Abstract

Studies of polluted white dwarf stars indicate that melting of small planetesimals was common outside the solar system. This in turn requires solar-like concentrations of 26Al (mean life = 1 Myr) in other planetary systems and implies that the solar system was not unusual in its complement of short-lived radionuclides. Indeed, apparent excesses in early-solar 26Al, 36Cl, 41Ca, and 60Fe relative to expectations for the ISM disappear if one accounts for ejecta from massive-star winds enriching star-forming regions. The removal of apparent excesses is evident when wind yields from Wolf-Rayet stars are included in the plot of radionuclide abundances normalized by their production vs. mean life. The resulting trend indicates that the solar radionuclides were inherited from parental molecular clouds with a characteristic cloud residence time of 10^8 years. This residence time is of the same order as the present-day timescale for conversion of molecular cloud material into stars. The concentrations of these extinct isotopes in the early solar system need not signify injection from unusual proximal stellar sources, but instead are well explained by normal concentrations in average star-forming clouds. The results imply that the efficiency of capture is greater for stellar winds than for supernova ejecta proximal to star-forming regions. They also suggest that enrichment of short-lived radionuclides is common in large and long-lived star-forming regions such as Cygnus X.

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