Zirconium and Molybdenum in Individual Circumstellar Graphite Grains: New Isotopic Data on the Nucleosynthesis of Heavy Elements
- Enrico Fermi Institute, University of Chicago, Chicago, IL 60637 (United States)
- Materials Science and Chemistry Divisions, Argonne National Laboratory, Argonne, IL 60439 (United States)
We have analyzed 32 individual graphite grains from the Murchison meteorite for their Mo and/or Zr isotopic compositions by resonant ionization mass spectrometry. Enormous isotopic anomalies were observed in some of these grains for both elements. The data for Zr revealed the largest isotopic anomalies, with {sup 96}Zr/{sup 94}Zr ratios ranging from 0.074 times to 10 times the solar value. The isotopic data on Mo show one population of graphite grains with close-to-terrestrial Mo composition in all isotopes and five grains with an {ital s}-process nucleosynthesis signature, i.e., correlated depletions in the {ital p}- and {ital r}-process isotopes. For eight grains we were able to measure both Mo and Zr isotopic compositions. Three of these eight graphite grains have {ital s}-process isotopic characteristics for both Zr and Mo, which suggests low-mass, thermally pulsed asymptotic giant branch stars as their origin. Four grains are puzzling, since they have nearly normal Mo compositions but significant anomalies in Zr, in particular, large depletions or enhancements in the {sup 96}Zr/{sup 94}Zr ratio. Two of these grains have extraordinary enrichments in {sup 96}Zr, with {sup 96}Zr/{sup 94}Zr ratios 10.4 {plus_minus} 1.3 and 2.5 {plus_minus} 0.3 times the solar system value. These enrichments are suggestive of the {ital r}-process, implying that these grains condensed from the ejecta of core-collapse supernovae, but these enrichments could also be made by the {ital s}-process if the neutron density were unusually high. {copyright} {ital {copyright} 1998.} {ital The American Astronomical Society}
- OSTI ID:
- 303872
- Journal Information:
- Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 504; ISSN ASJOAB; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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