Isotopic compositions of strontium, zirconium, molybdenum, and barium in single presolar SiC grains and asymptotic giant branch stars.
The strontium, zirconium, molybdenum, and barium isotopic compositions predicted in the mass-losing envelopes of asymptotic giant branch (AGB) stars of solar metallicity and mass 1.5, 3, and 5 M{sup {circle_dot}} are discussed and compared with recent measurements in single presolar silicon carbide (SiC) grains from the Murchison meteorite. Heavy-element nucleosynthesis via the s-process occurs in the helium intershell, the region between the helium-burning and hydrogen-burning shells, producing heavy elements beyond iron. After a limited number of thermal runaways of the helium shell (thermal pulses), at the quenching of each instability, the convective envelope penetrates into the top layers of the helium intershell (third dredge-up), mixing newly synthesized {sup 12}C and s-process material to the stellar surface. Eventually, the envelope becomes carbon-rich (C {>=} O), a necessary condition for SiC grains to condense. In the helium intershell, neutrons are released by (a, n) reactions on {sup 13}C and {sup 22}Ne during interpulse phases and the thermal pulses, respectively. A {sup 13}C pocket is assumed to form in a tiny region in the top layers of the helium intershell by injection of a small amount of protons from the envelope during each third dredge-up episode. This {sup 13}C then burns radiately during the interpulse phase. The average neutron density produced is low, but of long duration, so the total neutron exposure is high. We have explored a large range of possible {sup 13}C abundances in the pocket. In low-mass AGB stars (1.5 M{sup {circle_dot}} {<=} M {<=} 4 M{sup {circle_dot}}), a second small burst of neutrons is released by marginal {sup 22}Ne burning in the thermal pulse. The neutron density reaches quite a high peak value but is of short duration, so the neutron exposure is low. In intermediate-mass AGB stars (4 M{sup {circle_dot}} < M {<=} 8 M{sup {circle_dot}}), the {sup 22}Ne neutron source is more efficiently activated. The neutron capture process has been followed with a postprocessing code that considers all relevant nuclei from {sup 4}He to {sup 210}Po. The predicted isotopic compositions of strontium, zirconium, molybdenum, and barium in the envelopes of low-mass AGB stars of solar metallicity are in agreement with the isotopic ratios measured in individual presolar SiC grains, whereas predictions for intermediate-mass stars exclude them as the sources of these grains. A multiplicity of low-mass AGB stars with metallicity around solar, having different masses and experiencing different neutron exposures, are required to account for the measured spread in heavy-element isotopic compositions among single presolar SiC grains. The range of neutron exposures corresponds, on average, to a lower mean neutron exposure than that required to reproduce the s-process main component in the solar system.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC); National Aeronautics and Space Administration (NASA)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 961067
- Report Number(s):
- ANL/MSD/JA-44329; ASJOAB; TRN: US1003248
- Journal Information:
- Astrophys. J., Vol. 593, Issue 1, Pt. 1 ; Aug. 10, 2003; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- ENGLISH
Similar Records
Evolution and nucleosynthesis of asymptotic giant branch stellar models of low metallicity
Mainstream silicon carbide grains from meteorites
Related Subjects
70 PLASMA PHYSICS AND FUSION TECHNOLOGY
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
BARIUM
CAPTURE
CARBON
ELEMENTS
HELIUM
HELIUM BURNING
HYDROGEN BURNING
INJECTION
INSTABILITY
IRON
LAYERS
MASS
MATERIALS
MIXING
MOLYBDENUM
MULTIPLICITY
NEUTRON DENSITY
NEUTRON REACTIONS
NEUTRON SOURCES
NEUTRONS
NUCLEAR REACTIONS
NUCLEI
NUCLEOSYNTHESIS
PROTONS
PULSES
QUENCHING
S PROCESS
SILICON CARBIDES
SOLAR SYSTEM
STAR EVOLUTION
STARS
STRONTIUM
ZIRCONIUM