EVIDENCE FOR ~(92g)Nb IN THE EARLY SOLAR SYSTEM AND EVALUATION OF A NEW p-PROCESS COSMOCHRONOMETER FROM ~(92g)Nb/~(92)Mo

摘要

Initial abundances of extinct radionuclides in the solar system constrain both the history of nucleosynthesis in the Galaxy and the age of the solar system's parental molecular cloud complex (PMCC). ~(92g)Nb is a p-process radionuclide with a half-life of ～36 Myr. Evidence for the presence of ~(92g)Nb in the early solar system is based upon a well-resolved ~(92)Zr excess observed in Zr separated from a 110 μg sample of rare high-Nb/Zr rutile from the Toluca iron meteorite. The initial ~(92g)Nb/~(93)Nb ratio in the rutile was (1.6 ± 0.3) x 10~(-5), no later than ～10 Myr after the formation of the solar system. ~(92g)Nb is indexed to stable p-only ~(92)Mo to infer the extent of its decay during presolar Galactic history: ~(92g)Nb/ ~(92)Mo = (2.9 ± 0.6) x 10~(-5) in the solar abundance distribution. This is 0.7% of the theoretically estimated nucleosynthetic production ratio (～4 x 10~(-3)) for the p-process in both Type Ia and Type II supernova models, indicating a 9-29 Gyr model age range for the p-process in the Galactic disk at the solar Galactocentric radius. A best estimate of 15 Gyr is closely consistent with a 12 ± 2 Gyr disk age determined independently from nuclear cosmochronology, photometry-isochrone stellar ages, and the white dwarf luminosity function. Alternatively, if the age of the disk is known, then ~(92g)Nb/~(92)Mo can be used to estimate the age of the PMCC. The results suggest that the Sun formed 25 ± 15 Myr after the formation of its parental complex and therefore likely in a highly evolved cloud in the vicinity of an OB association. Other shorter lived extinct radionuclide abundances are consistent with self-contamination of the cloud by one or more massive star supernovae and provide further independent support for the OB association model for the origin of the solar system.