Evidence for Nb-92g in-the early solar system and evaluation of a new p-process cosmochronometer from Nb-92g/Mo-92

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). Nb-92g i, a p-process radionuclide with a half-life of similar to 36 Myr. Evidence for the presence of Nb-92g in the early solar system is based upon a well-resolved Zr-92 excess observed in Zr separated from a 110 mu g sample of rare high-Nb/Zr rutile from the Toluca iron meteorite. The initial Nb-92g/Nb-93 ratio in the rutile was (1.6 +/- 0.3) x 10(-5), no later than similar to 10 Myr after the formation of the solar system. Nb-92g is indexed to stable p-only Mo-92 to infer the extent of its decay during presolar Galactic history: Nb-92g/ Mo-92 = (2.9 +/- 0.6) x 10(-5) in the solar abundance distribution. This is 0.7% of the theoretically estimated nucleosynthetic production ratio (similar to 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 Nb-92g/Mo-92 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.