Li. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47344

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A new explanation of Xe isotopic composition in the terrestrial mantle has been suggested on the basis of Xe isotopic analyses in the n‐irradiated pitchblende, in the natural nuclear reactor Oklo, in Colorado type deposit, in the sandstone from the nuclear test epicenter. Xe isotopic anomalies in these samples and in some meteorites as well as in mantle rocks and well gases can be explained by the migration processes of &bgr;−‐active xenon precursors in the isobaric chains formed during heavy nuclei fission. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47346

出版商:AIP    

年代:1995

数据来源: AIP