科研之友
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摘要
In agreement with previous work, we show that the presence of the short-lived radionuclide (SLR) Al-26 in the early solar system was unlikely (less than 2% a priori probability) to be the result of direct introduction of supernova (SN) ejecta into the gaseous disk during the Class II stage of protosolar evolution. We also show that Bondi-Hoyle accretion of any contaminated residual gas from the Sun's natal star cluster contributed negligible Al-26 to the primordial solar system. Our calculations are consistent with the absence of the oxygen isotopic signature expected with any late introduction of SN ejecta into the protoplanetary disk. Instead, the presence of Al-26 in the oldest solar system solids (calcium-aluminum-rich inclusions (CAIs)) and its apparent uniform distribution with the inferred canonical Al-26/Al-27 ratio of (4.5-5) x 10(-5) support the inheritance of Al-26 from the Sun's parent giant molecular cloud. We propose that this radionuclide originated in a prior generation of massive stars that formed in the same molecular cloud and contaminated that cloud by Wolf-Rayet winds. We calculated the Galactic distribution of Al-26/Al-27 ratios that arise from such contamination using the established embedded cluster mass and stellar initial mass functions, published nucleosynthetic yields from the winds of massive stars, and by assuming rapid and uniform mixing into the cloud. Although our model predicts that the majority of stellar systems contain no Al-26 from massive stars, and that the a priori probability that the Al-26/Al-27 ratio will reach or exceed the canonical solar system value is only similar to 6%, the maximum in the distribution of nonzero values is close to the canonical Al-26/Al-27 ratio. We find that the Sun most likely formed 4-5 million years (Myr) after the massive stars that were the source of Al-26. Furthermore, our model can explain the initial solar system abundance of a second, co-occurring SLR, Ca-41, if similar to 5 x 10(5) yr elapsed between ejection of the radionuclides and the formation of CAIs. The presence of a third radionuclide, Fe-60, can be quantitatively explained if (1) the Sun formed immediately after the first SNe from the earlier generation of stars; (2) only 5% of SN ejecta was incorporated into the molecular cloud, or (3) the radionuclide originated in an even earlier generation of stars whose contributions to other radionuclides with a shorter half-life had completely decayed.
- 出版日期2009-5-10
